Determinants of the dual cofactor specificity and substrate cooperativity of the human mitochondrial NAD(P)+-dependent malic enzyme: functional roles of glutamine 362

Suppression of the human malic enzyme 2 modifies energy metabolism and inhibits cellular respiration

Human mitochondrial NAD(P)⁺-dependent malic enzyme (ME2) is well-known for its role in cell metabolism, which may be involved in cancer or epilepsy. We present potent ME2 inhibitors based on cyro-EM structures that target ME2 enzyme activity. Two structures of ME2-inhibitor complexes demonstrate that 5,5'-Methylenedisalicylic acid (MDSA) and embonic acid (EA) bind allosterically to ME2's fumarate-binding site. Mutagenesis studies demonstrate that Asn35 and the Gln64-Tyr562 network are required for both inhibitors' binding. ME2 overexpression increases pyruvate and NADH production while decreasing the cell's NAD⁺/NADH ratio; however, ME2 knockdown has the opposite effect. MDSA and EA inhibit pyruvate synthesis and thus increase the NAD⁺/NADH ratio, implying that these two inhibitors interfere with metabolic changes by inhibiting cellular ME2 activity. ME2 silence or inhibiting ME2 activity with MDSA or EA decreases cellular respiration and ATP synthesis. Our findings suggest that ME2 is crucial for mitochondrial pyruvate and energy metabolism, as well as cellular respiration, and that ME2 inhibitors could be useful in the treatment of cancer or other diseases that involve these processes.

New insights into the substrate inhibition of human 17β-hydroxysteroid dehydrogenase type 1

Human type 1 17β-hydroxysteroid dehydrogenase (17β-HSD1),a member of the short-chain dehydrogenase/reductase family, catalyzes the last step in the bioactivation of the most potent estrogen estradiol with high specificity and is thus involved in estrogen-dependent diseases. As an oxidoreductase, 17β-HSD1 can utilize both triphosphate and diphosphate cofactors in reaction at the molecular level, but more specific with triphosphate cofactor. The NADPH is much higher than NADP+ in living cells leading to preliminary reduction action. The enzyme also showed substrate-induced inhibition unprecedented in other members of 17β-HSDs. Our previous study elucidated the structural mechanism of substrate inhibition is due to the reversely bound estrone (E1) in the substrate-binding pocket of the enzyme resulting in a dead-end complex. However, the effect of the cofactor preference on the substrate inhibition of the enzyme is not yet clear. In the present study, we solved the ternary crystal structures of 17β-HSD1 in complex with E1 and cofactor analog NAD+ . Combined with molecular dynamics simulation using the enzyme with NADH/NADPH and different oriented E1 (normally oriented, E1N; reversely oriented, E1R), such ternary structure provides a complete picture of enzyme-substrate-cofactor interactions. The results reveal that different cofactors and substrate binding mode affect the allosteric effect between the two subunits of the enzyme. And the results from MD simulations confirmed that His²²¹ plays a key role in the formation of dead-end complex in NADPH complex, and the absence of stable interaction between His²²¹ and E1R in the NADH complex should be the main reason for its lack of substrate inhibition.

Integrative structural and functional analysis of human malic enzyme 3: A potential therapeutic target for pancreatic cancer

Malic enzymes (ME1, ME2, and ME3) are involved in cellular energy regulation, redox homeostasis, and biosynthetic processes, through the production of pyruvate and reducing agent NAD(P)H. Recent studies have implicated the third and least well-characterized isoform, mitochondrial NADP⁺-dependent malic enzyme 3 (ME3), as a therapeutic target for pancreatic cancers. Here, we utilized an integrated structure approach to determine the structures of ME3 in various ligand-binding states at near-atomic resolutions. ME3 is captured in the open form existing as a stable tetramer and its dynamic Domain C is critical for activity. Catalytic assay results reveal that ME3 is a non-allosteric enzyme and does not require modulators for activity while structural analysis suggests that the inner stability of ME3 Domain A relative to ME2 disables allostery in ME3. With structural information available for all three malic enzymes, the foundation has been laid to understand the structural and biochemical differences of these enzymes and could aid in the development of specific malic enzyme small molecule drugs.

Single nucleotide variants lead to dysregulation of the human mitochondrial NAD(P)+-dependent malic enzyme

Human mitochondrial NAD(P)⁺-dependent malic enzyme (ME2) is well recognized to associate with cancer cell metabolism, and the single nucleotide variants (SNVs) of ME2 may play a role in enzyme regulation. Here we reported that the SNVs of ME2 occurring in the allosteric sites lead to inactivation or overactivation of ME2. Two ME2-SNVs, ME2_R67Q and ME2-R484W, that demonstrated inactivating or overactivating enzyme activities of ME2, respectively, have different impact toward the cells. The cells with overactivating SNV enzyme, ME2_R484W, grow more rapidly and are more resistant to cellular senescence than the cells with wild-type or inactivating SNV enzyme, ME2_R67Q. Crystal structures of these two ME2-SNVs reveal that ME2_R67Q was an inactivating "dead form," and ME2_R484W was an overactivating "closed form" of the enzyme. The resolved ME2-SNV structures provide a molecular basis to explain the abnormal kinetic properties of these SNV enzymes.

Structural and Molecular Dynamics of Mycobacterium tuberculosis Malic Enzyme, a Potential Anti-TB Drug Target

Tuberculosis (TB) is the most lethal bacterial infectious disease worldwide. It is notoriously difficult to treat, requiring a cocktail of antibiotics administered over many months. The dense, waxy outer membrane of the TB-causing agent, Mycobacterium tuberculosis (Mtb), acts as a formidable barrier against uptake of antibiotics. Subsequently, enzymes involved in maintaining the integrity of the Mtb cell wall are promising drug targets. Recently, we demonstrated that Mtb lacking malic enzyme (MEZ) has altered cell wall lipid composition and attenuated uptake by macrophages. These results suggest that MEZ contributes to lipid biosynthesis by providing reductants in the form of NAD(P)H. Here, we present the X-ray crystal structure of MEZ to 3.6 Å. We use biochemical assays to demonstrate MEZ is dimeric in solution and to evaluate the effects of pH and allosteric regulators on its kinetics and thermal stability. To assess the interactions between MEZ and its substrate malate and cofactors, Mn²⁺ and NAD(P)⁺, we ran a series of molecular dynamics (MD) simulations. First, the MD analysis corroborates our empirical observations that MEZ is unusually flexible, which persists even with the addition of substrate and cofactors. Second, the MD simulations reveal that dimeric MEZ subunits alternate between open and closed states, and that MEZ can stably bind its NAD(P)⁺ cofactor in multiple conformations, including an inactive, compact NAD⁺ form. Together the structure of MEZ and insights from its dynamics can be harnessed to inform the design of MEZ inhibitors that target Mtb and not human malic enzyme homologues.

