Enzymatic properties of mutant forms of RNase Rh from Rhizopus niveus as to Asp51

In order to determine the role of Asp51 of RNase Rh from Rhizopus niveus, enzymes with mutations at the 51st position, D51N, D51E, D51Q, D51S, D51T, D51A, and D51K, were prepared, and their enzymatic properties were investigated as to specific activity and base specificity. All the mutant enzymes showed relatively high activity toward poly I and poly C, and markedly reduced activity toward poly A and poly U. In particular, the enzymatic activities toward poly I of D51T and D51S were higher than that of RNase RNAP Rh. Among the mutant enzymes, D51N, D51S, and D51T showed more than ca. 30% of the activity of RNase Rh, when RNA, poly I and poly C were used as substrates, respectively. The substitution of Ala, Glu, or Lys at Asp51 is unfavorable for enzymatic activity. Among XpGs (X = A, G, U, or C), D51N, D51S, and D51T showed higher activity toward GpG then CpG. Therefore, Asp51 in RNase Rh plays a critical role in the adenylic acid preference of RNase T2 family enzymes. Our results obtained with a protein engineering technique provide basic insights into the control of the base specificity of RNase Rh.

Inhibition of lipoprotein lipase induced cholesterol ester accumulation in human hepatoma HepG2 cells

It has been suggested previously that lipoprotein lipase may act as a ligand to enhance binding and uptake of lipoprotein particles. In the present study we have examined the capacity of bovine milk lipoprotein lipase to induce intracellular accumulation of triglyceride and cholesterol ester by VLDL (Sr 60-400) isolated from Type IV hypertriglyceridemic subject (HTg-VLDL) in HepG2 cells, independent of its lipolytic activity. We have also attempted to elucidate the cellular receptor mechanisms responsible for these effects. HTg-VLDL-mediated increases in intracellular triglyceride and cholesterol ester were dependent on the presence of an active lipase. Bovine milk lipoprotein lipase (LPL) increases triglyceride mass by 301% +/- 28% (P < 0.0005) and cholesterol ester mass by 176% +/- 12% (P < 0.0005). These HTg-VLDL-mediated increases in intracellular triglyceride and cholesterol ester did not occur when heat-inactivated lipase was used. Rhizopus lipase could replace LPL and cause equivalent increases in intracellular triglyceride and cholesterol ester (472% +/- 61%(P < 0.005) and 202% +/- 25% (P < 0.025) respectively vs. control). HTg-VLDL treated with LPL and reisolated also caused equivalent increases (274% +/- 18%(P < 0.01) and 177% +/- 12% (P < 0.005) for triglyceride and cholesterol ester). LDL also caused increases in intracellular cholesterol ester (189% +/- 20%(P < 0.005)), although three times more LDL cholesterol had to be added to achieve the same effect. These LDL-induced increases were effectively blocked by monoclonal antibodies directed against the B,E receptor binding domains of apo B (-97% +/- 13% (P < 0.0005) with anti-apo B 5E11 and -68% +/- 13% (P < 0.05) for anti-apo B B1B3) or by anti-B,E receptor antibodies (-77% +/- 7% (P < 0.01) antibody C7). These same antibodies had little effect on the HTg-VLDL+LPL-induced increases in cholesterol ester (+21%, +15% and -22% for 5E11, B1B3 and C7, respectively). Monoclonal anti-apo E antibodies also had no effect on LDL-mediated increases in intracellular cholesterol ester, but had a small and significant effect on VLDL-mediated increases in cholesterol ester. However, heparin, which interferes with cell surface proteoglycan interaction, was very effective at blocking HTg-VLDL-mediated increases in cholesterol ester in the presence of LPL (-86% +/- 8% P < 0.0005). Heparin was also effective in the presence of Rhizopus lipase (-79%) or lipolyzed re-isolated HTg-VLDL (-95%). These results suggest that lipoprotein lipase may enhance the uptake process beyond its role in lipolytic remodelling but does not appear to be an absolute requirement. In contrast, heparin had no effect on LDL-mediated cholesterol ester accumulation. Lactoferrin, which inhibits interaction with the low density lipoprotein receptor-related protein (LRP), was also very effective at inhibiting HTg-VLDL increases in intracellular cholesterol ester (-95% +/- 6%, P < 0.01). However, there was no effect of either heparin or lactoferrin on HTg-VLDL-mediated triglyceride accumulation. Thus cell surface heparin sulphate may facilitate intracellular lipid acquisition by providing a stabilizing bridge with the lipoproteins and enhance uptake through receptor-mediated processes such as LRP.

