1
|
Menefee AL, Zeczycki TN. Nearly 50 years in the making: defining the catalytic mechanism of the multifunctional enzyme, pyruvate carboxylase. FEBS J 2014; 281:1333-1354. [PMID: 24476417 DOI: 10.1111/febs.12713] [Citation(s) in RCA: 21] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/29/2013] [Revised: 01/02/2014] [Accepted: 01/06/2014] [Indexed: 01/04/2023]
Abstract
Numerous steady-state kinetic studies have examined the complex catalytic reaction mechanism of the multifunctional enzyme, pyruvate carboxylase (PC). Through initial velocity, product inhibition, isotopic exchange and alternate substrate experiments, early investigators established that PC catalyzes the MgATP-dependent carboxylation of pyruvate by HCO3 (-) through a nonclassical sequential Bi Bi Uni Uni reaction mechanism. This review surveys previous steady-state kinetic investigations of PC and evaluates the proposed hypotheses concerning the overall catalytic mechanism, nonlinear kinetics and active site coupling in the context of recent structural and mutagenic analyses of this multifunctional enzyme. The determination several PC holoenzyme structures have aided in corroborating the proposed molecular mechanisms by which catalysis occurs and established the inextricable link between the dynamic protein motions and complex kinetic mechanisms associated with PC activity. Unexpectedly, the conclusions drawn from these early steady-state kinetic investigations have consistently proven to be in fundamental agreement with our current understanding of PC catalysis, which is a testament to the overarching sophistication of the methods pioneered by Michaelis and Menten and further developed by Northrop, Cleland and others.
Collapse
Affiliation(s)
- Ann L Menefee
- Department of Biochemistry and Molecular Biology, Brody School of Medicine at East Carolina University, Greenville, NC, USA; The East Carolina Diabetes and Obesity Institute, Brody School of Medicine at East Carolina University, Greenville, NC, USA
| | | |
Collapse
|
2
|
Zeczycki TN, Menefee AL, Adina-Zada A, Jitrapakdee S, Surinya KH, Wallace JC, Attwood PV, St. Maurice M, Cleland WW. Novel insights into the biotin carboxylase domain reactions of pyruvate carboxylase from Rhizobium etli. Biochemistry 2011; 50:9724-37. [PMID: 21957995 PMCID: PMC3211089 DOI: 10.1021/bi2012788] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/15/2023]
Abstract
The catalytic mechanism of the MgATP-dependent carboxylation of biotin in the biotin carboxylase domain of pyruvate carboxylase from R. etli (RePC) is common to the biotin-dependent carboxylases. The current site-directed mutagenesis study has clarified the catalytic functions of several residues proposed to be pivotal in MgATP-binding and cleavage (Glu218 and Lys245), HCO(3)(-) deprotonation (Glu305 and Arg301), and biotin enolization (Arg353). The E218A mutant was inactive for any reaction involving the BC domain and the E218Q mutant exhibited a 75-fold decrease in k(cat) for both pyruvate carboxylation and the full reverse reaction. The E305A mutant also showed a 75- and 80-fold decrease in k(cat) for both pyruvate carboxylation and the full reverse reaction, respectively. While Glu305 appears to be the active site base which deprotonates HCO(3)(-), Lys245, Glu218, and Arg301 are proposed to contribute to catalysis through substrate binding interactions. The reactions of the biotin carboxylase and carboxyl transferase domains were uncoupled in the R353M-catalyzed reactions, indicating that Arg353 may not only facilitate the formation of the biotin enolate but also assist in coordinating catalysis between the two spatially distinct active sites. The 2.5- and 4-fold increase in k(cat) for the full reverse reaction with the R353K and R353M mutants, respectively, suggests that mutation of Arg353 allows carboxybiotin increased access to the biotin carboxylase domain active site. The proposed chemical mechanism is initiated by the deprotonation of HCO(3)(-) by Glu305 and concurrent nucleophilic attack on the γ-phosphate of MgATP. The trianionic carboxyphosphate intermediate formed reversibly decomposes in the active site to CO(2) and PO(4)(3-). PO(4)(3-) then acts as the base to deprotonate the tethered biotin at the N(1)-position. Stabilized by interactions between the ureido oxygen and Arg353, the biotin-enolate reacts with CO(2) to give carboxybiotin. The formation of a distinct salt bridge between Arg353 and Glu248 is proposed to aid in partially precluding carboxybiotin from reentering the biotin carboxylase active site, thus preventing its premature decarboxylation prior to the binding of a carboxyl acceptor in the carboxyl transferase domain.