Increasing lipid production using an NADP+-dependent malic enzyme from Rhodococcus jostii

The occurrence of NADP⁺-dependent malic enzymes (NADP⁺-MEs) in several Rhodococcus strains was analysed. The NADP⁺-ME number in Rhodococcus genomes seemed to be a strain-dependent property. Total NADP⁺-ME activity increased by 1.8- and 2.6-fold in the oleaginous Rhodococcus jostii RHA1 and Rhodococcus opacus PD630 strains during cultivation under nitrogen-limiting conditions. Total NADP⁺-ME activity inhibition by sesamol resulted in a significant decrease of the cellular biomass and lipid production in oleaginous rhodococci. A non-redundant ME coded by the RHA1_RS44255 gene located in a megaplasmid (pRHL3) of R. jostii RHA1 was characterized and its heterologous expression in Escherichia coli resulted in a twofold increase in ME activity in an NADP⁺-dependent manner. The overexpression of RHA1_RS44255 in RHA1 and PD630 strains grown on glucose promoted an increase in total NADP⁺-ME activity and an up to 1.9-foldincrease in total fatty acid production without sacrificing cellular biomass. On the other hand, its expression in Rhodococcus fascians F7 grown on glycerol resulted in a 1.3-1.4-foldincrease in total fatty acid content. The results of this study confirmed the contribution of NADP⁺-MEs to TAG accumulation in oleaginous rhodococci and the utility of these enzymes as an alternative approach to increase bacterial oil production from different carbon sources.

Protein Engineering for Nicotinamide Coenzyme Specificity in Oxidoreductases: Attempts and Challenges

Oxidoreductases are ubiquitous enzymes that catalyze an extensive range of chemical reactions with great specificity, efficiency, and selectivity. Most oxidoreductases are nicotinamide cofactor-dependent enzymes with a strong preference for NADP or NAD. Because these coenzymes differ in stability, bioavailability and costs, the enzyme preference for a specific coenzyme is an important issue for practical applications. Different approaches for the manipulation of coenzyme specificity have been reported, with different degrees of success. Here we present various attempts for the switching of nicotinamide coenzyme preference in oxidoreductases by protein engineering. This review covers 103 enzyme engineering studies from 82 articles and evaluates the accomplishments in terms of coenzyme specificity and catalytic efficiency compared to wild type enzymes of different classes. We analyzed different protein engineering strategies and related them with the degree of success in inverting the cofactor specificity. In general, catalytic activity is compromised when coenzyme specificity is reversed, however when switching from NAD to NADP, better results are obtained. In most of the cases, rational strategies were used, predominantly with loop exchange generating the best results. In general, the tendency of removing acidic residues and incorporating basic residues is the strategy of choice when trying to change specificity from NAD to NADP, and vice versa. Computational strategies and algorithms are also covered as helpful tools to guide protein engineering strategies. This mini review aims to give a general introduction to the topic, giving an overview of tools and information to work in protein engineering for the reversal of coenzyme specificity.

Molecular and functional characterization of two malic enzymes from Leishmania parasites

Leishmania parasites cause a broad spectrum of clinical manifestations in humans and the available clinical treatments are far from satisfactory. Since these pathogens require large amounts of NADPH to maintain intracellular redox homeostasis, oxidoreductases that catalyze the production of NADPH are considered as potential drug targets against these diseases. In the sequenced genomes of most Leishmania spp. two putative malic enzymes (MEs) with an identity of about 55% have been identified. In this work, the ME from L. major (LmjF24.0770, Lmj_ME-70) and its less similar homolog from L. mexicana (LmxM.24.0761, Lmex_ME-61) were cloned and functionally characterized. Both MEs specifically catalyzed NADPH production, but only Lmex_ME-61 was activated by l-aspartate. Unlike the allosterically activated human ME, Lmex_ME-61 exhibited typical hyperbolic curves without any sign of cooperativity in the absence of l-aspartate. Moreover, Lmex_ME-61 and Lmj_ME-70 differ from higher eukaryotic homologs in that they display dimeric instead of tetrameric molecular organization. Homology modeling analysis showed that Lmex_ME-61 and Lmj_ME-70 notably differ in their surface charge distribution; this feature encompasses the coenzyme binding pockets as well. However, in both isozymes, the residues directly involved in the coenzyme binding exhibited a good degree of conservation. Besides, only Lmex_ME-61 and its closest homologs were immunodetected in cell-free extracts from L. mexicana, L. amazonensis and L. braziliensis promastigotes. Our findings provide a first glimpse into the biochemical properties of leishmanial MEs and suggest that MEs could be potentially related to the metabolic differences among the species of Leishmania parasites.

Engineering the cofactor specificity of an alcohol dehydrogenase via single mutations or insertions distal to the 2'-phosphate group of NADP(H)

There have been many reports exploring the engineering of the cofactor specificity of aldo-keto reductases (AKRs), as this class of proteins is ubiquitous and exhibits many useful activities. A common approach is the mutagenesis of amino acids involved in interactions with the 2'-phosphate group of NADP(H) in the cofactor binding pocket. We recently performed a 'loop-grafting' approach to engineer the substrate specificity of the thermostable alcohol dehydrogenase D (AdhD) from Pyrococcus furiosus and we found that a loop insertion after residue 211, which is on the back side of the cofactor binding pocket, could also alter cofactor specificity. Here, we further explore this approach by introducing single point mutations and single amino acid insertions at the loop insertion site. Six different mutants of AdhD were created by either converting glycine 211 to cysteine or serine or by inserting alanine, serine, glycine or cysteine between the 211 and 212 residues. Several mutants gained activity with NADP+ above the wild-type enzyme. And remarkably, it was found that all of the mutants investigated resulted in some degree of reversal of cofactor specificity in the oxidative direction. These changes were generally a result of changes in conformations of the ternary enzyme/cofactor/substrate complexes as opposed to changes in affinities or binding energies of the cofactors. This study highlights the role that amino acids which are distal to the cofactor binding pocket but are involved in substrate interactions can influence cofactor specificity in AdhD, and this strategy should translate to other AKR family members.