The crystal structure of ribonuclease Rh from Rhizopus niveus at 2.0 A resolution

The three-dimensional structure of ribonuclease Rh (RNase Rh), a new class of microbial ribonuclease from Rhizopus niveus, has been determined at 2.0 A resolution. The overall structure of RNase Rh is completely different from those of other previously studied RNases, such as RNase A from bovine pancreas and RNase T1 from Aspergillus oryzae. In the structure of RNase Rh, two histidine residues (His46 and His109) and one glutamic acid residue (Glu105), which were predicted to be critical to the activity from the chemical modification and mutagenesis experiments, are found to be located close together, constructing the active site. The indole ring of Trp49 plays an important role in preserving the active site structure by its stacking interactions with the imidazole ring of His 109, and by hydrogen bonding with the carboxyl group of Glu105. There exists a hydrophobic pocket around the active site, which contains the aromatic side-chain of Trp49 and Tyr57. The results of mutagenesis studies suggest that this pocket is the base binding site of the substrate.

Enzymatic properties of mutant enzymes at Trp49 and Tyr57 of RNase Rh from Rhizopus niveus

In order to establish the role of Tyr57 and Trp49 in the enzymatic reaction of RNase Rh, several mutant enzymes at Tyr57 and Trp49 were prepared by protein engineering and their enzymatic properties were investigated. Among the four mutant enzymes at Trp49 (W49F, W49Y, W49A, and W49I), W49F showed 16% of the activity of the native enzyme, but the others (W49Y, W49A, and W49I) showed greatly decreased activity. The data showed that Trp49 is very important for the enzyme activity. Among 8 mutant enzymes at the 57th position, Y57F and Y57W showed similar enzymatic activity toward RNA to that of the wild-type enzyme, but the others (Y57G, Y57A, Y57V, Y57M, and Y57K) are more active toward RNA and less active toward XpGs. The reason for the apparent increase for RNA activity is discussed from the view point of substrate inhibition. It is noteworthy that W49F and Y57W became more pyrimidine base- and purine base-preferential, respectively.

The fumR gene encoding fumarase in the filamentous fungus Rhizopus oryzae: cloning, structure and expression

The filamentous fungus Rhizopus oryzae (Ro) is known for its ability to overproduce and accumulate high levels of fumaric acid (FA) under stress conditions. In order to study the molecular mechanisms involved in the increased biosynthesis of FA, the gene (designated fumR) encoding Ro fumarase was cloned and analysed for its structure and expression. Nucleotide (nt) sequence and comparison of the fumR product with fumarases from various sources established that fumR contains nine introns and encodes a deduced product of 494 amino acids (aa), related to class-II fumarases. A fumarase protein of 50 kDa was immuno-detected in crude Ro extracts. Primer extension experiments mapped the 5' end of the fumR RNA 159 nt upstream from the putative translation start codon. Both primer extension and Northern analysis showed the existence of one transcript of fumR. The level of fumR RNA increased in cells producing FA under stress conditions (high carbon and low nitrogen levels in the medium), suggesting that transcriptional regulation of fumR might be involved in the overproduction and accumulation of FA by Ro cells under stress conditions. The possibility that additional mechanisms are responsible for this phenomenon is discussed.

