BIOENERGETICS
FREE ENERGY
It is the portion of the total energy change in a
system that is available for doing work at
constant temperature and pressure; it is
represented as ΔG.
z Reactions involving free energy:
1. Exergonic
2. Endergonic
z
EXERGONIC REACTIONS
z
z
z
z
Reactions in which the
free energy of the final
state is less than the free
energy of the initial state.
This represents energy
that can be used to do
biological work
Reaction is spontaneous
or favorable.
ΔG is - or <0
ENDERGONIC REACTIONS
z
z
z
z
Reactions in which the
free energy of the initial
state is less than the free
energy of the final state.
ΔG is + or >0
Reaction is
nonspontaneous or
unfavorable
Considerable amount of
energy must be imparted
to the system
COUPLED REACTIONS: 2
TYPES
1.
Coupling involves a common obligatory intermediate (I)
A+CÆIÆB+D
2. Synthesizing a compound of high-energy potential in the
exergonic reaction and incorporate this new compound
into endergonic reaction.
AH2
A
Carrier
Carrier-H2
BH2
B
™The overall free energy change for the reaction is negative
(ΔG < 0)
HIGH ENERGY PHOSPHATES
z
High energy phosphates play central role in
energy capture & transfer.
E
X
E
R
G
O
N
I
C
SYNTHESIS
1
2
E
MUSCULAR CONTRACTION
NERVOUS EXCITATION
3
4
ACTIVE TRANSPORT
HIGH ENERGY COMPOUNDS
Compound
Phosphoenolpyruvate
1,3 bisphosphoglycerate
Phosphocreatine
ATP(ÆADP+Pi)
AMP(Adenosine+Pi)
PPi(Æ2Pi)
Glucose-1-phosphate
Fructose-6-phosphate
Glucose-6-phosphate
ΔG (Kcal/mol)
-14.8
-11.8
-10.3
-7.3
-3.4
-4.0
-5.0
-3.8
-3.3
ENERGY CURRENCY OF CELL
z
z
High energy phosphates act as energy currency of
cell.
3 major sources of high energy phosphates taking
part in energy conservation or energy capture.
1. Oxidative phosphorylation: Free energy to drive
this process comes from Respiratory chain
oxidation using molecular O2 in mitochondria.
2. Glycolysis:
3. TCA Cycle:
ROLE OF ATP/ADP CYCLE IN
TRANSFER OF HIGH ENERGY
PHOSPHATES
1,3 BPG
PEP
SUCCINYL Co-A
OXIDATIVE PHOSPHORYLATION
CREATINE- P
P
store of
P
CREATINE
ATP
P
ADP
G6P
Glycerol3P
Other
Phosphorylations
Gl 1,6 BP
BIOLOGICAL OXIDATION
BIOMEDICAL IMPORTANCE
z
Respiration
z
Xenobiotics (metabolism by Cytochrome P450
system).
z
Hyperbaric oxygen therapy in patients with
respiratory or circular failure.
z
May result oxygen toxicity.
ENZYMES INVOLVED IN
OXIDATION & REDUCTION:
OXIDOREDUCTASES
z
z
z
z
Oxidases
Dehydrogenases
Hydroperoxidases
Oxygenases
OXIDASES
z Catalyse the removal of hydrogen from
a subtrate using oxygen as a hydrogen
acceptor.
AH2
A
AH2
A
1/2 O2
e.g.
1. Cyt. Oxidase
H2O 2. L-AA oxidase
O2
H2O2
3. Xanthine
oxidase
4. Glucose
oxidase
DEHYDROGENASES
zTransfer of hydrogen from one substrate to another in a
coupled oxidation - reduction reaction. Can’t use O2 as H2
acceptor.
AH2
A
Carrier
BH2
Carrier -H2
B
Depend on:
1. Nicotinamide coenzymes
2. Flavin coenzymes
3. Cytochromes
HYDROPEROXIDASES
z
Use H2O2 or organic peroxides as substrates.
z
2 types:
1.
Peroxidases
2.
Catalases
H2O2+ AH2 Peroxidases 2H2O + A
2GSH
+
2H2O2
Glutathione peroxidase
H2O2
2H2O+ GSSG
Catalases
2H2O
+ O2
OXYGENASES
z Catalyze direct incorporation of oxygen into a
substrate. Takes place in 2 steps:
1. O2 binding to the enzyme at active site, &
2. The reaction in which bound O2 is
reduced/transferred to substrate.
z 2 subgroup of oxygenases:
i.
Dioxygenases
ii. Monooxygenases
zDioxygenases: Incorporate both atoms of
molecular oxygen into the substrate.
A+O2→AO2
ze.g.
i. Homogentisate oxidase
ii. L-tryptophan dioxygenase
iii. 3-hydroxyanthranilate dioxygenase
z Monooxygenases: Incorporate only one atom of
molecular oxygen into the substrate.
z May be Microsomal or Mitochondrial.
A-H + O2 + ZH2 →A-OH + H2O + Z
DRUG-H + O2 + 2Fe2+ + 2H+ → DRUG-OH + H2O + 2Fe3+
Hydroxylase
RESPIRATORY
CHAIN & OXIDATIVE
PHOSPHORYLATION
z
z
Respiratory chain oxidizes reducing equivalents and acts as a
proton pump.
Oxidative phosphorylation is the process by which liberated
free energy is trapped as high-energy phosphate.
ELECTRON TRANSPORT
CHAIN
z 4 sequential complexes found in the inner side of inner
mitochondrial membrane.
z They accept e- from e- donors such as NADH or succinate,
shuttle these e- across the membrane creating an electrical
& chemical gradient (+1.1V).
z Through the proton driven chemistry of the ATP synthase,
generate ATP.
