Methionine transaminase

Methionine transaminase (EC 2.6.1.88, methionine-oxo-acid transaminase) is an enzyme with systematic name L-methionine:2-oxo-acid aminotransferase.^[1] It catalyses a general chemical reaction that is used in the pathway to recycle the amino acid L-methionine:

L-methionine + an α-keto acid ${\displaystyle \rightleftharpoons }$ 4-(methylthio)-2-oxobutanoic acid + an L-amino acid

The enzyme characterised from Klebsiella pneumoniae operates in the direction towards methionine and can use a variety of L-amino acids as amino group donors to react with 4-(methylthio)-2-oxobutanoic acid. L-tyrosine was found to be most effective.^[2] A crystal structure of the corresponding enzyme in Escherichia coli has been determined.^[3]

In plants such as Arabidopsis thaliana, the enzyme operates in the forward direction, as part of the pathway to glucosinolates which contain extra methylene groups (CH₂) between the amino acid part and the thioether of methionine. These intermediates are formed from 4-(methylthio)-2-oxobutanoic acid.^[4]