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    STUDIA CHEMIA - Issue no. 2 / 2006  
         
  Article:   HEMES REVISITED BY DENSITY FUNCTIONAL APPROACHES. 2. A PARADIGM FOR AXIAL LIGATION IN HEMOPROTEINS.

Authors:  RADU SILAGHI-DUMITRESCU. 		 
       
         
  Abstract:  Reported here are DFT descriptions of dioxygen reduction at the active site of cytochr]ome cd1 nitrite reductase, an enzyme with a histidine-ligated heme active site, known to act as cytochrome oxidase. Also explored is the possibility of nitric oxide reduction by histidine-ligated hemes. The energetics of these two processes correlate well with previous findings on other hemoproteins, and allow further elaboration on the newly proposed “thiolate obstruction” theory [Silaghi-Dumitrescu, Eur. J. Inorg. Chem. 2003, 1048]. This theory, orthogonal if not opposed to the classical “thiolate push” dogma [Dawson et al., J. Am. Chem. Soc. 1976, 98, 3707] , argues that efficient reduction of diatomics such as dioxygen and nitric oxide is more readily accomplished by histidine-ligated hemes than by thiolate– ligated hemes. Generalizing, for reductive processes involving a small diatomic as the sole substrate, neutral ligands (e.g., histidine in cytochrome oxidases and heme oxygenase, lysine in cytochrome c nitrite reductase) are found to always be preferable over anionic ligands. By contrast, in enzymes designed to deal with more than one substrate, anionic ligands are preferable (e.g., cysteine or tyrosine in monooxygenases), since they allow the safety switches needed to avoid uncoupling in their race against entropy.  
         
     
         
         
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