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Investigation of the copper-binding centre of Panulirus interruptus haemocyanin
led to the discovery of a pseudo 2-fold axis relating two helical pairs
surrounding and co-ordinating the two copper ions. The pseudo 2-fold symmetry
relating one helical pair, co-ordinating Cu-A, to the second helical pair
co-ordinating Cu-B is quite precise with 31 equivalent C alpha atoms having a
root-mean-square deviation of only 1.47 A. The 2-fold consists of a rotation of
174.6 degrees and a translation parallel to the rotation axis of 0.7 A. After
superposition of the helical pairs, the two copper ions are within 1.1 A and the
three C alpha atoms of the histidine ligands of Cu-A are within a
root-mean-square deviation of 1.0 A from the C alpha atoms of the histidine
residues co-ordinating Cu-B. Of the superimposed residues, 26% are identical in
sequence. These data suggest that the current oxygen-binding centre of
arthropodan haemocyanins is the result of dimerization, gene duplication and
gene fusion of an ancestral mono-copper-binding helical pair. This suggestion is
supported by the recent discovery that in the sequence of functional domains of
molluscan haemocyanins only amino acid sequence homology with the arthropodan
Cu-B helical pair has been found and no evidence for similarity with a Cu-A
binding helical pair was observed. This provides strong evidence that a
mono-copper-binding helical pair has been the ancestor of both the arthropodan
and molluscan haemocyanins. Turning to the Fe-binding helical pairs in
haemerythrins, it appears that they are less similar to each other than the two
Cu-binding helical pairs in arthropodan haemocyanins. Nevertheless, the Fe-B
haemerythrin helical pair superimposes well onto the Cu-A helical pair of
Panulirus haemocyanin. A root-mean-square deviation of 1.9 A for 24 equivalent C
alpha carbon atoms is obtained, while Fe-B deviates 1.4 A from Cu-A after
superposition of the helices. Moreover, the three histidine ligands of the Cu-A
helical pair are equivalent with three histidine ligands of the Fe-B pair. The
structural similarity and correspondence in metal-binding ligands suggests that
both haemocyanins and haemerythrins have originated from an ancestral
mono-metal-binding helical pair having two ligands provided by the first helix
and one ligand by the second helix.(ABSTRACT TRUNCATED AT 400 WORDS)
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