COMPLEXES OF ETC
Complex II- Succinate Coenzyme Q reductase
Complex I NADH
dehydrogenase/
NADH
Coenzyme Q
reductase.
Fp
Q
Complex III Coenzyme Q cytochrome c
oxidoreductase
½ O2 + H+
H2O
Cyt-C
Complex IV Cytochrome c
oxidase.
ETC COMPONENTS
Complex
Components
Prosthetic group
I
NADH-Q oxidoreductase
FMN
Fe-S
II
Succinate Q reductase
FAD
Fe-S
III
IV
Q-Cytochrome C oxidoreductase
Cytochrome c oxidase
ATP synthase
Heme bH
Heme bL
Heme C1
Fe-s
Heme a
Heme a3
CuA & CuB
Oxidative phosphorylation- Two phases
H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
2. Using the
gradient's
energy to make
ATP
H+ ATP
H+
H+
H+
H+
H+
H+
1. Generation of
the proton
gradient.
H+
H+
TRANSPORT OF REDUCING
EQUIVALENTS THROUGH ETC
AH2
NAD+
FpH2
2Fe3+
A
NADH
Fp
2Fe2+
H+
H+
2H+
H2O
½ O2
2H+
Succinate
Choline
Proline
3- Hydroxyacyl - CoA
3- Hydroxybutyrate
Glutamate
Malate
Isocitrate
Pyruvate
Lipoate
α - Ketoglutarate
Fp
[FAD]
NAD
Glycerol 3 phosphate
Fp
[FAD]
FeS
I
Fp
[FMN]
FeS
Fp
[FAD]
FeS
Acyl - CoA
Sarcosine
Dimethylglycine
Q
FeS
ETF
[FAD]
Fp
[FAD]
II
III
Q
Cyt b Cyt c1
FeS
IV
Cyt c
Cyt aa3
Cu
O2
Q CYCLE
CYTOSOL
(OUTSIDE)
MATRIX
(INSIDE)
INNER MITOCHONDRIAL MEMBRANE
H+
e-
QH2
H+
QHy
e-
b566
b562
QHy
eH+
C1
H+
Q
ATP Synthase
‘a’ subunit binds
to outside of ring
Exterior column
has 1’ a’ subunit
2’ b’ subunits, &
the δ subunit
Moving unit (rotor) is c ring & γε
Remainder is stationary (stator)
F1 subunit has 5 types of
polypeptide chains
(α3, β3, γ, δ, ε)
F0 contains the proton channel
ring of 10-14 c subunits
Subunit a
Proton enters
Proton exits
The Binding Change Mechanism (Paul Boyer)
P:O RATIO
‰
When substrates oxidized by NADdehydrogenase, 3 mol ATP is produced per ½
mol of O2 consumed. P:O= 3.
‰
When substrates oxidized by AFDdehydrogenase, 2 mol ATP is produced per ½
mol of O2 consumed. P:O= 2.
INHIBITORS OF ETC
Complex II
Malonate
Succinate
N
A
D
FAD
FeS
Carboxin
TTFA
CN, CO
Azide, H2S
BAL
Antimycin A
Complex IV
Complex I
Q
FMN,FeS
Complex III
Cyt b, FeS, Cyt c1
Cyt c
H
Piericidine
Amobarbital
Rotenone
O
Cyt a Cyt a3
Cu
Cu
H2O
Uncouplers
Oligomycin
ADP + Pi
ATP
ADP + Pi
ATP
ADP + Pi
ATP
CHEMIOSMOTIC THEORY (Mitchell)
H+
OLIGOMYCIN
Inner
Mitochondrial
membrane
ATP
synthase
ADP+Pi
H+
Uncouplers
_
NADH
+H+
I
ATP
NAD+
Proton
H+ translocation
1/2O2
H2O
H+
Q
III
C
IV
H+
+
H+
H+
Transport of Reducing Equivalents
SHUTTLE PATHWAYS
SHUTTLE PATHWAYS
zTwo pathways:
1. Glycerol Phosphate Shuttle - Muscle & Brain
2. Malate-Aspartate Shuttle - Liver, kidney &
heart
zThey transport the reducing equivalents from
cytosol to mitochondria and not vice versa.
Malate aspartate shuttle
Cytosol
NAD+
Mitochondria
Malate
Oxaloacetate
1
Malate dehydrogenase
α - KG
α - KG
Oxaloacetate
NADH
+ H+
Transaminase
Transaminase
Glutamate
NAD+
Malate
Malate dehydrogenase
NADH
+ H+
Liver, kidney & heart
Asp
Asp
2
H+
H+
Glutamate
Glycerophosphate shuttle
Muscle & brain
Cytosol
NAD+
NADH
+
H+
Glycerol 3
phosphate
Glycerol 3 PO4
dehydrogenase
Dihydroxy acetone
phosphate
Mitochondria
Glycerol 3
phosphate
Glycerol 3 PO4
dehydrogenase
Dihydroxy acetone
phosphate
FAD
FADH2
Resp.
chain
TRANSPORTER SYSTEMS
N-Ethylmaleimide
H+
H2PO4
Malate-2
OUTSIDE
ATP4-
α-KG-2
Citrate-3+H+
Atractiloside
1
2
3
4
5
6
INSIDE
OH-
Pyruvate
HPO4-2
Malate-2
Malate-2
ADP3-
1. Phosphate 2. Pyruvate 3. Dicarboxylate 4. Tricarboxylate 5. α-KG 6. Adenine nucleotide