dues from pmoA (glu195) and pmoC (asp156, his160, his173) hold this zinc
ion in place. These are shown in stick format in Figure 7.51.
Since the nuclearity of biological copper centers does not correlate with
reactivity,^189 any of the pMMO sites could be catalytic. The authors believe
that the dinuclear site looks promising, because it is unusual. The dinuclear
site has an adjacent cavity lined by several conserved hydrophobic residues —
pro94 from pmoB and leu78, ile163, val164 from pmoC — that could be a
binding site for methane. Similarites between the dinuclear copper and the
cytochrome c oxidase (CcO) subunit II sites could indicate that this site is
involved in electron transfer. However, the CcO dinuclear copper site has an
entirely different coordination sphere that includes bridging cysteine sulfur
atoms and terminal coordination to two histidines, a methionine, and a car-
bonyl oxygen. Other arguments can be put forward for the mononuclear
copper center as well as the zinc center as catalytic sites in pMMO. For
instance, the zinc site is rich in amino acids with carboxylic acid side chains
and could accommodate a diiron center similar to that described for sMMO.
The physiological source of electrons for pMMO is unknown, but a possible
docking site is found in a negatively charged patch on the outer surface of the
soluble pmoB domains close to the dinuclear copper site — conserved pmoB
residues glu35 and asp368 as well as pmoC residue asp45 are found here.
Conclusions from the PDB: 1YEW X - ray crystallographic study of pMMO
are as follows: (1) The pMMO structure reveals an unexpected trimeric
arrangement; (2) three metal ions, a mononuclear copper(II) center, and a
dinuclear Cu – Cu site reside within the soluble regions of the pmoB subunit;
(3) the third metal center, occupied by a zinc ion, lies within the membrane
and is coordinated by aa residues from pmoA and pmoC; (4) metal centers lie
close enough together to possibly foster electron transfer; and (5) neither the
site of methane oxidation nor the pathway for methane ingress or egress can
be determined from these results.
In 2005, the Rosenzweig group published an article in Inorganic Chemistry
entitled: “ X - ray Crystallography and Biological Metal Centers: Is Seeing
Believing? ”.^190 This article serves as an excellent guide to bioinorganic
chemists. First, the article lays out restrictions inherent in X - ray crystallo-
graphic techniques. Some of these are as follows: (1) Very few protein crystal
structures are determined to atomic resolution, and therefore bond distances
and angles between small atoms such as carbon, nitrogen, oxygen, and defi -
nitely hydrogen cannot be known with certainty; (2) the number and type of
metal ion centers in biological molecules cannot be known with certainty from
X - ray crystallography only, because metal ions may be adventitious, the actual
biological metal ion may be replaced by other metal ions during crystallization,
or the metal ion may be lost altogether; (3) the metal ion ’ s oxidation state
cannot be known with certainty from X - ray crystallographic results alone;
(4) the metal ion can be reduced by the X - ray beam, and the true oxidation
state and resulting coordination sphere in a studied “ catalytic intermediate ”
may not be the one observed; and (5) exogenous ligands, such as O 2 , must
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