Collapse
Affiliation(s)
- Tonya N. Zeczycki
- Institute for Enzyme Research and Department of Biochemistry, University of Wisconsin-Madison, Madison, Wisconsin 53726
| | - Ann L. Menefee
- Department of Biological Sciences, Marquette University, Milwaukee, Wisconsin 53201
| | - Abdussalam Adina-Zada
- School of Biomedical, Biomolecular and Chemical Sciences, University of Western Australia, Crawley WA, 6009 Australia
| | - Sarawut Jitrapakdee
- Department of Biochemistry, Faculty of Science, Mahidol University, Bangkok 10400, Thailand
| | - Kathy H. Surinya
- School of Molecular and Biomedical Science, University of Adelaide, Adelaide S.A., 5005, Australia
| | - John C. Wallace
- School of Molecular and Biomedical Science, University of Adelaide, Adelaide S.A., 5005, Australia
| | - Paul V. Attwood
- School of Biomedical, Biomolecular and Chemical Sciences, University of Western Australia, Crawley WA, 6009 Australia
| | - Martin St. Maurice
- Department of Biological Sciences, Marquette University, Milwaukee, Wisconsin 53201
| | - W. Wallace Cleland
- Institute for Enzyme Research and Department of Biochemistry, University of Wisconsin-Madison, Madison, Wisconsin 53726
| |
Collapse
|
3
|
Jitrapakdee S, Maurice MS, Rayment I, Cleland WW, Wallace JC, Attwood PV. Structure, mechanism and regulation of pyruvate carboxylase. Biochem J 2008; 413:369-87. [PMID: 18613815 PMCID: PMC2859305 DOI: 10.1042/bj20080709] [Citation(s) in RCA: 289] [Impact Index Per Article: 18.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022]
Abstract
PC (pyruvate carboxylase) is a biotin-containing enzyme that catalyses the HCO(3)(-)- and MgATP-dependent carboxylation of pyruvate to form oxaloacetate. This is a very important anaplerotic reaction, replenishing oxaloacetate withdrawn from the tricarboxylic acid cycle for various pivotal biochemical pathways. PC is therefore considered as an enzyme that is crucial for intermediary metabolism, controlling fuel partitioning toward gluconeogenesis or lipogenesis and in insulin secretion. The enzyme was discovered in 1959 and over the last decade there has been much progress in understanding its structure and function. PC from most organisms is a tetrameric protein that is allosterically regulated by acetyl-CoA and aspartate. High-resolution crystal structures of the holoenzyme with various ligands bound have recently been determined, and have revealed details of the binding sites and the relative positions of the biotin carboxylase, carboxyltransferase and biotin carboxyl carrier domains, and also a unique allosteric effector domain. In the presence of the allosteric effector, acetyl-CoA, the biotin moiety transfers the carboxy group between the biotin carboxylase domain active site on one polypeptide chain and the carboxyltransferase active site on the adjacent antiparallel polypeptide chain. In addition, the bona fide role of PC in the non-gluconeogenic tissues has been studied using a combination of classical biochemistry and genetic approaches. The first cloning of the promoter of the PC gene in mammals and subsequent transcriptional studies reveal some key cognate transcription factors regulating tissue-specific expression. The present review summarizes these advances and also offers some prospects in terms of future directions for the study of this important enzyme.
