Hemocyanin reaction with oxygen

Manganese compounds are used as oxidant and catalyst in the various electron-transfer reactions in vivo involving the Mn(II), Mn(III), or Mn(IV) oxidation states. In particular, manganese complexes play an important role as oxygen carrier, similar to iron and copper porphyrins such as hemoglobin and hemocyanin. Manganese compounds have a pecuhar nature for the reaction with molecular oxygen. 

Discrepancies exist in the literature about the reaction of hemocyanin with carbon monoxide. It has been known for a long time that, in contrast to oxyhemocyanin which is blue, solutions of hemocyanin treated with CO are colorless (Craifalenau, 1919). Root (1934) showed that Limulus hemocyanin combines with carbon monoxide in proportions similar to those of oxygen. However, besides the loss of color, the affinity of this hemocyanin for carbon monoxide is only one-twentieth of that for oxygen. In contrast with these results, Rawlinson (1940) found no evidence for combination with carbon monoxide by hemocyanin from Palinurus vulgaris. 

The veiy different reactivities of hemocyanin and tyrosinase toward oxidizable substrates se n to be due to the presence of a substrate binding site in the latter. It appears, therefore, that the oxygenation of the phenol substrate occurs either by reaction of copper-bound pooxide or hydrc ieroxide with the ortho position of the copper-bound phenol (16) or that the 0-0 bond of the peroxide ligand is cleaved... 

In the following sections, examples of all the classes of reaction given above will be discussed, with emphasis on those that have been well studied. Many of the enzymes involved in these processes are hemoproteins, but non-heme prosthetic groups are important in oxygenases and are also present in some oxidases. Copper is an important metal in this context, and is present in the oxygen transport protein hemocyanin, and in oxidases such as cytochrome oxidase and laccase. Some flavoenzymes are important too, but will not be covered in this discussion. 

The copper in hemocyanin is directly and quantitatively involved in the oxygenation reaction. All incoming hemocyanin solutions are analyzed for copper with a Perkin-Elmer atomic absorption spectrophotometer. The analyses are carried out by diluting the stock solution with distilled water and feeding the solution to the burner. 

Theorell [4], and hemocyanin. The rate of the oxygen-myoglobin reaction of 2 X lOVrosec was accurately determined, a reaction as fast as any which has been studied by flow-mixing methods. This line of work came to an end with Millikan s untimely death in a climbing accident, and his apparatus has never been replicated, although it was preserved for many years in Roughton s laboratory. 

In certain copper-containing proteins the copper appears to serve principally in electron transport with no evidence of CU-O2 interaction, such as in cytochrome oxidase. Of importance, however, is that many copper proteins and enzymes participate in reactions in which the oxygen molecule is directly or indirectly involved. An example is hemocyanin, the oxygen carrier in the blood of certain sea animals such as snails, octopus, and Crustacea. Oxygenated hemocyanin is blue and the cephalopods (crabs and lobsters) are literally the blue bloods of the animal kingdom. Hemocyanins are giant molecules of MW > 10 that occur free in solution. 

Warburg (1949), developing the analogy of the behavior of phenol oxidase and hemocyanin against carbon monoxide and the difference of their absorption spectra, predicted that laccase, which is blue and is uninhibited by carbon monoxide, must be a nondissociating compound of oxygen with the monovalent copper present. But the reaction of hemocyanin with carbon monoxide was re-examined carefully by Rawlinson (1940) who found, contrarily to Root (1934), that hemocyanin does not form a carbon monoxide compound at all. 

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