Transaminase B from Escherichia coli : Quaternary Structure, Amino-Terminal Sequence, Substrate Specificity, and Absence of a Separate Valine-α-Ketoglutarate Activity

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Abstract
Transaminase B (branched-chain amino acid aminotransferase, EC 2.6.1.42), the ilvE gene product, was purified to apparent homogeneity from an E. coli K-12 strain which carries the ilvE gene both on the host chromosome and on a plasmid. The oligomeric structure of the enzyme, as determined by analytical ultracentrifugation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis, was that of a hexamer with a MW of about 182,000 and apparently identical subunits. Cross-linking with dimethylsuberimidate yielded trimers, dimers and monomers, but essentially no species of higher MW. These results are consistent with a double-trimer arrangement of the subunits in native enzyme. The amino-terminal sequence was: Gly Thr Lys Lys Ala Asp Tyr Ile (Trp) Phe Asn Gly (Thr) (Met) Val. Purified transaminase B catalyzed transamination between .alpha.-ketoglutarate and L-isoleucine, L-leucine, L-valine, and, to a lesser extent, L-phenylalanine and L-tyrosine, the latter reacting very sluggishly. The enzyme was free of aspartate transaminase and of transaminase C. The apparent Km values for the branched-chain .alpha.-ketoacids were smaller than those for the corresponding amino acids. The lowest Km was recorded for DL-.alpha.-keto-.beta.-methyl-n-valerate, and the highest was recorded for L-valine. The ratio of the valine- and isoleucine-.alpha.-ketoglutarate activities did not change significantly during purification, and both activities ere quantitatively removed from crude extract by antibody raised against purified transaminase B. These observations argue against the existence of a separate valine-.alpha.-ketoglutarate transaminase. Anti-E. coli transaminase B antibody cross-reacted with crude extract from Salmonella typhimurium, but not with extract obtained from Pseudomonas aeruginosa.