The function of the unreactive thiol groups of normal adult human haemoglobin

Abstract

Native human globin has the same number of SH groups as native hemoglobin, i.e., 2 reactive and 4 unreactive. It is therefore unlikely that the SH groups are involved in hem-globin bonds. The hem-group in cytochrome c is known to be linked to the protein by thio ether bonds. The reactivity of the sulphur groups of this protein was shown to be different from that of hemoglobin, thus supporting the conclusion that this type of bond does not exist in haemoglobin. Haemoglobin derivatives undergo slow changes in absorption spectra and reduced viscosity when exposed to 6 M-urea. These changes are accelerated by the addition of phenylmercuric hydroxide and mercuric chloride, suggesting that the SH groups are contributing to the stability of the protein. The rate at which these changes occur are in the order methaemoglogin>oxyhemoglobin>reduced hemoglobin>carboxyhemoglobin. The mean molecular weights of some of the products of reaction of hemoglobin with urea and phenylmercuric hydroxide were measured. These showed that there was no increase in the tendency of hemoglobin to dissociate under these conditions. It was concluded that these observations are consistent with the idea that the unreactive SH groups in adult human hemoglobin are concerned in intra-chain bonds.