Anesthetic-like interactions of nitric oxide with albumin and hemeproteins- A mechanism for control of protein function

Noncovalent bonding interactions of nitric oxide (NO) with human serum albu min (HSA), human hemoglobin A,bovine myoglobin, and bovine cytochrome c oxi dase (CcO) have been explored. The anesthetic nitrous oxide (NNO) occupies multiple sites within each protein, but does not bind to heme iron. Infrare d (IR) spectra of NNO molecules sequestered within albumin, with NO present , support the binding of NO and NNO to the same sites with comparable affin ities. Perturbations of IR spectra of the Cys(34) thiol of HSA indicate NO, NNO, halothane, and chloroform can induce similar changes in protein struc ture. Experiments evaluating the relative affinities of binding of NO and c arbon monoxide (CO) to iron(II) sites of the hemeproteins led to evidence o f NO binding to noniron, nonsulfur sites as well. With HbA, IR spectra of c ysteine thiols and/or the iron(II) N-O stretching region denote changes in protein structure due to NO, NNO, or CO occupying noniron sites with an ord er of decreasing affinities of NO > NNO > CO. Loss of NO from some, not all , noniron sites in hemeproteins is very slow (t(1/2) similar to hours). The se findings provide examples in which NO and anesthetics alter the structur e and properties of protein similarly, and support the hypothesis that some physiological effects of NO land possibly CO) result from anesthetic-like noncovalent bonding to sites within protein or other tissue components. Suc h bonding may be involved in mechanisms for control of oxygen transport, mi tochondrial respiration, and activation of soluble guanylate cyclase by NO.