120 UNITY AND DIVERSITY IN BIOCHEMISTRY 



the sulphur atom. The imidazole nitrogen atoms of the two histidine 

 residues are firmly attached to the iron, they take the place of the two water 

 molecules in the haem complex. 



Of the a and h cytochromes little is known save that the haem of cyto- 

 chrome-a is similar to that in chlorocruorin and that the haem of cyto- 

 chrome-6 is protohaem, Cytochrome-a is not autoxidizable (modification 

 of the valency of the iron by the action of molecular oxygen) whilst cyto- 

 chrome-6 is. It has been possible to identify three new haemochromogens 

 whose spectra are similar to the spectrum of cytochrome -a ; these are cyto- 

 chromes-^i, -ag and -a^. Cytochromes-ai and -a^ replace cytochrome-a in 

 certain bacteria where this latter substance is missing. Cytochrome-ag is 

 identical with cytochrome-oxidase, otherwise known as Warburg's respira- 

 tory enzyme. In the ferrous state it is autoxidizable, that is, it is oxidized 

 to the ferric state by molecular oxygen. Cytochrome-«, which is not 

 autoxidizable, is oxidized by cytochrome-^g (cytochrome-oxidase). 



Molecular oxygen oxidizes ferrous cytochrome-flg to the ferric state. An 

 electron is lost by the iron (which becomes trivalent) and passes to oxygen. 

 Then the ferricytochrome-ag receives an electron from ferrocytochrome-a 

 (which becomes ferri-) and is reconverted into ferrocytochrome-^g. 



2 Fe++ (cyt. tf ) + 2 Fe+++ (cyt. a^) -^ 

 2 Fe+++ (cyt. a) ^ 1 Fe++ (cyt. a^) 

 2 Fe++ (cyt. ^g) + O2 -|- 2 H+ -* 2 Fe+-^^- (cyt. a^) + H2O. 



7. Hydroper oxidases 



In these haemoproteins the iron is in the ferric state and remains in this 

 state. The reaction catalysed by the hydroperoxidase enzymes is the 

 following : 



AH2 + H2O2 -> A + 2H2O 



(A — a phenol, ascorbic acid, etc.) 



These enzymes can be divided into two groups : the peroxidases and 

 the catalases. The peroxidases are principally to be found in plants, but 

 they have been discovered in milk and in leucocytes. The catalytic action 

 of peroxidase has been elucidated by B. Chance. He showed that when 

 H2O2 is added to the peroxidase, a primary addition product is formed 

 which is green. This enzyme-substrate complex is transformed into a pale 

 red compound. 