Extracellular production of alpha-rhamnosidase by Rhizopus nigricans

Extracellular production of alpha-rhamnosidase [EC 3.2.1.40] from an indigeneous fungal strain of R. nigricans has been demonstrated. The enzyme has been shown to follow Michaelis-Menten kinetics using p-nitrophenyl-alpha-L-rhamnopyranosid as a substrate. The pH and the temperature optima of the enzyme have been found to be around 6.5 and 60 degrees-80 degrees C respectively.

Synthesis of 2-deoxy-glucooligosaccharides through condensation of 2-deoxy-D-glucose by glucoamylase and alpha-glucosidase

Glucoamylases from Aspergillus niger and Rhizopus niveus catalyzed condensation of 2-deoxy-D-glucose (dGlc) to yield deoxy-glucooligosaccharides with polymerization degrees of 2-5. The enzymes also gave a small amount of products from 3-deoxy-D-glucose, but no products from 6-deoxy-D-glucose. A. niger alpha-glucosidase also catalyzed condensation of dGlc, while Torula and Saccharomyces alpha-glucosidases had low activity. alpha-1,4-, 1,6-, and 1,3-linked deoxy-glucobioses were isolated and identified as the products of A. niger glucoamylase and A. niger alpha-glucosidase. In the reaction of the glucoamylase, 1,4- and 1,3-linked saccharides decreased with an increase of 1,6-linked one. A. niger alpha-glucosidase produced alpha-1,6-linked disaccharide predominantly during the whole course of the reaction.

The role of cytochrome P450(11 alpha) in detoxification of steroids in the filamentous fungus Rhizopus nigricans

The evidence was presented that steroid hydroxylating enzyme complex induced by substrate in the filamentous fungus Rhizopus nigricans (R. nigricans) alleviated toxic effect(s) of the steroid on fungal growth. The growth inhibition of fungal mycelium observed in steroid-containing culture(s) became much more obvious when fungal mycelia were grown in the simultaneous presence of inducing steroid and the P450(11 alpha) inhibitor metyrapone. On the other hand, in experiments where we followed the fate of radioactively labelled progesterone added to the mycelial suspension, we noticed that steroid, after being initially accumulated in the microorganism, was, after some time, released from it; the latter phenomenon was not observed if induction of 11 alpha-hydroxylase was prevented by cycloheximide. Results of experiments presented in this communication can be regarded as the first strong indication that the biological role of P450(11 alpha) induction in R. nigricans is in removal of steroids which are toxic for the mycelium.

Molecular and enzymatic properties of an aspartic proteinase from Rhizopus hangchow

An aspartic proteinase, rhizopuspepsin (EC 3.4.23.21), from Rhizopus hangchow was purified. The M(r) and isoelectric point were determined as ca 37,000 and 4.5, respectively. The first 19 amino acids in the N-terminal region were SGSGVVPMTDYEYDIEYYG. The contents of the alpha-helix, beta-structure and random coil were calculated to be ca 7.5, 88.9 and 2.7%, respectively. The enzyme can activate trypsinogen at pH 3.0. The activity was completely inactivated by pepstatin A. The specificity and mode of action of the enzyme were investigated with oxidized insulin B-chain at pH 3. The enzyme hydrolysed primarily two peptide bonds, the Leu15-Tyr16 bond and the Tyr16-Leu17 bond, while additional cleavage of the bonds, Ala14-Leu15 and Phe24-Phe25 was also noted.

