Journal
BIOCHEMISTRY
Volume 41, Issue 39, Pages 11711-11720Publisher
AMER CHEMICAL SOC
DOI: 10.1021/bi020393+
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- NIGMS NIH HHS [P50 GM 62529] Funding Source: Medline
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L-Threonine acetaldehyde-lyase (threonine aldolase, TA) is a pyridoxal-5'-phosphate-dependent (PLP) enzyme that catalyzes conversion Of L-threonine or L-allo-threonine to glycine and acetaldehyde in a secondary glycine biosynthetic pathway. X-ray structures of Thermatoga maritima TA have been determined as the apo-enzyme at 1.8 Angstrom resolution and bound to substrate L-allo-threonine and product glycine at 1.9 and 2.0 Angstrom resolution, respectively. Despite low pairwise sequence identities, TA is a member of aspartate aminotransferase (AATase) fold family of PLP enzymes. The enzyme forms a 222 homotetramer with the PLP cofactor bound via a Schiff-base linkage to Lys199 within a domain interface. The structure reveals bound calcium and chloride ions that appear to contribute to catalysis and oligomerization, respectively. Although L-threonine and L-allo-threonine are substrates for T. maritima TA, enzymatic assays revealed a strong preference for L-allo-threonine. Structures of the external aldimines with substrate/product reveal a pair of histidines that may provide flexibility in substrate recognition. Variation in the threonine binding pocket may explain preferences for L-allo-threonine versus L-threonine among TA family members.
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