A small-molecule inhibitor suppresses the tumor-associated mitochondrial NAD(P)+-dependent malic enzyme (ME2) and induces cellular senescence

Here, we found a natural compound, embonic acid (EA), that can specifically inhibit the enzymatic activity of mitochondrial NAD(P)+-dependent malic enzyme (m-NAD(P)-ME, ME2) either in vitro or in vivo. The in vitro IC50 value of EA for m-NAD(P)-ME was 1.4 ± 0.4 μM. Mutagenesis and binding studies revealed that the putative binding site of EA on m-NAD(P)-ME is located at the fumarate binding site or at the dimer interface near the site. Inhibition studies reveal that EA displayed a non-competitive inhibition pattern, which demonstrated that the binding site of EA was distinct from the active site of the enzyme. Therefore, EA is thought to be an allosteric inhibitor of m-NAD(P)-ME. Both EA treatment and knockdown of m-NAD(P)-ME by shRNA inhibited the growth of H1299 cancer cells. The protein expression and mRNA synthesis of m-NAD(P)-ME in H1299 cells were not influenced by EA, suggesting that the EA-inhibited H1299 cell growth occurs through the suppression of in vivo m-NAD(P)-ME activity EA treatment further induced the cellular senescence of H1299 cells. However, down-regulation of the enzyme-induced cellular senescence was not through p53. Therefore, the EA-evoked senescence of H1299 cells may occur directly through the inhibition of ME2 or a p53-independent pathway.

A computational strategy for altering an enzyme in its cofactor preference to NAD(H) and/or NADP(H)

Coenzyme engineering, especially for altered coenzyme specificity, has been a research hotspot for more than a decade. In the present study, a novel computational strategy that enhances the hydrogen-bond interaction between an enzyme and a coenzyme was developed and utilized to alter the coenzyme preference. This novel computational strategy only required the structure of the target enzyme. No other homologous enzymes were needed to achieve alteration in the coenzyme preference of a certain enzyme. Using our novel strategy, Gox2181 was reconstructed from exhibiting complete NADPH preference to exhibiting dual cofactor specificity for NADH and NADPH. Structure-guided Gox2181 mutants were designed in silico and molecular dynamics simulations were performed to evaluate the strength of hydrogen-bond interactions between the enzyme and the coenzyme NADPH. Three Gox2181 mutants displaying high structure stability and structural compatibility to NADH/NADPH were chosen for experimental confirmation. Among the three Gox2181 mutants, Gox2181-Q20R&D43S showed the highest enzymatic activity by utilizing NADPH as its coenzyme, which was even better than the wild-type enzyme. In addition, isothermal titration calorimetry analysis further verified that Gox2181-Q20R&D43S was able to interact with NADPH but the wild-type enzyme could not. This novel computational strategy represents an insightful approach for altering the cofactor preference of target enzymes.

Structure-based conversion of the coenzyme requirement of a short-chain dehydrogenase/reductase involved in bacterial alginate metabolism

The alginate-assimilating bacterium, Sphingomonas sp. strain A1, degrades the polysaccharides to monosaccharides through four alginate lyase reactions. The resultant monosaccharide, which is nonenzymatically converted to 4-deoxy-L-erythro-5-hexoseulose uronate (DEH), is further metabolized to 2-keto-3-deoxy-D-gluconate by NADPH-dependent reductase A1-R in the short-chain dehydrogenase/reductase (SDR) family. A1-R-deficient cells produced another DEH reductase, designated A1-R', with a preference for NADH. Here, we show the identification of a novel NADH-dependent DEH reductase A1-R' in strain A1, structural determination of A1-R' by x-ray crystallography, and structure-based conversion of a coenzyme requirement in SDR enzymes, A1-R and A1-R'. A1-R' was purified from strain A1 cells and enzymatically characterized. Except for the coenzyme requirement, there was no significant difference in enzyme characteristics between A1-R and A1-R'. Crystal structures of A1-R' and A1-R'·NAD(+) complex were determined at 1.8 and 2.7 Å resolutions, respectively. Because of a 64% sequence identity, overall structures of A1-R' and A1-R were similar, although a difference in the coenzyme-binding site (particularly the nucleoside ribose 2' region) was observed. Distinct from A1-R, A1-R' included a negatively charged, shallower binding site. These differences were caused by amino acid residues on the two loops around the site. The A1-R' mutant with the two A1-R-typed loops maintained potent enzyme activity with specificity for NADPH rather than NADH, demonstrating that the two loops determine the coenzyme requirement, and loop exchange is a promising method for conversion of coenzyme requirement in the SDR family.

Human mitochondrial NAD(P)(+)-dependent malic enzyme participates in cutaneous melanoma progression and invasion

Cutaneous melanoma is the most life-threatening neoplasm of the skin, accounting for most of the skin cancer deaths. Accumulating evidence suggests that targeting metabolism is an appealing strategy for melanoma therapy. Mitochondrial NAD(P)(+)-dependent malic enzyme (ME2), an oxidative decarboxylase, was evaluated for its biological significance in cutaneous melanoma progression. ME2 mRNA and protein expression significantly increased during melanoma progression, as evidenced by Gene Expression Omnibus analysis and immunohistochemistry on clinically annotated tissue microarrays, respectively. In addition, ME2 knockdown attenuated melanoma cell proliferation in vitro. ME2 ablation resulted in reduced cellular ATP levels and elevated cellular reactive oxygen species production, which activated the AMP-activated protein kinase pathway and inhibited acetyl-CoA carboxylase. Furthermore, ME2 expression was associated with cell migration and invasion. ME2 knockdown decreased anchorage-independent growth in vitro and tumor cell growth in vivo. These results suggested that ME2 might be an important factor in melanoma progression and a novel biomarker of invasion.

Structural characteristics of the nonallosteric human cytosolic malic enzyme

Human cytosolic NADP(+)-dependent malic enzyme (c-NADP-ME) is neither a cooperative nor an allosteric enzyme, whereas mitochondrial NAD(P)(+)-dependent malic enzyme (m-NAD(P)-ME) is allosterically activated by fumarate. This study examines the molecular basis for the different allosteric properties and quaternary structural stability of m-NAD(P)-ME and c-NADP-ME. Multiple residues corresponding to the fumarate-binding site were mutated in human c-NADP-ME to correspond to those found in human m-NAD(P)-ME. Additionally, the crystal structure of the apo (ligand-free) human c-NADP-ME conformation was determined. Kinetic studies indicated no significant difference between the wild-type and mutant enzymes in Km,NADP, Km,malate, and kcat. A chimeric enzyme, [51-105]_c-NADP-ME, was designed to include the putative fumarate-binding site of m-NAD(P)-ME at the dimer interface of c-NADP-ME; however, this chimera remained nonallosteric. In addition to fumarate activation, the quaternary structural stability of c-NADP-ME and m-NAD(P)-ME is quite different; c-NADP-ME is a stable tetramer, whereas m-NAD(P)-ME exists in equilibrium between a dimer and a tetramer. The quaternary structures for the S57K/N59E/E73K/S102D and S57K/N59E/E73K/S102D/H74K/D78P/D80E/D87G mutants of c-NADP-ME are tetrameric, whereas the K57S/E59N/K73E/D102S m-NAD(P)-ME quadruple mutant is primarily monomeric with some dimer formation. These results strongly suggest that the structural features near the fumarate-binding site and the dimer interface are highly related to the quaternary structural stability of c-NADP-ME and m-NAD(P)-ME. In this study, we attempt to delineate the structural features governing the fumarate-induced allosteric activation of malic enzyme.