Role of Lys108 in the enzymatic activity of RNase Rh from Rhizopus niveus

In order to elucidate on the mechanism of action of RNase Rh from Rhizopus niveus, we investigated the role of Lys108, which is conserved among the RNase T2 family RNases except for two cases. The RNase activities of Lys108 mutant RNases, RNase RNAP K108R and K108L, are about 33.5 and 3.1% of that of the wild type enzyme, respectively. The relative rates of cleavage of dinucleoside phosphates by these two mutant enzymes were comparable to those with RNA as a substrate. The kinetic parameters of RNases RNAP K108R and K108L towards XpGs (where X is one of A, G, U, and C) were measured. The data indicated that the Km values of the two mutant enzymes are similar to those of the wild-type enzyme. The rates of release of the four nucleotides from RNA by digestion with the mutant enzymes were in the order A > G > U > C, which is qualitatively the same as that of the wild-type enzyme. From these data, we concluded that the Lys108 residue participates in the catalytic process, but not in the binding, and the positive charge of Lys108 is indispensable for the catalytic process, that is, the positive charge of Lys108 may stabilize the pentacoordinated intermediate in the transition state as proposed in the case of Lys41 in RNase A, or may polarize the phosphate moiety of the substrate.

Bindability and digestibility of high-pressure-treated starch with glucoamylases from Rhizopus sp

The effect of hydrostatic pressure on the bindability and digestibility of raw corn starch with two types of glucoamylase [EC 3.2.1.3] from Rhizopus sp., Gluc1 and Gluc2, was investigated in the range of 100 to 600 MPa. Pressurization of raw starch at 100 to 300 MPa for 1 h had no detectable effect, whereas pressurization at higher pressures than 400 MPa markedly enhanced the bindability and digestibility with Gluc1 and Gluc2, especially with Gluc2, which scarcely binds to raw starch and has much lower activity than Gluc1 towards raw starch. The binding constants Ks of Gluc1 and Gluc2 for the 400-MPa-pressurized starch reached 16 and 1.6 x 10(5) M-1, respectively, as compared with the Ks of 2.1 and 0.082 x 10(5) M-1 for raw starch. The 500- to 600-MPa-pressurized starch was digested by Gluc1 and Gluc2 at about 4.2 and 80 times higher rates, respectively, than raw starch. Thus, the high-pressure-treated starch was hardly distinguishable from high temperature (75 to 100 degrees C)-treated starch at least with respect to behavior with the enzymes, although some difference was observed between these starches by scanning electron microscopy and differential scanning calorimetry.

Isolation and characterization of two chitin synthase genes of Rhizopus oligosporus

Two chitin synthase genes (chs1 and chs2) were isolated from Rhizopus oligosporus by plaque hybridization probed with the chitin synthase 2 gene of Saccharomyces cerevisiae. From their deduced amino acid sequences, they were both class II chitin synthases according to the classification proposed by Bowen et al. The expression of these genes was controlled differently in each stage of differentiation. It was suggested that the gene products of chs1 and chs2 function mainly in the hyphae growing stage but not in the late stage of spore formation. When each of these genes was expressed in S. cerevisiae, elevation of chitin synthase activity was observed in both cases.

Continuous determination of endopolygalacturonase activity by means of rotational viscosimeters

A continuous viscosimetric method for the determination of endopolygalacturonase activity has been developed from theoretical considerations. The method is based upon the decrease in the specific viscosity of a sodium polygalacturonate solution caused by the action of the enzyme and measured by means of a rotational viscosimeter. The assay has advantages over other viscosimetric methods previously described and over those which rely on the reducing groups assay. The suitability of the newly developed method for measuring the heat resistance of endopolygalacturonase samples has been tested and proved to be satisfactory.

Purification, characterization, and crystallization of two types of lipase from Rhizopus niveus

The purification and some properties of two types of lipase (Lipase I and Lipase II) from Rhizopus niveus are described. The enzymes were purified to homogeneity by column chromatographies on DEAE-Toyopearl (1 pass) and CM-Toyopearl (2 passes). Lipase I consists of two polypeptide chains [a small peptide with sugar moiety (A-chain) and a large peptide of molecular weight 34,000 (B-chain)]. Lipase II has a molecular weight of 30,000 consisting of a single polypeptide chain. Lipase I appeared to be converted to Lipase II by limited proteolysis by a specific protease a small amount of which is in the culture supernatant from Rh. niveus, because one of the peptides formed has the same N-terminal sequence and C-terminal amino acid as Lipase II, as well as the molecular mass estimated by SDS-PAGE. Lipase I had a pH optimum of 6.0-6.5 and a temperature optimum of 35 degrees C, while, for Lipase II these values were pH 6.0 and 40 degrees C. Both enzymes were obtained in the crystalline state using the hanging drop method of vapor diffusion and PEG as the precipitating agents.