Collapse
Affiliation(s)
- Sarawut Jitrapakdee
- Department of Biochemistry, Faculty of Science, Mahidol University, Bangkok 10400, Thailand
| | - Martin St. Maurice
- Department of Biochemistry, University of Wisconsin, Madison, WI 53706, USA
| | - Ivan Rayment
- Department of Biochemistry, University of Wisconsin, Madison, WI 53706, USA
| | - W. Wallace Cleland
- Department of Biochemistry, University of Wisconsin, Madison, WI 53706, USA
| | - John C. Wallace
- School of Molecular & Biomedical Science, University of Adelaide, SA 5005, Australia
| | - Paul V. Attwood
- School of Biomedical, Biomolecular and Chemical Sciences, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6100, Australia
| |
Collapse
|
4
|
Ito Y, Kondo H, Shiota Y, Yoshizawa K. Theoretical Analysis of the Reaction Mechanism of Biotin Carboxylase. J Chem Theory Comput 2008; 4:366-74. [DOI: 10.1021/ct700260f] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Yuko Ito
- Department of Bioscience and Bioinformatics, Kyushu Institute of Technology, 680-4 Kawazu, Iizuka 820-8502, Japan, and Institute for Materials Chemistry and Engineering, Kyushu University, Nishi-ku, Fukuoka 819-0395, Japan
| | - Hiroki Kondo
- Department of Bioscience and Bioinformatics, Kyushu Institute of Technology, 680-4 Kawazu, Iizuka 820-8502, Japan, and Institute for Materials Chemistry and Engineering, Kyushu University, Nishi-ku, Fukuoka 819-0395, Japan
| | - Yoshihito Shiota
- Department of Bioscience and Bioinformatics, Kyushu Institute of Technology, 680-4 Kawazu, Iizuka 820-8502, Japan, and Institute for Materials Chemistry and Engineering, Kyushu University, Nishi-ku, Fukuoka 819-0395, Japan
| | - Kazunari Yoshizawa
- Department of Bioscience and Bioinformatics, Kyushu Institute of Technology, 680-4 Kawazu, Iizuka 820-8502, Japan, and Institute for Materials Chemistry and Engineering, Kyushu University, Nishi-ku, Fukuoka 819-0395, Japan
| |
Collapse
|
5
|
St Maurice M, Reinhardt L, Surinya KH, Attwood PV, Wallace JC, Cleland WW, Rayment I. Domain architecture of pyruvate carboxylase, a biotin-dependent multifunctional enzyme. Science 2007; 317:1076-9. [PMID: 17717183 DOI: 10.1126/science.1144504] [Citation(s) in RCA: 105] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/02/2022]
Abstract
Biotin-dependent multifunctional enzymes carry out metabolically important carboxyl group transfer reactions and are potential targets for the treatment of obesity and type 2 diabetes. These enzymes use a tethered biotin cofactor to carry an activated carboxyl group between distantly spaced active sites. The mechanism of this transfer has remained poorly understood. Here we report the complete structure of pyruvate carboxylase at 2.0 angstroms resolution, which shows its domain arrangement. The structure, when combined with mutagenic analysis, shows that intermediate transfer occurs between active sites on separate polypeptide chains. In addition, domain rearrangements associated with activator binding decrease the distance between active-site pairs, providing a mechanism for allosteric activation. This description provides insight into the function of biotin-dependent enzymes and presents a new paradigm for multifunctional enzyme catalysis.
Collapse
Affiliation(s)
- Martin St Maurice
- Department of Biochemistry, University of Wisconsin, Madison, WI 53706, USA
| | | | | | | | | | | | | |
Collapse
|
6
|
Abstract
Biotin (1), a water-soluble B series vitamin, distributes widely in microorganisms, plants, and animals. Biosynthesis of 1 involves five steps sequence starting from pimelic acid. The last step, a transformation from dethiobiotin (DTB) to 1, includes an iron clusters-mediated radical process. The compound 1 is a cofactor of carboxylation enzymes and plays crucial roles in the metabolism of fatty acids, sugars, and alpha-amino acids. In addition to the increasing application to feed additives, recent reports have revealed that 1 enhances insulin secretion in animals, suggesting it for a promising therapeutic candidate for an anti-diabetes drug. The remarkably strong affinity of 1 with avidin and streptavidin has been extensively applied for such technologies as photoaffinity labeling. Among the number of approaches to 1 so far developed in 50 years, a synthesis using L-cysteine and thiolactone as a starting material and a key intermediate, respectively, represents one of the best routes leading to 1, because of short steps, high yield, use of inexpensive reagents, and ease of operation.