Fumarate analogs act as allosteric inhibitors of the human mitochondrial NAD(P)+-dependent malic enzyme

Human mitochondrial NAD(P)⁺-dependent malic enzyme (m-NAD(P)-ME) is allosterically activated by the four-carbon trans dicarboxylic acid, fumarate. Previous studies have suggested that the dicarboxylic acid in a trans conformation around the carbon-carbon double bond is required for the allosteric activation of the enzyme. In this paper, the allosteric effects of fumarate analogs on m-NAD(P)-ME are investigated. Two fumarate-insensitive mutants, m-NAD(P)-ME_R67A/R91A and m-NAD(P)-ME_K57S/E59N/K73E/D102S, as well as c-NADP-ME, were used as the negative controls. Among these analogs, mesaconate, trans-aconitate, monomethyl fumarate and monoethyl fumarate were allosteric activators of the enzyme, while oxaloacetate, diethyl oxalacetate, and dimethyl fumarate were found to be allosteric inhibitors of human m-NAD(P)-ME. The IC₅₀ value for diethyl oxalacetate was approximately 2.5 mM. This paper suggests that the allosteric inhibitors may impede the conformational change from open form to closed form and therefore inhibit m-NAD(P)-ME enzyme activity.

Cofactory: sequence-based prediction of cofactor specificity of Rossmann folds

Obtaining optimal cofactor balance to drive production is a challenge in metabolically engineered microbial production strains. To facilitate identification of heterologous enzymes with desirable altered cofactor requirements from native content, we have developed Cofactory, a method for prediction of enzyme cofactor specificity using only primary amino acid sequence information. The algorithm identifies potential cofactor binding Rossmann folds and predicts the specificity for the cofactors FAD(H2), NAD(H), and NADP(H). The Rossmann fold sequence search is carried out using hidden Markov models whereas artificial neural networks are used for specificity prediction. Training was carried out using experimental data from protein-cofactor structure complexes. The overall performance was benchmarked against an independent evaluation set obtaining Matthews correlation coefficients of 0.94, 0.79, and 0.65 for FAD(H2), NAD(H), and NADP(H), respectively. The Cofactory method is made publicly available at http://www.cbs.dtu.dk/services/Cofactory.

Computational design of short-chain dehydrogenase Gox2181 for altered coenzyme specificity

Short-chain dehydrogenase Gox2181 from Gluconobacter oxydans catalyzes the reduction of 2,3-pentanedione by using NADH as the physiological electron donor. To realize its synthetic biological application for coenzyme recycling use, computational design and site-directed mutagenesis have been used to engineer Gox2181 to utilize not only NADH but also NADPH as the electron donor. Single and double mutations at residues Q20 and D43 were made in a recombinant expression system that corresponded to Gox2181-D43Q and Gox2181-Q20R&D43Q, respectively. The design of mutant Q20R not only resolved the hydrogen bond interaction and electrostatic interaction between R and 2'-phosphate of NADPH, but also could enhance the binding with 2'-phophated of NADPH by combining with D43Q. Molecular dynamics simulation has been carried out to testify the hydrogen bond interactions between mutation sites and 2'-phosphate of NADPH. Steady-state turnover measurement results indicated that Gox2181-D43Q could use both NADH and NADPH as its coenzyme, and so could Gox2181-Q20R&D43Q. Meanwhile, compared to the wild-type enzyme, Gox2181-D43Q exhibited dramatically reduced enzymatic activity while Gox2181-Q20R&D43Q successfully retained the majority of enzymatic activity.

Conserved and non-conserved residues and their role in the structure and function of p-hydroxybenzoate hydroxylase

In order to elucidate the molecular mechanism of the catalytic reaction and enzyme conformation, we substituted 53 conserved residues identified by aligning 92 p-hydroxybenzoate hydroxylase sequences and 19 non-conserved residues selected from crystallographic studies of Pseudomonas fluorescens NBRC14160 p-hydroxybenzoate hydroxylase with 19 other naturally occurring amino acids, yielding a database of 619 active single mutants. The database contained 365 and 254 active single mutants for 44/53 conserved residues and 19 non-conserved residues, respectively; the data included main activity, sub-activity for NADPH and NADPH reaction specificity. Active mutations were not observed for the G14, Q102, G160, E198, R220, R246, N300, F342 and G387 conserved residues, and only one active mutant was obtained at the G9, G11, G187, D286, Y201, R214 and G295 conserved residues and the S13, E32 and R42 non-conserved residues. Only seven active mutants with higher activity than the wild-type enzyme were observed at conserved residues, and only two were observed at non-conserved residues. The 365 mutants at conserved residues included 64 active mutants with higher NADPH reaction specificity than the wild-type enzyme, and some Y181X single mutants exhibited considerable changes in NADPH reaction specificity. A Y181X/L268G double-mutant database was constructed to computationally analyze the effects of these substitutions on structural conformation and function. These results indicated that some conserved or non-conserved residues are important for structural stability or enzyme function.

Impact of malic enzymes on antibiotic and triacylglycerol production in Streptomyces coelicolor

In this paper, we have characterized two malic enzymes (ME), SCO2951 and SCO5261, from Streptomyces coelicolor and analyzed their role in antibiotic and triacylglycerol (TAG) production. Biochemical studies have demonstrated that Sco2951 and Sco5261 genes encode NAD(+)- and NADP(+)-dependent malic enzymes, respectively. Single or double mutants in the ME-encoding genes show no effect on growth rate compared to the parental M145 strain. However, the single Sco2951 and the double Sco2951 Sco5261 mutants display a strong reduction in the production of the polyketide antibiotic actinorhodin; additionally, the Sco2951 Sco5261 mutant shows a decrease in stored TAGs during exponential growth. The lower production of actinorhodin in the double mutant occurs as a consequence of a decrease in the expression of actII-ORF4, the transcriptional activator of the actinorhodin gene cluster. On the other hand, the reduced TAG accumulation is not due to reduced transcript levels of fatty acid biosynthetic genes nor to changes in the amount of the precursor acetyl coenzyme A (acetyl-CoA). This mutant accumulates intermediates of the tricarboxylic acid (TCA) cycle that could alter the regulation of the actinorhodin biosynthetic pathway, suggesting that MEs are important anaplerotic enzymes that redirect C4 intermediates from the TCA cycle to maintain secondary metabolism and TAG production in Streptomyces.