pH profile of kinetic constants of RNase Rh from Rhizopus niveus and its mutant enzymes towards UpU, and possible mechanisms of RNase Rh

In order to elucidate the mechanism of action of Rhizopus niveus RNase Rh, we investigated the pH profiles of the kinetic parameters of RNase RNAP Rh, a derivative of RNase Rh, and its mutant enzymes, i.e., RNase RNAP Rh H104F, RNase RNAP Rh E105Q, and RNase RNAP Rh D51N. Based on comparisons of their profiles we concluded that protonation of His104 is indispensable for the enzymatic activity and Glu105 accelerates the enzymatic activity, especially at acid pH centered at pH 3.5. Based on these data and the previous data on the chemical modification and enzymatic properties of other mutant enzymes, we propose the following as a possible mechanisms of RNase Rh action. (i) His109 participates in enzymatic action as a general base catalyst which removes the hydrogen of the 2'-OH of the ribose moiety. (ii) His46 participates in the reaction as a general acid catalyst which interacts with the 5'-oxygen atom of the scissile phosphodiester bond and becomes a proton donor to the departing nucleoside or nucleotide. (iii) His104 interacts with phosphate anion and its protonation is favorable for the enzymatic activity. (iv) Since the protonated form of Glu105 is more favorable for activity, we postulate two possible roles for Glu105: (a) its stabilizes the intermediate, and (b) it interacts with the oxygen atom of P = O and polarizes the phosphorus atom.

Structural similarities in glucoamylase by hydrophobic cluster analysis

The model of the catalytic domain of Aspergillus awamori var. X100 glucoamylase was related to 14 other glucoamylase protein sequences belonging to five subfamilies. Structural features of the different sequences were revealed by multisequence alignment following hydrophobic cluster analysis. The alignment agreed with the hydrophobic microdomains, normally conserved throughout evolution, evaluated from the 3-D model. Saccharomyces and Clostridium glucoamylases lack the alpha-helix exterior to the catalytic domain. A different catalytic base was found in the Saccharomyces glucoamylase subfamily. The starch binding domain of fungal glucoamylases has identical structural features and substrate interacting residues as the C-terminal domain of models of Bacillus circulans cyclodextrin glucosyltransferases. Three putative N-glycosylation sites were found in the same turns in glucoamylases of different subfamilies. O-Glycosylation is present at different levels in the catalytic domain and in the linker between the catalytic and starch binding domains.

Alteration of chain length selectivity of a Rhizopus delemar lipase through site-directed mutagenesis

