Kinetic Studies on<i>α</i>-Aminoisobutyrate Decomposing Enzyme

Honma, Mamoru; Shimomura, Tokuji

doi:10.1080/00021369.1974.10861266

Cited by 4 publications

(6 citation statements)

References 2 publications

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“…This is illustrated in Figure 7. 3 The unreactivity of AC3C is surprising given that of AC5C and AC6C. The X-ray structure of the AC3C complex (V. N. Malashkevich, M. D. Toney, P. Strop, J. N. Jansonius, unpublished results) shows an aldimine bond that is twisted fully out of the plane of the PLP ring toward the substrate binding site.…”

Section: Discussionmentioning

confidence: 99%

“…All 2,2-dialkylamino acids tested (AIB, AC5C, AC6C) give undetectable amounts of nonoxidative decarboxylation. 3 On the other hand, the monoalkylamino acids all give substantial amounts of nonoxidative decarboxylation. The ratio of nonoxidative decarboxylation for L-alanine vs AIB is >10 4 ; the presence of two vs one alkyl groups on C R strongly influences reaction specificity in the decarboxylation reaction.…”

Section: Discussionmentioning

confidence: 99%

“…The catalytic cycle is completed by transamination of an R-keto acid, preferentially pyruvate (1)(2)(3). The overall reaction catalyzed by DGD is shown below.…”

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confidence: 99%

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Reactions of Alternate Substrates Demonstrate Stereoelectronic Control of Reactivity in Dialkylglycine Decarboxylase

1998

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Kinetic and product analyses of the reactions of dialkylglycine decarboxylase with several alternative substrates are presented. Rate constants for the reactions of amino and keto acids of several substrates decrease logarithmically with increasing side-chain size. Conversely, k cat for L-amino acid decarboxylation increases with side-chain size. These and other data confirm a proposed model for three binding subsites in the active site. In this model, bond making and breaking in both the decarboxylation and transamination half-reactions occurs at the "A" subsite, which maintains the scissile bond aligned with the p orbitals of the conjugated aldimine and thus maximizes stereoelectronic effects. This strongly supports the proposal by Dunathan (Proc. Natl. Acad. Sci. U.S.A. 55,[712][713][714][715][716]) that PLP-dependent enzymes can largely control reaction specificity by specific orientation about C R in the external aldimine intermediate. The "B" subsite can accept either an alkyl or a carboxylate group, while the "C" subsite accepts only small alkyl groups. This model predicts the existence of nonproductive binding modes for amino acids, which is proposed to be the ultimate origin of the k cat increase with side-chain size for L-amino acid decarboxylation. The specificity of the 2-aminoisobutyrate decarboxylation half-reaction toward oxidative decarboxylation is very high (<1 in 10 5 turnovers yields nonoxidative decarboxylation). The origin of this specificity is explored with the reactions of amino-and methylaminomalonate. These substrates exhibit high yields of nonoxidative decarboxylation, providing support for a model in which the interaction between a carboxylate group in the B subsite and Arg406 is a prerequisite to proton donation to and removal from C R .Dialkylglycine decarboxylase (DGD) 1,2 from Pseudomonas cepacia is a PLP dependent enzyme that catalyzes the oxidative decarboxylation of 2,2-dialkylglycines, yielding CO 2 , a ketone, and the PMP form of the enzyme (Scheme 1). The catalytic cycle is completed by transamination of an R-keto acid, preferentially pyruvate (1-3). The overall reaction catalyzed by DGD is shown below. The unusual ability of DGD to catalyze rapidly two classical PLPdependent reactions has rekindled an interest in this enzyme.The X-ray structure of DGD has been determined to 2.1-Å resolution (4, 5). The fold of the enzyme is roughly similar to the well-studied aspartate aminotransferase (6), which is a member of evolutionary subgroup I of aminotransferases (7). DGD is a representative member of evolutionary subgroup II (8), and its fold is closely similar to the other known structures of subgroup II enzymes (9, 10). The active site structure of DGD is similar to that of aspartate aminotransferase in that the interactions made between coenzyme and protein are highly conserved (5).Toney et al. (5) proposed a structurally based model of the active site of DGD in which there are three binding subsites (A-C) that differ in their functional group specificities. One of t...

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

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Reactions of Alternate Substrates Demonstrate Stereoelectronic Control of Reactivity in Dialkylglycine Decarboxylase

1998

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“…6 ) This enzyme catalyzed the cleavage of a-methyl-L-serine to formaldehyde moiety and Dalanine. Another enzyme, a-aminoisobutyrate (AIB) decomposing enzyme which was purified from bacteria grown on AIB4) or isovaline 3 ) was also active on amethyl-DL-serine.')…”

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confidence: 99%

“…6 ) Determination of acetol was carried out by the Somogyi-Nelson's method. n For assay of a-methylserine hydroxymethyltransferase activity, [I_HC] DLalanine was used as the hydroxymethyl group acceptor according to the method described by Chen and Schirch 8 ) for serine hydroxymethyltransferase.…”

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confidence: 99%