Molecular determinants of the cofactor specificity of ribitol dehydrogenase, a short-chain dehydrogenase/reductase

Ribitol dehydrogenase from Zymomonas mobilis (ZmRDH) catalyzes the conversion of ribitol to d-ribulose and concomitantly reduces NAD(P)(+) to NAD(P)H. A systematic approach involving an initial sequence alignment-based residue screening, followed by a homology model-based screening and site-directed mutagenesis of the screened residues, was used to study the molecular determinants of the cofactor specificity of ZmRDH. A homologous conserved amino acid, Ser156, in the substrate-binding pocket of the wild-type ZmRDH was identified as an important residue affecting the cofactor specificity of ZmRDH. Further insights into the function of the Ser156 residue were obtained by substituting it with other hydrophobic nonpolar or polar amino acids. Substituting Ser156 with the negatively charged amino acids (Asp and Glu) altered the cofactor specificity of ZmRDH toward NAD(+) (S156D, [k(cat)/K(m)(,NAD)]/[k(cat)/K(m)(,NADP)] = 10.9, where K(m)(,NAD) is the K(m) for NAD(+) and K(m)(,NADP) is the K(m) for NADP(+)). In contrast, the mutants containing positively charged amino acids (His, Lys, or Arg) at position 156 showed a higher efficiency with NADP(+) as the cofactor (S156H, [k(cat)/K(m)(,NAD)]/[k(cat)/K(m)(,NADP)] = 0.11). These data, in addition to those of molecular dynamics and isothermal titration calorimetry studies, suggest that the cofactor specificity of ZmRDH can be modulated by manipulating the amino acid residue at position 156.

Mitochondrial NAD+-dependent malic enzyme from Anopheles stephensi: a possible novel target for malaria mosquito control

Background

Anopheles stephensi mitochondrial malic enzyme (ME) emerged as having a relevant role in the provision of pyruvate for the Krebs' cycle because inhibition of this enzyme results in the complete abrogation of oxygen uptake by mitochondria. Therefore, the identification of ME in mitochondria from immortalized A. stephensi (ASE) cells and the investigation of the stereoselectivity of malate analogues are relevant in understanding the physiological role of ME in cells of this important malaria parasite vector and its potential as a possible novel target for insecticide development.

Methods

To characterize the mitochondrial ME from immortalized ASE cells (Mos. 43; ASE), mass spectrometry analyses of trypsin fragments of ME, genomic sequence analysis and biochemical assays were performed to identify the enzyme and evaluate its activity in terms of cofactor dependency and inhibitor preference.

Results

The encoding gene sequence and primary sequences of several peptides from mitochondrial ME were found to be highly homologous to the mitochondrial ME from Anopheles gambiae (98%) and 59% homologous to the mitochondrial NADP⁺-dependent ME isoform from Homo sapiens. Measurements of ME activity in mosquito mitochondria isolated from ASE cells showed that (i) V_max with NAD⁺ was 3-fold higher than that with NADP⁺, (ii) addition of Mg²⁺ or Mn²⁺ increased the V_max by 9- to 21-fold, with Mn²⁺ 2.3-fold more effective than Mg²⁺, (iii) succinate and fumarate increased the activity by 2- and 5-fold, respectively, at sub-saturating concentrations of malate, (iv) among the analogs of L-malate tested as inhibitors of the NAD⁺-dependent ME catalyzed reaction, small (2- to 3-carbons) organic diacids carrying a 2-hydroxyl/keto group behaved as the most potent inhibitors of ME activity (e.g., oxaloacetate, tartronic acid and oxalate).

Conclusions

The biochemical characterization of Anopheles stephensi ME is of critical relevance given its important role in bioenergetics, suggesting that it is a suitable target for insecticide development.

Determinants of nucleotide-binding selectivity of malic enzyme

Malic enzymes have high cofactor selectivity. An isoform-specific distribution of residues 314, 346, 347 and 362 implies that they may play key roles in determining the cofactor specificity. Currently, Glu314, Ser346, Lys347 and Lys362 in human c-NADP-ME were changed to the corresponding residues of human m-NAD(P)-ME (Glu, Lys, Tyr and Gln, respectively) or Ascaris suum m-NAD-ME (Ala, Ile, Asp and His, respectively). Kinetic data demonstrated that the S346K/K347Y/K362Q c-NADP-ME was transformed into a debilitated NAD⁺-utilizing enzyme, as shown by a severe decrease in catalytic efficiency using NADP⁺ as the cofactor without a significant increase in catalysis using NAD⁺ as the cofactor. However, the S346K/K347Y/K362H enzyme displayed an enhanced value for k(cat,NAD), suggesting that His at residue 362 may be more beneficial than Gln for NAD⁺ binding. Furthermore, the S346I/K347D/K362H mutant had a very large K(m,NADP) value compared to other mutants, suggesting that this mutant exclusively utilizes NAD⁺ as its cofactor. Since the S346K/K347Y/K362Q, S346K/K347Y/K362H and S346I/K347D/K362H c-NADP-ME mutants did not show significant reductions in their K(m,NAD) values, the E314A mutation was then introduced into these triple mutants. Comparison of the kinetic parameters of each triple-quadruple mutant pair (for example, S346K/K347Y/K362Q versus E314A/S346K/K347Y/K362Q) revealed that all of the K(m) values for NAD⁺ and NADP(+) of the quadruple mutants were significantly decreased, while either k(cat,NAD) or k(cat,NADP) was substantially increased. By adding the E314A mutation to these triple mutant enzymes, the E314A/S346K/K347Y/K362Q, E314A/S346K/K347Y/K362H and E314A/S346I/K347D/K362H c-NADP-ME variants are no longer debilitated but become mainly NAD⁺-utilizing enzymes by a considerable increase in catalysis using NAD⁺ as the cofactor. These results suggest that abolishing the repulsive effect of Glu314 in these quadruple mutants increases the binding affinity of NAD⁺. Here, we demonstrate that a series of E314A-containing c-NADP-ME quadruple mutants have been changed to NAD⁺-utilizing enzymes by abrogating NADP⁺ binding and increasing NAD⁺ binding.

