14
1 Redox Proteins and Bioelectrocatalysis
(+0.153 V). On the other hand, stellacyanin from Rhus vernicifera with axial Gln
shows the most negative side value of E
⊕ (+0.184 V) among blue copper proteins
[35]. This is predominantly ascribed to the electrostatic attractive-interaction between
Gln and Cu
2+ . As anticipated from a simple electrostatics argument, replacement of
the axial Met in BOD by Gln causes a −0.25 V-shift of E
⊕ of T1Cu [36]. Similar
effects were reported for a Met510Gln mutant in copper efflux oxidase (CueO) (a
shift of −0.13 V) [37], a Met121Asp mutant in azurin (a shift of −0.19 V) [38].
When the axial site of the T1Cu site is vacant (with Phe, Leu, or Ala at the corresponding axis residue position), E
⊕ of the T1Cu site shifts to the direction of the
positive potential [37].
“Non-blue” type II copper sites (T2Cu) exhibit a square planar coordination by
N or N/O ligand (of His and Tyr or H 2 O), and are present in oxidases and oxygenases, nitric (NO 3
− ) reductase, and Cu, Zn-superoxide dismutase (SOD). T2Cu gives
normal EPR spectra with axial symmetry. Type III copper sites (T3Cu) consist of
oxygen-bridged dimer of copper ion centers, each coordinated by three His residues.
They occur in an oxygen-transporting protein hemocyanine and a monooxygenase
tyrosinase. Trinuclear copper centers (T2/3Cu) are trinuclear copper ion sites coordinated by eight His ligands as a coupled two T2Cu and a T3Cu ligands. T2/3Cu is
present in blue copper oxidases (i.e. MCOs) together with T1Cu. Binuclear Copper
A centres (Cu A ) are found in cyt c oxidase and nitrous oxide (N 2 O) reductase. The
two copper ions are coordinated by two His ligands, one Met, one protein backbone
carbonyl oxygen, and two bridging Cys residues. Cupper B centers (Cu B ) are also
found in cyt c oxidase. The copper ion is coordinated by three His ligands in a trigonal
pyramidal geometry.
Cupper ion also exists in non-blue oxidases (TPQ-containing amine oxidases and
galactose oxidases).
1.2.7 Nickel [39]
Hydrogenases (H 2 ases) are ubiquitous and key components among anaerobic (occasionally aerobic) bacterial and archaea, and catalyze the reversible oxidation of dihydrogen (H 2 ) and reduction of proton (H
+ ). One major family of H 2 ases contain binuclear Ni-Fe active sites, in which two thiolates of Cys residues are bridging the two
metals, and are called [NiFe] H 2 ases (Fig. 1.13 [40]). NAD(P)
+ -reducing [NiFe]
H 2 ases have a Dp subunit containing an FMN, which catalyzes electrochemical
communication between electron-transfer-type FeS in the subunit and hydride ion
transfer-type NAD(P)(H) in solution. Some [NiFe] enzymes have a selenocysteinyl
(Sec) residue instead of one of the Cys residues supporting the Ni-Fe center and are
called [NiFeSe] H 2 ases. Other families of H 2 ases are [FeFe] H 2 ases and [Fe] H 2 ases.
1 Redox Proteins and Bioelectrocatalysis
(+0.153 V). On the other hand, stellacyanin from Rhus vernicifera with axial Gln
shows the most negative side value of E
⊕ (+0.184 V) among blue copper proteins
[35]. This is predominantly ascribed to the electrostatic attractive-interaction between
Gln and Cu
2+ . As anticipated from a simple electrostatics argument, replacement of
the axial Met in BOD by Gln causes a −0.25 V-shift of E
⊕ of T1Cu [36]. Similar
effects were reported for a Met510Gln mutant in copper efflux oxidase (CueO) (a
shift of −0.13 V) [37], a Met121Asp mutant in azurin (a shift of −0.19 V) [38].
When the axial site of the T1Cu site is vacant (with Phe, Leu, or Ala at the corresponding axis residue position), E
⊕ of the T1Cu site shifts to the direction of the
positive potential [37].
“Non-blue” type II copper sites (T2Cu) exhibit a square planar coordination by
N or N/O ligand (of His and Tyr or H 2 O), and are present in oxidases and oxygenases, nitric (NO 3
− ) reductase, and Cu, Zn-superoxide dismutase (SOD). T2Cu gives
normal EPR spectra with axial symmetry. Type III copper sites (T3Cu) consist of
oxygen-bridged dimer of copper ion centers, each coordinated by three His residues.
They occur in an oxygen-transporting protein hemocyanine and a monooxygenase
tyrosinase. Trinuclear copper centers (T2/3Cu) are trinuclear copper ion sites coordinated by eight His ligands as a coupled two T2Cu and a T3Cu ligands. T2/3Cu is
present in blue copper oxidases (i.e. MCOs) together with T1Cu. Binuclear Copper
A centres (Cu A ) are found in cyt c oxidase and nitrous oxide (N 2 O) reductase. The
two copper ions are coordinated by two His ligands, one Met, one protein backbone
carbonyl oxygen, and two bridging Cys residues. Cupper B centers (Cu B ) are also
found in cyt c oxidase. The copper ion is coordinated by three His ligands in a trigonal
pyramidal geometry.
Cupper ion also exists in non-blue oxidases (TPQ-containing amine oxidases and
galactose oxidases).
1.2.7 Nickel [39]
Hydrogenases (H 2 ases) are ubiquitous and key components among anaerobic (occasionally aerobic) bacterial and archaea, and catalyze the reversible oxidation of dihydrogen (H 2 ) and reduction of proton (H
+ ). One major family of H 2 ases contain binuclear Ni-Fe active sites, in which two thiolates of Cys residues are bridging the two
metals, and are called [NiFe] H 2 ases (Fig. 1.13 [40]). NAD(P)
+ -reducing [NiFe]
H 2 ases have a Dp subunit containing an FMN, which catalyzes electrochemical
communication between electron-transfer-type FeS in the subunit and hydride ion
transfer-type NAD(P)(H) in solution. Some [NiFe] enzymes have a selenocysteinyl
(Sec) residue instead of one of the Cys residues supporting the Ni-Fe center and are
called [NiFeSe] H 2 ases. Other families of H 2 ases are [FeFe] H 2 ases and [Fe] H 2 ases.