Engineering Chemistry S Datta

566 ENGINEERING CHEMISTRY
Three types of redox centres are found in biology—protein, small molecules and redox
cofactors. Mainly in the class of small molecules for electron transfers nicotinamide and quinone
coenzymes are found in biosphere.
Nicotinamide adenine dinucleotide (NAD) and NADP participate with the 4-position of
the pyridine ring being active.
Coenzyme A, also called ubiquinone, found in all cells both in free and in protein bound
forms functions as follows:
O
HCO
3 CH 3
OH
HCO
3 CH 3
CH 3
+ –
HCO 3 [CH 2 CH = CCH 2] n H + 2H + 2e
O
CH 3
HCO 3 [CH 2 CH = CCH 2] n H
OH
Metalloproteins act as electron-transfer proteins, also called ‘electron transferases’,
possess some characteristics like:
(a) Possess a suitable cofactor which acts as electron sink.
(b) Possess one hydrophobic shell close to cofactor which forms hydrogen bonds with the
hydrophobic environment to stabilize the oxidised and the reduced forms.
(c) Small structural changes accompanying electron transfer. These transferases are of
four classes—flavodoxins, blue copper proteins, iron-sulfur proteins, cytochromes.
The flavodoxins contain organic redox cofactor flavin mononucleotide (FMN). They are
found mainly in bacteria and algae. FMN can act as 1 or 2 elctron redox center and functions as
Me
2–
CH
2(CHOH) 3CH2OPO3
N
N
O
Me
2–
CH
2(CHOH) 3CH2OPO3
N
N
O
Me
N
O
NH
Me
N
H
O
NH
Me
2–
CH
2(CHOH) 3CH2PO3
H
N N O
The blue copper proteins function in bacteria and plants. The geometry of the copper
site is trigonal planar and three ligands bind to copper and have a relatively high reduction
potential.
Me
N
H
O
NH