1. CYTOCHROMES M.Prasad Naidu MSc Medical Biochemistry,
Ph.D.Research Scholar

2. Cytochromes
Cytochromes are electron carriers containing hemes . Hemes in the 3 classes of cytochrome (a, b, c) differ in substituents on the porphyrin ring. Some cytochromes(b,c1,a,a3) are part of large integral membrane protein complexes. Cytochrome c is a small, water-soluble protein.

4. Heme is a prosthetic group of cytochromes.
Heme contains an iron atom in a porphyrin ring system.
The heme iron can undergo 1 e- transition between ferric and ferrous states: Fe3+ + e-  Fe2+
Copper ions besides two heme A groups (a and a3) act as electron carriers in Cyta,a3
Cu2++e-  Cu+

5. Cytochromes proteins that accept electrons from QH2 or FeS
Ultimately transfers the electrons to oxygen

6. Mitochondrial Complexes
NADH Dehydrogenase
Succinate dehydrogenase
Cytochrome Oxidase
CoQ-cyt c Reductase

7. Mitochondrial respiratory chain:
Complex I:
- Transfers e- from NADH to quinone pool & pumps H+.
Complex II:
- Transfers e- from succinate to quinone pool.
Complex III:
- Transfers e- from quinol to cytochrome c & pumps H+.
Complex IV:
- Accepts e- from cytochrome c, reduces O2 to H2O & pumps H+.
Complex V:
- Harvests H+ gradient & regenerates ATP.

8. Complex III (or bc1-complex)
Catalyses the transfer of e- to cytochrome c.
Pumps protons through redox coupled Q-cycle.
- Coupled ubiquinone/ubiquinol redox reactions occur either side of the membrane.
- Quinone/quinol binding sites labeled QP and QN (P=positive; N=negative).
Selectively diverts e- from QP to either cytochrome c or to QN

9. Q-cycle
Quinol binds at QP and e- transfer to heme c1.
Second e- transfer to bound quinone at QN via hemes bL and bH.
Quinone replaced by quinol at QP.
Above steps repeated.
Heme c1 fully reduced (passes 2e- to cyt. c).
One quinol consumed (ie. two consumed but one regenerated).
4 H+ released to cytoplasm and 2 H+ taken up from matrix.

11. Complex IV (or cytochrome c oxidase)
Catalyses the transfer of e- from cytochrome c to O2.
Energy liberated pumps protons through conformational changes.
- Reduction of oxygen to water one of the most important reactions in biology.
More H+s taken up from matrix side, which balances bc1-complex, for which more H+ released to cytoplasmic side.

12. Heme A and Cu act together to transfer electrons to oxygen
Cytochrome c oxidase
Cu(II)  Cu(I)
e- from cyt c to a
Heme A and Cu act together to
transfer electrons to oxygen

13. Distribution of cofactors
Subunit I: heme a and the binuclear centre (heme a3 & CuB).
- It is the binuclear centre which forms the active site for O2 reduction.
Subunit II: dinuclear centre (CuA which is two Cu atoms).
- Electrons first transferred from cytochrome c to CuA.
- Passed onto the binuclear centre via heme a.
Also a Mg ion present at the interface between subunits.

14. C Y P Xenobiotics Endogenous Substrates Sterols CYP51 CYP7A1
CYP27A1 CYP46
CYP39
Vitamin D
CYP27 A1, B1, C1 CYP24
Steroid(hormone)s
CYP11 A1,B CYP17 CYP19 CYP21A2
CYP7B1
CYP26A1,B1,C1
Arachidonic acid
CYP5A1 CYP8 A1,B1
NO synthase
Endogenous Substrates
CYP3A
(4, 5, 7, 43)
CYP2 A(6,7,13),B6,C(8,9,18,19)D6,E1,F1,J2,R1,S1,U1,W1
CYP1 A1, A2, B1
Xenobiotics
CYP4 A(11,22),B1, F(2,3,8,12,22), V2,X1,Z1
C Y P
Retinoic acid

15. Ethanol Metabolism
Ethanol is oxidized to acetaldehyde through several enzymatic pathways:
Alcohol dehydrogenase
CH3CH2OH + NAD+ → CH3CHO + NADH + H+
Catalase
CH3CH2OH + H2O2 → CH3CHO + 2 H2O
Cytochrome P-450
The “Microsomal Ethanol Oxidizing System” (MEOS)
CH3CH2OH + NADPH + H+ + O2
→ CH3CHO + 2 H2O + NADP+
And through a non-enzymatic free radical pathway:

16. Apoptotic Pathways Effectors and Modulators
There are two major apoptotic pathways in mammalian cells.
The death receptor pathway, exemplified by FasL binding to an extracellular receptor, causes the formation of the DISC that results in the activation of caspase-8.
The mitochondrial pathway is activated by most cellular stresses. A resulting signal or intracellular change causes the release of cytochrome c into the cytosol. Cytochrome c binds to Apaf-1 and procaspase-9 to form the apoptosome and catalyzes the activation of caspase-9.

17. Major Apoptotic Pathways in Mammalian Cells
Mitochondrial Pathway
Death Receptor Pathway
FasL
Caspase 3 D
Fas/Apo1
/CD95
FADD
Procaspase 8
DISC
Caspase 8
BID
oxidants
ceramide
others
Bcl-2
Cytochrome c
dATP
Procaspase 9
Apaf -1
Caspase 9
Procaspase 3
apoptosome
DNA damage
Cellular targets

18. Initiator caspases, such as 8 and 9, activate effector caspases that cleave multiple cellular proteins. Caspases are characterized by an active site cysteine.
Bcl-2 is a proto-oncogene that was first discovered in B-cell lymphoma. Bcl-2 prevents apoptosis by blocking the release of cytochrome c from the mitochondrion by an unknown mechanism. There are many Bcl-2 homologs, some with pro- and others with anti-apoptotic functions. The ratio between these two types helps determine the fate of the cell. Additional information about Bcl-2 family members can be found

19. Cytochrome c and Cellular Redox Environment
Cytochrome c in solution can act as an antioxidant and an ROS scavenging function for cytochrome c in the intermembrane space has been proposed by Skulachev.
Release of cytochrome c into the cytosol from the mitochondrion interrupts the electron transport chain resulting in increased production of superoxide from the mitochondrion.

20. Binding of cytochrome c to form the apoptosome and activate caspase-9 does not appear to depend on the ability of cytochrome c to transfer or accept electrons.
However, the reduction state of cytochrome c may still be important because reduction and oxidation cause conformational changes that may be critical for cytochrome c binding to Apaf-1 and procaspase-9.

25. THAN’Q