Amino-acid-functionalized gold clusters modulate the catalytic behavior of alpha-chymotrypsin (ChT) toward cationic, neutral, and anionic substrates. Kinetic studies reveal that the substrate specificity (k(cat)/K(M)) of ChT-nanoparticle complexes increases by approximately 3-fold for the cationic substrate but decreases by 95% for the anionic substrate as compared with that of free ChT, providing enhanced substrate selectivity. Concurrently, the catalytic constants (k(cat)) of ChT show slight augmentation for the cationic substrate and significant attenuation for the anionic substrate in the presence of amino-acid-functionalized nanoparticles. The amino acid monolayer on the nanoparticle is proposed to control both the capture of substrate by the active site and release of product through electrostatic interactions, leading to the observed substrate specificities and catalytic constants.