KINETICS OF NITRIC-OXIDE DISSOCIATION FROM 5-COORDINATE AND 6-COORDINATE NITROSYL HEMES AND HEME-PROTEINS, INCLUDING SOLUBLE GUANYLATE-CYCLASE

Kinetics of NO dissociation were characterized for three five-coordina te systems, heme-NO, HSA-heme-NO (human serum albumin), GC-NO (soluble guanylate cyclase), and for the six-coordinate system, Im-heme-NO. Ni trosyl myoglobin was redetermined for comparison. Previously known, si x-coordinate R and T state nitrosyl hemoglobins are also included in t he comparison. The data indicate that NO dissociates more than 1000 ti mes faster from five-coordinate model heme than it does from the six-c oordinate analog. Such a negative trans-effect between NO and a proxim al base is in sharp contrast to carboxy heme derivatives, in which lig and dissociation rates are greatly slowed in when a trans base is pres ent. As a result of opposite trans-effects, six-coordinate carboxy and nitrosyl derivatives have comparable dissociation rates, even though the five-coordinate species are very different. In proteins, five- and six-coordinate forms do not show a large difference in dissociation r ates. Part of the reason may be due to different probabilities for gem inate recombination in the different proteins, but this cannot explain all the facts. There must also be influences of the protein structure on bond-breaking rate constants themselves. With the exception of hem oglobin in the T state, nitrosyl guanylate cyclase shows the highest N O dissociation rate constant, k(obs) = 6 x 10(-4) s(-1) This would yie ld a half-life of about 2 min at 37 degrees C for dissociation of NO f rom GC-NO, a number that has implications for the mechanism of regulat ion of the activity of this key heme enzyme.