Abstract
The reactivity of bromo- or iodoacetate with the methionyl, lysyl, and histidyl side chains of human myoglobin has been examined. Each of the side chains reacts as expected if it is assumed that the conformation of human myoglobin closely resembles that of sperm whale myoglobin. The three methionyl side chains, which are expected to be buried, are not carboxymethylated unless the conformation of native myoglobin is disrupted. Carboxymethylation of the lysyl side chains proceeds without difficulty at the appropriate pH, a result in accord with the predicted location of these side chains on the surface of the molecule. Four of the 9 histidyl residues cannot be carboxymethylated in native human myoglobin. This is to be expected if certain of the imidazole side chains are involved in internal bonding as in the case of sperm whale myoglobin.
Highlights
The carboxymethylmyoglobin obtained after 7day reaction in this experiment contained an average of 4 histidyl residues per molecule
These data show that it is primarily only the histidyl side chains which react with bromoacetate under these conditions and that little alkylation of methionine and lysine occurs
Homoserine was found in this acid hydrolysate, indicating alkylation of methionine least 4 histidyl residues are not carboxymethylated in aqueous solution, in urea an average of just more than 1 residue resists carboxymethylation after 7 days. These results suggest that in 8 M urea essentially all of the histidyl and methionyl side chains are available for reaction with bromoacetate
Summary
Present information suggests that the conformation of most vertebrate myoglobins may be very similar to, and perhaps identical with that described for sperm whale myoglobin (1) This view gains support on comparison of the amino acid sequence of the human (2) and the whale (3) proteins. From current knowledge of the role of individual side chains in determining the conformation of myoglobin (l), there is little reason to believe that the amino acid interchanges at 26 residues in the polypeptide chains are suflicient to produce markedly different conformations in the human and whale molecules. To determine whether human myoglobin, in solution, undergoes chemical modification as expected if the whale and human molecules possess nearly identical three-dimensional structures.
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