Biochemistry 2280A Lecture Notes - Lecture 11: Electron Affinity, Cytochrome C Oxidase, Phosphorylation
5 May 2018
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Topic 11: Oxidative Phosphorylation
Section 1: Mitochondrial Electron Carriers
Cofactor: protein-bound non-protein chemical (cofactors are not provided by amino acids)
- There are 4 protein-based complexes in the mitochondrial electron-transport system that work in sequence to deliver
electrons to oxygen. Each complex contains an enzyme that catalyzes electron transport and several non-protein groups
that accept and release electrons.
Energetics of the Mitochondrial ETC
- Electron flow is favourable
- Each transfer results in loss of free energy
- Movement of electrons is in a strictly defined order
Prosthetic Group: type of cofactor that is
tightly bound to a coenzyme or may interact
transiently w/proteins (prosthetic groups
are often metals - Fe, Mg, Co)
Coenzyme: orgaic cofactor that’s ofte a reactio
substrate (ex. vitamins); coenzyme ≠ enzyme
(Occurs at complexes I, II, and IV)
*NADH carries 2 electrons
* to fully reduce O2 to H2O
requires 4 electrons
(2 NADH)
2H
+
½
H2O
O2 has the greatest
electron affinity (final
acceptor)
Low affinity/redox potential High affinity/redox potential
(NADH) (O2)
Some people rely on prosthetic legs,
just like coenzymes rely on
prosthetic groups (prosthetic groups
tightly bound to coenzymes)
find more resources at oneclass.com
find more resources at oneclass.com
5 types of Electron Carriers in the ETC
1. Flavins
2. Iron-Sulfur Centres
3. Ubiquinone
4. Cytochromes
5. Copper centres
1. Flavins prosthetic group
Contain isoalloxazine ring
Flavin rings can pick up 2 electrons
Flavins are tightly bound to proteins (i.e. not free-flowing)
- Ex. FADH2 is a flavin (carries 2 e-s). It contains riboflavin/vitamin B2 (ribose + flavin) attached to adenine dinucleotide.
FADH2 is covalently attached to complex II.
2. Iron-Sulfur Centres prosthetic group
Clusters of iron and sulfur atoms covalently linked (tightly bound) to proteins by cysteine residues
*Iron-sulfur centres are the most numerous electron carriers in the mitochondria*
It’s iportat to get eough iro i our diet!
1 centre accepts 1 electron (reduce Fe3+ to Fe2+)
Fe2S4
Fe4S8 (This is 1 iron-sulfur centre, so it accepts 1 electron)
3. Ubiquinone (Coenzyme Q) coenzyme
Ubiquinone = lipid w/a polyisoprene tail (non-polar and hydrophobic) found w/in the IMM
Ubiquinone is not permanently bound to any protein, it freely diffuses w/in the hydrophobic phase of IMM
Ubiquinone picks up 2 electros i discrete steps thaks to it’s 2 carbonyl groups
oxidized Q ubisemiquinone fully reduced Q
(Q) (Q·) (QH2)
4. Cytochromes proteins
Cytochromes = proteins containing prosthetic heme (heme contains Fe that carries 1 e-)
Tetrapyrrole ring w/a coordinated iron ion at the centre (iron ion can only carry 1 e- at a time; alternates between
Fe3+ and Fe2+)
Cytochroe C is the ol ctochroe that is a peripheral protei of the IMM it’s polar ad hdrophilic. The other
cytochromes are integral membrane proteins (part of larger protein complexes).
Low Electron Affinity (found at the beginning of ETC)
High Electron Affinity (found at the end of ETC)
*general order electron affinity varies based on
environmental conditions.
Cysteine groups on
proteins have S-H groups
that covalently bond via
disulfide bridges to the
sulfurs in the iron-
sulfur centres
+1 e- +1 e-
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Cofactor: protein-bound non-protein chemical (cofactors are not provided by amino acids) Prosthetic group: type of cofactor that is tightly bound to a coenzyme or may interact transiently w/proteins (prosthetic groups are often metals - fe, mg, co) Coenzyme: orga(cid:374)ic cofactor that"s ofte(cid:374) a reactio(cid:374) substrate (ex. vitamins); coenzyme enzyme. Some people rely on prosthetic legs, just like coenzymes rely on prosthetic groups (prosthetic groups tightly bound to coenzymes) There are 4 protein-based complexes in the mitochondrial electron-transport system that work in sequence to deliver electrons to oxygen. Each complex contains an enzyme that catalyzes electron transport and several non-protein groups that accept and release electrons. *nadh carries 2 electrons (occurs at complexes i, ii, and iv) Each transfer results in loss of free energy. Movement of electrons is in a strictly defined order. * to fully reduce o2 to h2o requires 4 electrons (2 nadh) O2 has the greatest electron affinity (final acceptor)