Collapse
Affiliation(s)
- Masahiko Seki
- Tanabe Seiyaku Co., Ltd., 3-2-10, Dosho-Machi, Osaka 541-8505, Japan.
| |
Collapse
|
7
|
Chuakrut S, Arai H, Ishii M, Igarashi Y. Characterization of a bifunctional archaeal acyl coenzyme A carboxylase. J Bacteriol 2003; 185:938-47. [PMID: 12533469 PMCID: PMC142822 DOI: 10.1128/jb.185.3.938-947.2003] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Acyl coenzyme A carboxylase (acyl-CoA carboxylase) was purified from Acidianus brierleyi. The purified enzyme showed a unique subunit structure (three subunits with apparent molecular masses of 62, 59, and 20 kDa) and a molecular mass of approximately 540 kDa, indicating an alpha(4)beta(4)gamma(4) subunit structure. The optimum temperature for the enzyme was 60 to 70 degrees C, and the optimum pH was around 6.4 to 6.9. Interestingly, the purified enzyme also had propionyl-CoA carboxylase activity. The apparent K(m) for acetyl-CoA was 0.17 +/- 0.03 mM, with a V(max) of 43.3 +/- 2.8 U mg(-1), and the K(m) for propionyl-CoA was 0.10 +/- 0.008 mM, with a V(max) of 40.8 +/- 1.0 U mg(-1). This result showed that A. brierleyi acyl-CoA carboxylase is a bifunctional enzyme in the modified 3-hydroxypropionate cycle. Both enzymatic activities were inhibited by malonyl-CoA, methymalonyl-CoA, succinyl-CoA, or CoA but not by palmitoyl-CoA. The gene encoding acyl-CoA carboxylase was cloned and characterized. Homology searches of the deduced amino acid sequences of the 62-, 59-, and 20-kDa subunits indicated the presence of functional domains for carboxyltransferase, biotin carboxylase, and biotin carboxyl carrier protein, respectively. Amino acid sequence alignment of acetyl-CoA carboxylases revealed that archaeal acyl-CoA carboxylases are closer to those of Bacteria than to those of Eucarya. The substrate-binding motifs of the enzymes are highly conserved among the three domains. The ATP-binding residues were found in the biotin carboxylase subunit, whereas the conserved biotin-binding site was located on the biotin carboxyl carrier protein. The acyl-CoA-binding site and the carboxybiotin-binding site were found in the carboxyltransferase subunit.
Collapse
Affiliation(s)
- Songkran Chuakrut
- Laboratory of Applied Microbiology, Department of Biotechnology, The University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-8567, Japan
| | | | | | | |
Collapse
|
8
|
Abstract
Acetyl-CoA carboxylase (ACC) catalyses the first committed step of fatty acid synthesis, the carboxylation of acetyl-CoA to malonyl-CoA. Two physically distinct types of enzymes are found in nature. Bacterial and most plant chloroplasts contain a multi-subunit ACC (MS-ACC) enzyme that is readily dissociated into its component proteins. Mammals, fungi, and plant cytosols contain the second type of ACC, a single large multifunctional polypeptide. This review will focus on the structures, regulation, and enzymatic mechanisms of the bacterial and plant MS-ACCs.
Collapse
Affiliation(s)
- John E Cronan
- Department of Microbiology, B103 Chemical and Life Sciences Laboratory, University of Illinois, 601 S. Goodwin Avenue, Urbana, IL 61801, USA.
| | | |
Collapse
|
9
|
Attwood PV, Wallace JC. Chemical and catalytic mechanisms of carboxyl transfer reactions in biotin-dependent enzymes. Acc Chem Res 2002; 35:113-20. [PMID: 11851389 DOI: 10.1021/ar000049+] [Citation(s) in RCA: 81] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Biotin-dependent carboxylases catalyze a variety of carboxyl transfer reactions in a number of metabolic pathways and are found in all free-living organisms. They are large molecules which can comprise a single polypeptide chain with three domains or up to three subunits, each of which performs a particular part of the overall reaction. Biotin plays a central role in the action of these enzymes. In this Account we examine the current state of knowledge of the chemistry of catalysis and consider how the recent explosion of available protein sequence and structural information has assisted our understanding of the mechanisms of biotin-dependent enzymes.
