The atomic structure of the monomeric insect haemoglobin is closely similar to the structure of whale myoglobin, although only about 20% of the amino acids are identical in both protein molecules.
A quantitative comparison of both structures, however, showed that in the vicinity of some amino acid substitutions considerable changes occur. The comparison also revealed unexpected long range structural changes induced by amino acid deletions. In particular a deletion of four amino acids in the A B corner shifts the whole helix B considerably.
The general distribution of amino acids in erythrocruorin is analogous to that which has been found in other haemoglobin molecules. The large proportion of phenylalanines in the hydrophobic core of the molecule is remarkable. Also the haem pocket is lined with seven phenylalanine side chains. Four of these are unknown in vertebrate haemoglobins.
The distal histidine E 7, which is invariant in all known vertebrate haemoglobins, is replaced by a glutamic acid, which has no haem contact, but protrudes into the surrounding solution. This proves that a histidine or another polar residue is not essential for the function of a haemoglobin molecule as a reversible oxygen carrier.
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