Fern L-methionine decarboxylase: Kinetics and mechanism of decarboxylation and abortive transamination

Journal Article · Tue Aug 21 00:00:00 EDT 1990 · Biochemistry; (USA)

DOI:https://doi.org/10.1021/bi00485a014· OSTI ID:6058603

Akhtar, M; Stevenson, D E; Gani, D ^[1]

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1. Univ. of Southampton (England)

L-Methionine decarboxylase from Dryopteris filix-mas catalyzes the decarboxylation of L-methionine and a range of straight- and branched-chain L-amino acids to give the corresponding amine products. The deuterium solvent isotope effects for the decarboxylation of (2S)-methionine are {sup D}(V/K) = 6.5 and {sup D}V = 2.3, for (2S)-valine are {sup D}(V/K) = 1.9 and {sup D}V = 2.6, and for (2S)-lecuine are {sup D}(V/K) = 2.5 and {sup D}V = 1.0 at pL 5.5. At pL 6.0 and above, where the value of k{sub cat} for all of the substrates is low, the solvent isotope effects on V{sub max} for methionine are 1.1-1.2 whereas the effects on V/K remain unchanged, indicating that the solvent-sensitive transition state occurs before the first irreversible step, carbon dioxide desorption. At very high concentration, the product amine can promote transamination of the coenzyme. However, the reaction occurs infrequently and does not influence the partitioning between decarboxylation and substrate-mediated abortive transamination under steady-state turnover conditions. The partition ratio, normal catalytic versus abortive events, can be determined from the amount of substrate consumed by a known amount of enzyme at infinite time, and the rate of inactivation can be determined by measuring the decrease in enzyme activity with respect to time. Experiments conducted in deuterium oxide allowed the solvent isotope effects for the partition ratio and the abortive reaction to be determined. {sup 1}H NMR spectroscopic analysis of 3-(methylthio)-1-aminopropane isolated from incubations conducted in 50 molar % deuterium oxide at pL 4.8 and at pL 6.5 indicated that the proton donor was monoprotic and, therefore, is probably the imidazolium side chain of a histidine residue.

• OSTI ID: 6058603
• Journal Information: Biochemistry; (USA), Vol. 29:33; ISSN 0006-2960
• Country of Publication: United States
• Language: English

Related Subjects

59 BASIC BIOLOGICAL SCIENCES
AMINO ACIDS
METABOLISM
DECARBOXYLASES
BIOCHEMICAL REACTION KINETICS
DEUTERIUM COMPOUNDS
ISOTOPE EFFECTS
AMINATION
DECARBOXYLATION
HISTIDINE
LYSINE
METHIONINE
AZOLES
CARBON-CARBON LYASES
CARBOXY-LYASES
CARBOXYLIC ACIDS
CHEMICAL REACTIONS
DRUGS
ENZYMES
HETEROCYCLIC ACIDS
HETEROCYCLIC COMPOUNDS
HYDROGEN COMPOUNDS
IMIDAZOLES
KINETICS
LIPOTROPIC FACTORS
LYASES
ORGANIC ACIDS
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
ORGANIC SULFUR COMPOUNDS
REACTION KINETICS
550201* - Biochemistry- Tracer Techniques