THE ENOLASE SUPERFAMILY - A GENERAL STRATEGY FOR ENZYME-CATALYZED ABSTRACTION OF THE ALPHA-PROTONS OF CARBOXYLIC-ACIDS

We have discovered a superfamily of enzymes related by their ability t o catalyze the abstraction of the alpha-proton of a carboxylic acid to form an enolic intermediate. Although each reaction catalyzed by thes e enzymes is initiated by this common step, their overall reactions (i ncluding racemization, beta-elimination of water, beta-elimination of ammonia, and cycloisomerization) as well as the stereochemical consequ ences (syn vs anti) of the beta-elimination reactions are diverse. Ana lysis of sequence and structural similarities among these proteins sug gests that all of their chemical reactions are mediated by a common ac tive site architecture modified through evolution to allow the enolic intermediates to partition to different products in their respective a ctive sites via different overall mechanisms. All of these enzymes ret ain the ability to catalyze the thermodynamically difficult step of pr oton abstraction. These homologous proteins, designated the ''enolase superfamily'', include enolase as well as more metabolically specializ ed enzymes: mandelate racemase, galactonate dehydratase, glucarate deh ydratase, muconate-lactonizing enzymes, N-acylamino acid racemase, bet a-methylaspartate ammonia-lyase, and o-succinylbenzoate synthase. Comp arative analysis of structure-function relationships within the superf amily suggests that carboxy-phosphonoenolpyruvate synthase, another me mber of the superfamily, does not catalyze the reaction proposed in th e literature but catalyzes an enolase-like reaction instead. The estab lished and deduced structure-function relationships in the superfamily allow the prediction that other apparent members of the family for wh ich no catalytic functions have yet been assigned will also perform ch emistry involving abstraction of the alpha-protons of carboxylic acids .