BASIS OF GUANYLATE-CYCLASE ACTIVATION BY CARBON-MONOXIDE

Kinetics of CO association with guanylate cyclase [GTP pyrophosphate-l yase (cyclizing), EC 4,6,1,2] and dissociation from carboxy guanylate cyclase have been studied at pH 7.5 by flash photolysis, yielding rate constants at 23 degrees C of 1.2 +/- 0.1 x 10(5) M(-1). sec(-1), and 28 +/- 2 sec(-1), respectively. While the CO combination rate constant is the same as for the T state of hemoglobin, the CO dissociation rat e constant is much higher than expected for a sis coordinate carboxyhe me protein; yet the absorption spectrum is indicative of a six-coordin ate heme. The two observations are reconciled by a reaction mechanism in which CO dissociation proceeds via a five-coordinate intermediate. This intermediate is structurally very similar to the five-coordinate nitrosyl heme derivative of guanylate cyclase and is presumably respon sible for the observed 4-fold activation of guanylate cyclase by CO. T hus, we provide a model that explains enzyme activities of the nitrosy l and carboxy forms of the enzyme on the basis of a common mechanism.