Molecular cloning and characterization of a malic enzyme gene from the oleaginous yeast Lipomyces starkeyi

The malic enzyme-encoding cDNA (GQ372891) from the oleaginous yeast Lipomyces starkeyi AS 2.1560 was isolated, which has an 1719-bp open reading frame flanked by a 290-bp 5' untranslated sequence and a 92-bp 3' untranslated sequence. The proposed gene, LsME1, encoded a protein with 572 amino acid residues. The protein presented 58% sequence identity with the malic enzymes from Yarrowia lipolytica CLIB122 and Aspergillus fumigatus Af293. The LsME1 gene was cloned into the vector pMAL-p4x to express a fusion protein (MBP-LsME1) in Escherichia coli TB1. The fusion protein was purified and then cleaved by Factor Xa to give the recombinant LsME1. This purified enzyme took either NAD(+) or NADP(+) as the coenzyme but preferred NAD(+). The K (m) values for malic acid, NAD(+) and NADP(+) were 0.85 +/- 0.05 mM, 0.34 +/- 0.08 mM, and 7.4 +/- 0.32 mM, respectively, at pH 7.3.

Computational design of Candida boidinii xylose reductase for altered cofactor specificity

In this study we introduce a computationally-driven enzyme redesign workflow for altering cofactor specificity from NADPH to NADH. By compiling and comparing data from previous studies involving cofactor switching mutations, we show that their effect cannot be explained as straightforward changes in volume, hydrophobicity, charge, or BLOSUM62 scores of the residues populating the cofactor binding site. Instead, we find that the use of a detailed cofactor binding energy approximation is needed to adequately capture the relative affinity towards different cofactors. The implicit solvation models Generalized Born with molecular volume integration and Generalized Born with simple switching were integrated in the iterative protein redesign and optimization (IPRO) framework to drive the redesign of Candida boidinii xylose reductase (CbXR) to function using the non-native cofactor NADH. We identified 10 variants, out of the 8,000 possible combinations of mutations, that improve the computationally assessed binding affinity for NADH by introducing mutations in the CbXR binding pocket. Experimental testing revealed that seven out of ten possessed significant xylose reductase activity utilizing NADH, with the best experimental design (CbXR-GGD) being 27-fold more active on NADH. The NADPH-dependent activity for eight out of ten predicted designs was either completely abolished or significantly diminished by at least 90%, yielding a greater than 10(4)-fold change in specificity to NADH (CbXR-REG). The remaining two variants (CbXR-RTT and CBXR-EQR) had dual cofactor specificity for both nicotinamide cofactors.

Effects of structural analogues of the substrate and allosteric regulator of the human mitochondrial NAD(P)+-dependent malic enzyme

Fumarate, a four-carbon trans dicarboxylic acid, is the allosteric activator of the human mitochondrial NAD(P)(+)-dependent malic enzyme (m-NAD(P)-ME). In this paper, we discuss the effects of the structural analogues of fumarate on human m-NAD(P)-ME. Succinate, a dicarboxylic acid with a carbon-carbon single bond, can also activate the enzyme, but the activating effect of succinate is less than that of fumarate. Succinamide, a diamide of succinate, cannot activate the enzyme and is a poor active-site inhibitor. The cis isomer of fumarate, maleic acid, significantly inhibits the ME activity, suggesting that the trans configuration of fumarate is crucial for operating the allosteric regulation of the enzyme. Other dicarboxylic acids, including glutaconic acid, malonic acid and alpha-ketoglutarate, cannot activate the enzyme and inversely inhibit enzyme activity. Our data suggest that these structural analogues are mainly active-site inhibitors, although they may enter the allosteric site to inhibit the enzyme. Furthermore, these data also suggest that the dicarboxylic acid must be in a trans conformation for allosteric activation of the enzyme.

Long-range interaction between the enzyme active site and a distant allosteric site in the human mitochondrial NAD(P)+-dependent malic enzyme

Our previous study has suggested that mutation of the amino acid residue Asp102 has a significant effect on the fumarate-mediated activation of human mitochondrial NAD(P)+-dependent malic enzyme (m-NAD(P)-ME). In this paper, we examine the cationic amino acid residue Arg98, which is adjacent to Asp102 and is highly conserved in most m-NAD(P)-MEs. A series of R98/D102 mutants were created to examine the possible interactions between Arg98 and Asp102 using the double-mutant cycle analysis. Kinetic analysis revealed that the catalytic efficiency of the enzyme was severely affected by mutating both Arg98 and Asp102 residues. However, the binding energy of these mutant enzymes to fumarate as determined by analysis of the K(A,Fum) values, show insignificant differences, indicating that the mutation of Arg98 and Asp102 did not cause a significant decrease in the binding affinity of fumarate. The overall coupling energies for R98K/D102N as determined by analysis of the k(cat)/K(m) and K(A,Fum) values were -2.95 and -0.32kcal/mol, respectively. According to these results, we conclude that substitution of both Arg98 and Asp102 residues has a synergistic effect on the catalytic ability of the enzyme.

Dual roles of Lys(57) at the dimer interface of human mitochondrial NAD(P)+-dependent malic enzyme

Human m-NAD(P)-ME [mitochondrial NAD(P)+-dependent ME (malic enzyme)] is a homotetramer, which is allosterically activated by the binding of fumarate. The fumarate-binding site is located at the dimer interface of the NAD(P)-ME. In the present study, we decipher the functional role of the residue Lys57, which resides at the fumarate-binding site and dimer interface, and thus may be involved in the allosteric regulation and subunit-subunit interaction of the enzyme. In the present study, Lys57 is replaced with alanine, cysteine, serine and arginine residues. Site-directed mutagenesis and kinetic analysis strongly suggest that Lys57 is important for the fumarate-induced activation and quaternary structural organization of the enzyme. Lys57 mutant enzymes demonstrate a reduction of Km and an elevation of kcat following induction by fumarate binding, and also display a much higher maximal activation threshold than WT (wild-type), indicating that these Lys57 mutant enzymes have lower affinity for the effector fumarate. Furthermore, mutation of Lys57 in m-NAD(P)-ME causes the enzyme to become less active and lose co-operativity. It also increased K0.5,malate and decreased kcat values, indicating that the catalytic power of these mutant enzymes was significantly impaired following mutation of Lys57. Analytical ultracentrifugation analysis demonstrates that the K57A, K57S and K57C mutant enzymes dissociate predominantly into dimers, with some monomers present, whereas the K57R mutant forms a mixture of dimers and tetramers, with a small amount of the enzyme in monomeric form. The dimeric form of these Lys57 mutants, however, cannot be reconstituted into tetramers with the addition of fumarate. Modelling structures of the Lys57 mutant enzymes show that the hydrogen bond network in the dimer interface where Lys57 resides may be reduced compared with WT. Although the fumarate-induced activation effects are partially maintained in these Lys57 mutant enzymes, the mutant enzymes cannot be reconstituted into tetramers through fumarate binding and cannot recover their full enzymatic activity. In the present study, we demonstrate that the Lys57 residue plays dual functional roles in the structural integrity of the allosteric site and in the subunit-subunit interaction at the dimer interface of human m-NAD(P)-ME.

