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Figure 4.
Figure 4. Superposition of the dinuclear copper center of sweet
potato catechol oxidase with bound phenylthiourea (PTU) with the
oxygenated form of Limulus polyphemus hemocyanin^19. The side
chains of catechol oxidase are colored by atom type and the
metal-ligating histidine residues of lpHC are shown in green.
The metal-ligating residues forming the CuB binding site are
completely conserved (see also Fig. 6). For the CuA binding site
two amino acid substitutions are found. The HXXXH sequence motif
present in lpHC is changed to HXXXC^92 in catechol oxidase. In
catechol oxidase the side chain of Cys 92 is not coordinated to
CuA and the corresponding free co-ordination site is occupied by
His 109. In hemocyanin phenylalanine Phe 49, located on an  -helix
from the N-terminal domain, blocks the substrate access to the
dicopper center.
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