From no-confidence to nitric oxide acknowledgement: A story of bacterial nitric-oxide reductase

The review briefly summarizes current knowledge of the bacterial nitric-oxi de reductase(NOR) This membrane enzyme consists of two subunits, the smalle r one contains haem C and the larger one two haems B and nonhaem iron. The protein sequence and structure of metal centres demonstrate the relationshi p of NOR to the family of terminal oxidases. The binuclear Fe-Fe reaction c entre, consisting of antiferromagnetically coupled hem B and nonhaem iron, is analogous to Fe-Cu centre of terminal oxidases. The data on the structur e and function of NOR and terminal oxidases suggest that all these enzymes are closely evolutionally related. The catalytic properties are determined most of all by the relatively high toxicity of nitric oxide as a substrate and the resulting strong need to maintain its concentration at nanomolar le vels. A kinetic model of the action of the enzyme comprises substrate inhib ition. NOR does not conserve the free energy of nitric oxide reduction beca use it does not work as a proton pump and, moreover, the protons coming int o the reaction are taken from periplasm, i.e. they do nor cross the membran e.