CYTOCHROMES

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.

3
(No Transcript)
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
• Cu2e- ?? 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
  (Ppositive Nnegative).
• 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.

10
(No Transcript)
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 Hs taken up from matrix side, which
  balances bc1-complex, for which more H released
  to cytoplasmic side.

12
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
(No Transcript)
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
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.

21
(No Transcript)
22
(No Transcript)
23
(No Transcript)
24
(No Transcript)
25
THANQ