Electron transfer

Question 1

3 types of biological redox centres

Answer 1

1. Redox-active amino acids in proteins e.g. Cys, Tyr
2. Small organic molecules e.g. NAD, quinone-type molecules i.e. coenzyme Q, flavodoxins with cofactors like FAD
3. Metal-containing redox cofactors e.g. FeS proteins, blue copper proteins, cytochromes

Question 2

FeS proteins

Answer 2

Rubredoxin

Ferredoxin

Question 3

Redox potential of aqueous Fe(II)/Fe(III)

Answer 3

770 mV

Question 4

Ionic radii of Fe(II)/Fe(III)

Answer 4

77 pm / 63 pm

14 pm difference!

Question 5

Rubredoxin

Answer 5

6 kDa
Common in bacteria
Active site: Fe ion coordinated by 4 Cys residues in tetrahedron
[1Fe-0S] system
Performs one electron transfer processes - central Fe alternates between Fe(II) and Fe(III)
Reduction potential in the range +50 mV to -50 mV
6-8 pm difference in Fe-S bond length between Fe(II) and Fe(III)
State of Fe centre can be easily determined: Oxidised = red due to LMCT, reduced = colourless

Question 6

Ferredoxin

Answer 6

10 kDa
Can be classified according to the nature of the FeS clusters:
[2Fe-2S]: reduction potential -300 mV to -500 mV. Fe(III)Fe(III) —> Fe(II)Fe(III)
[3Fe-4S]: reduction potenital -100 mV. Fe(III)3 —> Fe(II)Fe(III)2
[4Fe-4S]: 2 families of these clusters are known - ferredoxin (Fd) and high potential iron-sulfur protein family (HiPIP). Both families share the same resting state, difference is in their active states. Fd forms by reduction Fe(II)2Fe(III)2 Fe(II)3Fe(III)
HiPIP forms by oxidation
Fe(II)2Fe(III)2 Fe(II)Fe(III)3

Question 7

Blue copper proteins

Answer 7

= cupredoxins
= type I copper centres
Cu-containing proteins involved in electron transfer

Question 8

Properties of blue copper proteins

Answer 8

1000x more intense blue colour than aqueous Cu2+ due to S to Cu charge transfer
Relatively high reduction potentials (+350 mV c.f. +170 mV for aq Cu2+) because Cu(I) is stabilised

Question 9

General structure of blue copper proteins

Answer 9

A single Cu coordinated by 2 His and 1 Cys in a trigonal planar structure with a variable axial ligand
(This is Met in type I copper centres)

Question 10

Examples of blue copper proteins

Answer 10

Plastocyanin

Azurin

Question 11

Plastocyanin

Answer 11

Electron transfer agent in photosynthesis
Cu-binding site has a “distorted” trigonal pyramidal geometry
Distortion occurs in the Cu-S bond lengths
Cu-Cys bond shorter than Cu-Met - elongated Cu-Met bond destabilises Cu(II), increasing the redox potential of the protein

Question 12

Azurin

Answer 12

Contains carbonyl group of Gly as a second axial ligand

Associated with cytochrome chain and can participate in denitrification processes in bacteria

Question 13

Cytochromes

Answer 13

= Fe-containing haemproteins

Responsible for the generation of ATP via electron transport