Collapse
Affiliation(s)
- Paul V Attwood
- Department of Biochemistry, The University of Western Australia, Nedlands, WA 6907, Australia.
| | | |
Collapse
|
10
|
Perham RN. Swinging arms and swinging domains in multifunctional enzymes: catalytic machines for multistep reactions. Annu Rev Biochem 2001; 69:961-1004. [PMID: 10966480 DOI: 10.1146/annurev.biochem.69.1.961] [Citation(s) in RCA: 489] [Impact Index Per Article: 21.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/30/2023]
Abstract
Multistep chemical reactions are increasingly seen as important in a growing number of complex biotransformations. Covalently attached prosthetic groups or swinging arms, and their associated protein domains, are essential to the mechanisms of active-site coupling and substrate channeling in a number of the multifunctional enzyme systems responsible. The protein domains, for which the posttranslational machinery in the cell is highly specific, are crucially important, contributing to the processes of molecular recognition that define and protect the substrates and the catalytic intermediates. The domains have novel folds and move by virtue of conformationally flexible linker regions that tether them to other components of their respective multienzyme complexes. Structural and mechanistic imperatives are becoming apparent as the assembly pathways and the coupling of multistep reactions catalyzed by these dauntingly complex molecular machines are unraveled.
Collapse
Affiliation(s)
- R N Perham
- Cambridge Centre for Molecular Recognition, Department of Biochemistry, University of Cambridge, Cambridge CB2 1GA, United Kingdom
| |
Collapse
|
11
|
Takahashi T, Ikeda Y, Tateishi A, Yamaguchi Y, Ishikawa M, Taniguchi N. A sequence motif involved in the donor substrate binding by alpha1,6-fucosyltransferase: the role of the conserved arginine residues. Glycobiology 2000; 10:503-10. [PMID: 10764839 DOI: 10.1093/glycob/10.5.503] [Citation(s) in RCA: 38] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
Abstract
Alpha1,6-fucosyltransferase catalyzes the transfer of fucose to the innermost GlcNAc residue of an N-linked oligosaccharide. In order to identify the amino acid residue(s) which are associated with the enzyme activity and to investigate their function, we prepared a series of mutant human alpha1,6-fucosyltransferases in which the conserved residues in the region homologous to alpha1,2-fucosyltransferase had been replaced. These proteins were then characterized by kinetic analyses. The wild-type and mutant alpha1,6-fucosyltransferases were expressed using a baculovirus-insect cell system. The activity assay showed that replacement of Arg-365 by Ala or Lys led to a complete loss of activity while substitution of Ala or Lys for the neighboring Arg-366 decreased the activity to about 3% that of the wild type. Kinetic analyses revealed that the replacements of Arg-366 lead to an increase in the apparent K (m) value for both GDP-fucose and the acceptor oligosaccharide but did not markedly affect the apparent V (max). When these mutants were inhibited by GDP in a competitive manner with respect to the donor substrate, the K (i) values were found to be 50-100 times higher than the value in the wild type. On the other hand, in the inhibition by GMP, the K (i) values for the mutants were very similar to that of the wild type. These findings suggest that Arg-366 contributes to the binding of GDP-fucose via an interaction with the beta-phosphoryl group of the GDP moiety of the donor, and that Arg-365 may also play an essential role in substrate binding. The results suggest that the motif common to alpha1,2- and alpha1,6-fucosyltransferases is critical for binding of the donor substrate, GDP-fucose.