The coding sequences of the Rhizopus delemar lipase and prolipase were altered by oligonucleotide-directed mutagenesis to introduce amino acid substitutions. The resulting mutant enzymes, synthesized by the bacterial host Escherichia coli BL21 (DE3), were tested for their ability to hydrolyze the triglycerides triolein (TO), tricaprylin (TC) and tributyrin (TB). Mutagenesis and lipase gene expression were carried out using plasmid vectors derived from previously described recombinant plasmids [Joerger and Haas (1993) Lipids 28, 81-88] by introduction of the origin of replication of bacteriophage f1. Substitution of threonine 83 (thr83), a residue thought to be involved in oxyanion binding, by alanine essentially eliminated lipolytic activity toward all substrates examined (TB, TO and TC). Replacement of thr83 with serine caused from two- to sevenfold reductions in the activity toward these substrates. Introduction of tryptophan (trp) at position 89, where such a residue is found in closely related fungal lipases, reduced the specific activity toward the three triglyceride substrates. For the mutagenesis of residues in the predicted acyl chain binding groove, mutagenic primers were designed to cause the replacement of a specific codon within the prolipase gene with codons for all other amino acids. Phenylalanine 95 (phe95), phe112, valine 206 (val206) and val209, were targeted. A phenotypic screen was successfully employed to identify cells producing prolipase with altered preference for olive oil, TC or TB. In assays involving equimolar mixtures of the three triglycerides, a prolipase with a phe95-->aspartate mutation showed an almost twofold increase in the relative activity toward TC. Substitution of trp for phe112 caused an almost threefold decrease in the relative preference for TC, but elevated relative TB hydrolysis. Replacement of val209 with trp resulted in an enzyme with a two- and fourfold enhanced preference for TC and TB, respectively.

Cloning and characterization of two 3-phosphoglycerate kinase genes of Rhizopus niveus and heterologous gene expression using their promoters

Two 3-phosphoglycerate kinase genes (pgk1 and pgk2) were cloned from Rhizopus niveus. It was deduced that both pgk genes have two introns. They have open reading frames of 1,355 bp and 1,356 bp, and code for proteins of 417 and 416 amino acids, respectively. The first introns of both genes are located at similar positions as those of pgk genes from other fungi based on the deduced amino-acid sequences of PGK proteins. The position of their second introns was similar to that of the seventh intron of the human pgk gene. The deduced amino-acid sequences of PGK proteins show high identity (64.8-72.2%) to those of PGKs of other filamentous fungi. When the promoters of each of the pgk genes were fused to the E. coli beta-glucuronidase (GUS) gene and introduced into R. niveus, significant GUS activities were detected in the cell lysates of the transformants, suggesting that GUS protein was expressed under the control of both pgk gene promoters in R. niveus. GUS activity was induced by glucose but not by glycerol, indicating that expression of R. niveus pgk genes was regulated by the carbon source.

The prosequence of Rhizopus niveus aspartic proteinase-I supports correct folding and secretion of its mature part in Saccharomyces cerevisiae

Extracellular Rhizopus niveus aspartic proteinase-I (RNAP-I) was secreted effectively by Saccharomyces cerevisiae when RNAP-I with its preprosequence was synthesized in this organism (Horiuchi, H., Ashikari, T., Amachi, T., Yoshizumi, H., Takagi, M., and Yano, K. (1990) Agric. Biol. Chem. 54, 1771-1779). Certain deletions (delta pro, delta 1, delta 2), and amino acid substitutions (M1) in the prosequence blocked secretion of RNAP-I, although the protease protection assay revealed that even delta pro could be translocated across the membrane of the endoplasmic reticulum. When delta pro or M1 was synthesized simultaneously with the wild-type preprosequence in S. cerevisiae, secretion of RNAP-I was recovered. Therefore, the physical linkage of the prosequence to the mature region is not a prerequisite for secretion of active RNAP-I. Purified RNAP-I with the prosequence once denatured in 6 M guanidine HCl could be renatured and activated to have its enzymatic activity by removing guanidine HCl in vitro, but RNAP-I without the prosequence could not. Furthermore, the wild-type prosequence helped the recovery of the activity of the denatured RNAP-I in trans, but the prosequences of M1 with which secretion of RNAP-I was not observed in vivo, did not. From these results we concluded that the prosequence of RNAP-I supports correct folding of RNAP-I in the endoplasmic reticulum lumen and its subsequent secretion in S. cerevisiae. The functional role of the prosequence of an aspartic proteinase was elucidated.

