Jd. Stewart et al., DISSECTION OF AN ANTIBODY-CATALYZED REACTION, Proceedings of the National Academy of Sciences of the United Statesof America, 91(16), 1994, pp. 7404-7409
Antibody 43C9 accelerates the hydrolysis of a p-nitroanilide by a fact
or of 2.5 x 10(5) over the background rate in addition to catalyzing t
he hydrolysis of a series of aromatic esters. Since this represents on
e of the largest rate accelerations achieved with an antibody, we have
undertaken a series of studies aimed at uncovering the catalytic mech
anism of 43C9. The immunogen, a phosphonamidate, was designed to mimic
the geometric and electronic characteristics of the tetrahedral inter
mediate that forms upon nucleophilic attack by hydroxide on the amide
substrate. Further studies, however, revealed that the catalytic mecha
nism is more complex and involves the fortuitous formation of a covale
nt acyl-antibody intermediate as a consequence of complementary side c
hain residues at the antibody-binding site. Several lines of evidence
indicate that the catalytic mechanism involves two key residues: His-L
91, which acts as a nucleophile to form the acyl-antibody intermediate
, and Arg-L96, which stabilizes the anionic tetrahedral moieties. Supp
ort for this mechanism derives from the results of site-directed mutag
enesis experiments and solvent deuterium isotope effects as well as di
rect detection of the acyl-antibody by electrospray mass spectrometry.
Despite its partial recapitulation of the course of action of enzymic
counterparts, the reactivity of 43C9, like other antibodies, is appar
ently limited by its affinity for the inducing immunogen. To go beyond
this level, one must introduce additional catalytic functionality, pa
rticularly general acid-base catalysis, through either improvements in
transition-state analog design or site-specific mutagenesis.