Collapse
Affiliation(s)
- T Takahashi
- Department of Biochemistry, Osaka University Medical School, 2-2 Yamadaoka, Suita, Osaka 565-0871, Japan
| | | | | | | | | | | |
Collapse
|
12
|
Levert KL, Lloyd RB, Waldrop GL. Do cysteine 230 and lysine 238 of biotin carboxylase play a role in the activation of biotin? Biochemistry 2000; 39:4122-8. [PMID: 10747803 DOI: 10.1021/bi992662a] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Biotin carboxylase from Escherichia coli catalyzes the ATP-dependent carboxylation of biotin and is one component of the multienzyme complex acetyl-CoA carboxylase, which catalyzes the committed step in long-chain fatty acid synthesis. For the carboxylation of biotin to occur, biotin must be deprotonated at its N1' position. Kinetic investigations, including solvent isotope effects and enzyme inactivation by N-ethylmaleimide, suggested a catalytic role for a cysteine residue and led to the proposal of a mechanism for the deprotonation of biotin. The proposed pathway suggests a catalytic base removes a proton from a nearby cysteine residue, forming a thiolate anion, which then abstracts the proton from biotin. Inactivation studies of pyruvate carboxylase, which has an analogous mode of action to biotin carboxylase, suggests the catalytic base in this reaction is a lysine residue. Using the crystal structure of biotin carboxylase, cysteine 230 and lysine 238 were identified as the likely active-site residues that act as this acid-base pair. To test the hypothesis that cysteine 230 and lysine 238 act as an acid-base pair to deprotonate biotin, site-directed mutagenesis was used to mutate cysteine 230 to alanine (C230A) and lysine 238 to glutamine (K238Q). Mutations at either residue resulted in a 50-fold increase in the K(m) for ATP. The C230A mutation had no effect on the formation of carboxybiotin, indicating that cysteine 230 does not play a role in the deprotonation of biotin. However, the K238Q mutation resulted in no formation of carboxybiotin, which showed that lysine 238 has a role in the carboxylation reaction. N-Ethylmaleimide was found to inactivate the C230A mutant but not the K238Q mutant, suggesting that N-ethylmaleimide is reacting with lysine 238 and not cysteine 230. The pH dependence of N-ethylmaleimide inactivation revealed that the pK value for lysine 238 was 9.4 or higher, suggesting lysine 238 is not a catalytic base. Thus, the results suggest that cysteine 230 and lysine 238 do not act as an acid-base pair in the deprotonation of biotin.
Collapse
Affiliation(s)
- K L Levert
- Division of Biochemistry, Louisiana State University, Baton Rouge, Louisiana 70803-1715, USA
| | | | | |
Collapse
|
13
|
Blanchard CZ, Lee YM, Frantom PA, Waldrop GL. Mutations at four active site residues of biotin carboxylase abolish substrate-induced synergism by biotin. Biochemistry 1999; 38:3393-400. [PMID: 10079084 DOI: 10.1021/bi982660a] [Citation(s) in RCA: 61] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Acetyl-CoA carboxylase catalyzes the first committed step in the biosynthesis of long-chain fatty acids. The Escherichia coli form of the enzyme consists of a biotin carboxylase protein, a biotin carboxyl carrier protein, and a carboxyltransferase protein. In this report a system for site-directed mutagenesis of the biotin carboxylase component is described. The wild-type copy of the enzyme, derived from the chromosomal gene, is separated from the mutant form of the enzyme which is coded on a plasmid. Separation of the two forms is accomplished using a histidine-tag attached to the amino terminus of the mutant form of the enzyme and nickel affinity chromatography. This system was used to mutate four active site residues, E211, E288, N290, and R292, to alanine followed by their characterization with respect to several different reactions catalyzed by biotin carboxylase. In comparison to wild-type biotin carboxylase, all four mutant enzymes gave very similar results in all the different assays, suggesting that the mutated residues have a common function. The mutations did not affect the bicarbonate-dependent ATPase reaction. In contrast, the mutations decreased the maximal velocity of the biotin-dependent ATPase reaction 1000-fold but did not affect the Km for biotin. The activity of the ATP synthesis reaction catalyzed by biotin carboxylase where carbamoyl phosphate reacts with ADP was decreased 100-fold by the mutations. The ATP synthesis reaction required biotin to stimulate the activity in the wild-type; however, biotin did not stimulate the activity of the mutant enzymes. The results showed that the mutations have abolished the ability of biotin to increase the activity of the enzyme. Thus, E211, E288, N290, and R292 were responsible, at least in part, for the substrate-induced synergism by biotin in biotin carboxylase.
Collapse
Affiliation(s)
- C Z Blanchard
- Division of Biochemistry and Molecular Biology, Louisiana State University, Baton Rouge 70803-1806, USA
| | | | | | | |
Collapse
|