Unexpected divergence of enzyme function and sequence: "N-acylamino acid racemase" is o-succinylbenzoate synthase

A protein identified as "N-acylamino acid racemase" from Amycolaptosis sp. is an inefficient enzyme (k(cat)/K-m = 3.7 x 10(2) M-1 s(-1)). Its sequence is 43% identical to that of an unidentified protein encoded by the Bacillu s subtilis genome. Both proteins efficiently catalyze the o-succinylbenzoat e synthase reaction in menaquinone biosynthesis (k(cat)/K-m = 2.5 x 10(5) a nd 7.5 x 10(5) M-1 s(-1), respectively), suggesting that this;is their "cor rect" metabolic function. Their membership in the mechanistically diverse e nolase superfamily provides an explanation for the catalytic promiscuity of the protein from Amycolaptosis. The adventitious promiscuity may provide a n example of a protein poised for:evolution of a new enzymatic function in the enolase superfamily. This study demonstrates that the correct assignmen t of function to new proteins in functional and structural genomics may req uire an understanding of the metabolism of the organism.