Arginine deiminase from Mycoplasma arthritidis. Structure-activity relationships among substrates and competitive inhibitors

Abstract

The arginine deiminase (L-arginine iminohydrolase, EC 3.5.3.6) from Mycoplasma arthritidis catalyzes the irreversible hydrolysis of arginine and related guanidine derivatives to ammonia and the corresponding ureido analog of the substrate. The kinetic constants Km, kcat, and kcat/Km for the arginine deiminase-catalyzed hydrolysis of L-arginine are equal to 4 micron, 29 s-1, and 7.4 X 10(7) M-1 s-1, respectively, at 25 degrees C and pH 7.2. The enzyme also catalyzes the hydrolysis of L-canavanine, Nalpha-methyl-L-argine, D-arginine, L-homoarginine, L-argininic acid, and guanidine, in order of decreasing second order rate constants (kcat/Km); the second order rate constants for these substrates are 10(-3) to 10(-10) smaller than the rate constant for L-arginine. Twenty-two arginine and guanidine analogs were tested for inhibitory capacity. Only 13 are competitive inhibitors having Ki values in the range 3.2 to 40 mM. These results show that binding of ligands to the enzyme is dominated by electrostatic or hydrogen bonding interactions, or both, of the guanidino and alpha-amino group. Neither citrulline nor ornithine, the end product of arginine degradation in M. arthritidis, is an inhibitor of arginine deiminase from this organism.