Functional roles of the tetramer organization of malic enzyme

Malic enzyme has a dimer of dimers quaternary structure in which the dimer interface associates more tightly than the tetramer interface. In addition, the enzyme has distinct active sites within each subunit. The mitochondrial NAD(P)(+)-dependent malic enzyme (m-NAD(P)-ME) isoform behaves cooperatively and allosterically and exhibits a quaternary structure in dimer-tetramer equilibrium. The cytosolic NADP(+)-dependent malic enzyme (c-NADP-ME) isoform is noncooperative and nonallosteric and exists as a stable tetramer. In this study, we analyze the essential factors governing the quaternary structure stability for human c-NADP-ME and m-NAD(P)-ME. Site-directed mutagenesis at the dimer and tetramer interfaces was employed to generate a series of dimers of c-NADP-ME and m-NAD(P)-ME. Size distribution analysis demonstrated that human c-NADP-ME exists mainly as a tetramer, whereas human m-NAD(P)-ME exists as a mixture of dimers and tetramers. Kinetic data indicated that the enzyme activity of c-NADP-ME is not affected by disruption of the interface. There are no significant differences in the kinetic properties between AB and AD dimers, and the dimeric form of c-NADP-ME is as active as tetramers. In contrast, disrupting the interface of m-NAD(P)-ME causes the enzyme to be less active than wild type and to become less cooperative for malate binding; the k(cat) values of mutants decreased with increasing K(d,24) values, indicating that the dissociation of subunits at the dimer or tetramer interfaces significantly affects the enzyme activity. The above results suggest that the tetramer is required for a fully functional m-NAD(P)-ME. Taken together, the analytical ultracentrifugation data and the kinetic analysis of these interface mutants demonstrate the differential role of tetramer organization for the c-NADP-ME and m-NAD(P)-ME isoforms. The regulatory mechanism of m-NAD(P)-ME is closely related to the tetramer formation of this isoform.

Functional role of fumarate site Glu59 involved in allosteric regulation and subunit-subunit interaction of human mitochondrial NAD(P)+-dependent malic enzyme

Here we report on the role of Glu59 in the fumarate-mediated allosteric regulation of the human mitochondrial NAD(P)+-dependent malic enzyme (m-NAD-ME). In the present study, Glu59 was substituted by Asp, Gln or Leu. Our kinetic data strongly indicated that the charge properties of this residue significantly affect the allosteric activation of the enzyme. The E59L enzyme shows nonallosteric kinetics and the E59Q enzyme displays a much higher threshold in enzyme activation with elevated activation constants, K(A,Fum) and alphaK(A,Fum). The E59D enzyme, although retaining the allosteric property, is quite different from the wild-type in enzyme activation. The K(A,Fum) and alphaK(A,Fum) of E59D are also much greater than those of the wild-type, indicating that not only the negative charge of this residue but also the group specificity and side chain interactions are important for fumarate binding. Analytical ultracentrifugation analysis shows that both the wild-type and E59Q enzymes exist as a dimer-tetramer equilibrium. In contrast to the E59Q mutant, the E59D mutant displays predominantly a dimer form, indicating that the quaternary stability in the dimer interface is changed by shortening one carbon side chain of Glu59 to Asp59. The E59L enzyme also shows a dimer-tetramer model similar to that of the wild-type, but it displays more dimers as well as monomers and polymers. Malate cooperativity is not significantly notable in the E59 mutant enzymes, suggesting that the cooperativity might be related to the molecular geometry of the fumarate-binding site. Glu59 can precisely maintain the geometric specificity for the substrate cooperativity. According to the sequence alignment analysis and our experimental data, we suggest that charge effect and geometric specificity are both critical factors in enzyme regulation. Glu59 discriminates human m-NAD-ME from mitochondrial NADP+-dependent malic enzyme and cytosolic NADP+-dependent malic enzyme in fumarate activation and malate cooperativity.

Engineering of the cofactor specificities and isoform-specific inhibition of malic enzyme

Malic enzyme (ME) is a family of enzymes that catalyze a reversible oxidative decarboxylation of l-malate to pyruvate with simultaneous reduction of NAD(P)(+) to NAD(P)H. According to the cofactor specificity, the mammalian enzyme can be categorized into three isoforms. The cytosolic (c) and mitochondrial (m) NADP(+)-dependent MEs utilize NADP(+) as the cofactor. The mitochondrial NAD(P)(+)-dependent ME can use either NAD(+) or NADP(+) as the cofactor. In addition, the m-NAD(P)-ME isoform can be inhibited by ATP and allosterically activated by fumarate. In this study, we delineated the determinants for cofactor specificity and isoform-specific inhibition among the ME isoforms. Our data strongly suggest that residue 362 is the decisive factor determining cofactor preference. All the mutants containing Q362K (Q362K, K346S/Q362K, Y347K/Q362K, and K346S/Y347K/Q362K) have a larger k(cat,NADP) value compared with the k(cat,NAD) value, indicating that the enzyme has changed to use NADP(+) as the preferred cofactor. Furthermore, we suggest that Lys-346 in m-NAD(P)-ME is crucial for the isoform-specific ATP inhibition. The enzymes containing the K346S mutation (K346S, K346S/Y347K, K346S/Q362K, and K346S/Y347K/Q362K) are much less inhibited by ATP and have a larger K(i,ATP) value. Kinetic analysis also suggests that residue 347 functions in cofactor specificity. Here we demonstrate that the human K346S/Y347K/Q362K m-NAD(P)-ME has completely shifted its cofactor preference to become an NADP(+)-specific ME. In the triple mutant, Lys-362, Lys-347, and Ser-346 work together and function synergistically to increase the binding affinity for NADP(+).