Crystallization and preliminary crystallographic studies of the precursor and mature forms of a neutral lipase from the fungus Rhizopus delemar

A neutral lipase from the filamentous fungus Rhizopus delemar has been crystallized in both its proenzyme and mature forms. Although the latter crystallizes readily and produces a variety of crystal forms, only one was found to be suitable for X-ray studies. It is monoclinic (C2, a = 92.8 A, b = 128.9 A, c = 78.3 A, beta = 135.8) with two molecules in the asymmetric unit related by a noncrystallographic diad. The prolipase crystals are orthorhombic (P2(1)2(1)2(1), with a = 79.8 A, b = 115.2 A, c = 73.0 A) and also contain a pair of molecules in the asymmetric unit. Initial results of molecular replacement calculations using the refined coordinates of the related lipase from Rhizomucor miehei identified the correct orientations and positions of the protein molecules in the unit cells of crystals of both proenzyme and the mature form.

Structure-function relationships in the catalytic and starch binding domains of glucoamylase

Sixteen primary sequences from five sub-families of fungal, yeast and bacterial glucoamylases were related to structural information from the model of the catalytic domain of Aspergillus awamori var. X100 glucoamylase obtained by protein crystallography. This domain is composed of thirteen alpha-helices, with five conserved regions defining the active site. Interactions between methyl alpha-maltoside and active site residues were modelled, and the importance of these residues on the catalytic action of different glucoamylases was shown by their presence in each primary sequence. The overall structure of the starch binding domain of some fungal glucoamylases was determined based on homology to the C-terminal domains of Bacillus cyclodextrin glucosyl-transferases. Crystallography indicated that this domain contains 6-8 beta-strands and homology allowed the attribution of a disulfide bridge in the glucoamylase starch binding domain. Glucoamylase residues Thr525, Asn530 and Trp560, homologous to Bacillus stearothermophilus cyclodextrin glucosyltransferase residues binding to maltose in the C-terminal domain, could be involved in raw-starch binding. The structure and length of the linker region between the catalytic and starch binding domains in fungal glucoamylases can vary substantially, a further indication of the functional independence of the two domains.

Conformational lability of lipases observed in the absence of an oil-water interface: crystallographic studies of enzymes from the fungi Humicola lanuginosa and Rhizopus delemar

Considerable controversy exists regarding the exact nature of the molecular mechanism of interfacial activation, a process by which most lipases achieve maximum catalytic activity upon adsorption to an oil water interface. X-ray crystallographic studies show that lipases contain buried active centers and that displacements of entire secondary structure elements, or "lids," take place when the enzymes assume active conformations [Derewenda, U., A. M. Brzozowski, D. M. Lawson, and Z. S. Derewenda. 1992. Biochemistry: 31: 1532-1541; van Tilbeurgh, H., M-P. Egloff, C. Martinez, N. Rugani, R. Verger, and C. Cambillau. 1993. Nature: 362: 814-820; Grochulski, P., L. Yunge, J. D. Schrag, F. Bouthillier, P. Smith, D. Harrison, B. Rubin, and M. Cygler. 1993. J. Biol. Chem. 268: 12843-12847]. A simple two-state model inferred from these results implies that the "closed" conformation is stable in an aqueous medium, rendering the active centers inaccessible to water soluble substrates. We now report that in crystals of the Humicola lanuginosa lipase the "lid" is significantly disordered irrespective of the ionic strength of the medium, while in a related enzyme from Rhizopus delemar, crystallized in the presence of a detergent, the two molecules that form the asymmetric unit show different "lid" conformations. These new results call into question the simplicity of the "enzyme theory" of interfacial activation.

An unusual buried polar cluster in a family of fungal lipases

The stability of globular proteins arises largely from the burial of non-polar amino acids in their interior. These residues are efficiently packed to eliminate energetically unfavorable cavities. Contrary to these observations, high resolution X-ray crystallographic analyses of four homologous lipases from filamentous fungi reveal an alpha/beta fold which contains a buried conserved constellation of charged and polar side chains with associated cavities containing ordered water molecules. It is possible that this structural arrangement plays an important role in interfacial catalysis.