Reaction of nitric oxide with heme proteins: studies on metmyoglobin, opossum methemoglobin, and microperoxidase

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Abstract
Kinetic and EPR studies show that the first step in the reaction of NO with ferric myoglobin [horse heart, Mb], opossum Hb [Hb OP] and microperoxidase [MP] is the reversible formation of the H-NO complex: H + NO .dblarw. H-NO (where H = Mb+, or Hb+ OP, or MP+). The NO-combination rates are markedly affected by the presence or absence of the distal histidine. The distal histidine significantly reduces the NO-combination rates, perhaps by interaction between the distal histidine and the ferric iron. Thus the .beta.-chains of Hb+ OP and metmyoglobin show similar combination rates. In the absence of a distal histidine, the NO-combination rates in the .alpha.-chains of Hb+ OP are much faster and similar to those observed for the 5-coordinate heme in MP. The loss of a water molecule from the 6-coordination site is assumed to be the rate-limiting step.

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