Influential factor contributing to the isoform-specific inhibition by ATP of human mitochondrial NAD(P)+-dependent malic enzyme: functional roles of the nucleotide binding site Lys346

Human mitochondrial NAD(P)(+)-dependent malic enzyme (m-NAD-ME) is a malic enzyme isoform with dual cofactor specificity, ATP inhibition and substrate cooperativity. The determinant of ATP inhibition in malic enzyme isoforms has not yet been identified. Sequence alignment of nucleotide-binding sites of ME isoforms revealed that Lys346 is conserved uniquely in m-NAD-ME. In other ME isoforms, this residue is serine. As the inhibitory effect of ATP is more pronounced on m-NAD-ME than on other ME isoforms, we have examined the possible role of Lys346 by replacing it to alanine, serine or arginine. Our kinetic data indicate that the K346S mutant enzyme displays a shift in its cofactor preference from NAD(+) to NADP(+) upon increasing k(cat,NADP) and decreasing K(m,NADP). Furthermore, the cooperative binding of malate becomes less significant in human m-NAD-ME after mutation of Lys346. The h value for the wild-type is close to 2, but those of the K346 mutants are approximately 1.5. The K346 mutants can also be activated by fumarate and the cooperative effect can be abolished by fumarate, suggesting that the allosteric property is retained in these mutants. Our data strongly suggest that Lys346 in human m-NAD-ME is required for ATP inhibition. Mutation of Lys346 to Ser or Ala causes the enzyme to be much less sensitive to ATP, similar to cytosolic NADP-dependent malic enzyme. Substitution of Lys to Arg did not change the isoform-specific inhibition of the enzyme by ATP. The inhibition constants of ATP are increased for K346S and K346A, but are similar to those of the wild-type for K346R, suggesting that the positive charge rather than group specificity is required for binding affinity of ATP. Thus, ATP inhibition is proposed to be determined by the electrostatic potential involving the positive charge on the side chain of Lys346.

Role of residues in the adenosine binding site of NAD of the Ascaris suum malic enzyme

Ascaris suum mitochondrial malic enzyme catalyzes the divalent metal ion dependent conversion of l-malate to pyruvate and CO(2), with concomitant reduction of NAD(P) to NAD(P)H. In this study, some of the residues that form the adenosine binding site of NAD were mutated to determine their role in binding of the cofactor and/or catalysis. D361, which is completely conserved among species, is located in the dinucleotide-binding Rossmann fold and makes a salt bridge with R370, which is also highly conserved. D361 was mutated to E, A and N. R370 was mutated to K and A. D361E and A mutant enzymes were inactive, likely a result of the increase in the volume in the case of the D361E mutant enzyme that caused clashes with the surrounding residues, and loss of the ionic interaction between D361 and R370, for D361A. Although the K(m) for the substrates and isotope effect values did not show significant changes for the D361N mutant enzyme, V/E(t) decreased by 1400-fold. Data suggested the nonproductive binding of the cofactor, giving a low fraction of active enzyme. The R370K mutant enzyme did not show any significant changes in the kinetic parameters, while the R370A mutant enzyme gave a slight change in V/E(t), contrary to expectations. Overall, results suggest that the salt bridge between D361 and R370 is important for maintaining the productive conformation of the NAD binding site. Mutation of residues involved leads to nonproductive binding of NAD. The interaction stabilizes one of the Rossmann fold loops that NAD binds. Mutation of H377 to lysine, which is conserved in NADP-specific malic enzymes and proposed to be a cofactor specificity determinant, did not cause a shift in cofactor specificity of the Ascaris malic enzyme from NAD to NADP. However, it is confirmed that this residue is an important second layer residue that affects the packing of the first layer residues that directly interact with the cofactor.

Isocitrate dehydrogenase: A NADPH-generating enzyme in the lumen of the endoplasmic reticulum

The aim of the present study was the investigation of the occurrence of NADPH-generating pathways in the endoplasmic reticulum others then hexose-6-phosphate dehydrogenase. A significant isocitrate and a moderate malate-dependent NADP+ reduction were observed in endoplasmic reticulum-derived rat liver microsomes. The isocitrate-dependent activity was very likely attributable to the appearance of the cytosolic isocitrate dehydrogenase isozyme in the lumen. The isocitrate dehydrogenase activity of microsomes was present in the luminal fraction; it showed a strong preference towards NADP+ versus NAD+, and it was almost completely latent. Antibodies against the cytosolic isoform of isocitrate dehydrogenase immunorevealed a microsomal protein of identical molecular weight; the microsomal enzyme showed similar kinetic parameters and oxalomalate inhibition as the cytosolic one. Measurable luminal isocitrate dehydrogenase activity was also present in microsomes from rat epididymal fat. The results suggest that isocitrate dehydrogenase is an important NADPH-generating enzyme in the endoplasmic reticulum.

Surfactant protein A2 (SP-A2) variants expressed in CHO cells stimulate phagocytosis of Pseudomonas aeruginosa more than do SP-A1 variants

Surfactant protein A (SP-A) enhances phagocytosis of Pseudomonas aeruginosa. Two functional genes, SP-A1 and SP-A2, encode human SP-A. As we showed before, baculovirus-mediated insect cell-expressed SP-A2 enhances the association of P. aeruginosa with rat alveolar macrophages (rAMs) more than does SP-A1. However, true phagocytosis (internalization) was not shown, and insect cell derived proteins lack or are defective in certain mammalian posttranslational modifications that may be important for SP-A1 and SP-A2 activity and specificity. Here we used SP-A1 (6A(2), 6A(4)) and SP-A2 (1A(0), 1A(1)) allele variants expressed by CHO (Chinese hamster ovary) mammalian cells to study their effect on association and/or internalization of P. aeruginosa by rAMs and/or human AMs (hAMs) and to study if phagocytosis can be modulated differentially and/or more effectively by CHO cell-expressed SP-A variants than by insect-cell expressed SP-A variants. For cell association and internalization assessments, light microscopy and fluorescence-activated cell sorter analyses were used, respectively. We found the following for the first time. (i) SP-A2 variants enhanced phagocytosis (cell association and/or internalization) of P. aeruginosa more than SP-A1 variants did, and the cell association correlated with internalization. (ii) Differences in the activities of SP-A variants were observed in the following order: 1A(1)>1A(0)>6A(2)>6A(4). (iii) rAMs, although more active than hAMs, are an appropriate model, as SP-A2 variants exhibited activity higher than that seen for SP-A1 variants with either rAMs or hAMs. (iv) CHO cell-expressed SP-A was considerably more active than insect cell-expressed variants. We conclude that SP-A2 variants stimulate phagocytosis of P. aeruginosa more effectively than SP-A1 variants and that posttranslational modifications positively influence the phagocytic activity of SP-A.