Bioenergetics and Biochemical Reaction Types
bioenergetics
is the study of the balance between energy intake in the form of food and energy utilization by organisms
for life-sustaining processes- tissue synthesis, osmoregulation, digestion, respiration, reproduction, locomotion, etc.
Photosynthetic autotrophs
Heterotrophs
Autotrophs
Use CO2 as sole carbon source
Photoautotrophs: Energy from sunlight (photosynthesis)
Chemoautotrophs: Energy from oxidation of inorganic compounds
e.g.,Fe ++-----> Fe+++
Heterotrophs: Use combined forms of carbon (sugars) for energy
Energy
The
measurement of energy requires converting it from one form to another
The
basic unit of energy is the calorie
international
unit: the joule - 1.0 joule = 0.239 calories or 1 calorie = 4.184 joule
Energy content of a substance
Gross
energy represents the energy present in dry matter (DM)-
Laws of Thermodynamics
1.
Conservation of energy. Energy may change form or be transported but cannot be created or
destroyed.
2.
Entropy. In all natural processes, the entropy of the universe increases.
Animals
are not engines and can’t use heat to perform work
Cells
are isothermal (Const temp and pressure)
Cells
obtain their energy from chemical bonds,
Free
energy G- amt of energy capable of work at const temp and pressure
Enthalpy
Entropy
H-Heat content of the system.
S- expression of randomness in the system-
DG=DH-TDS

DG’o = -RTlnK’eq (Std free energy change of a reaction) (pH7, 25C, 1M reactant/product, 1Atm)
Std versus actual
Std free energy change DG’o tells us direction of a reaction and how far it will go to
reach equilibrium WHEN Initial conc of reactant and product is 1M, pH is 7, temp is 25C,
pressure is 1Atm.
DG’o is a constant for that reaction
ACTUAL CHANGE
A+B <-----> C+D
DG = DG’o + RTln
[C][D]
[A][B]
A---->B DG’o = +13.8 kJ/mol
B---->C DG’o =-30.5 kJ/mol
A---->C
Sum= -16.7 kJ/mol (reaction is spontaneous because the two are coupled)
Keq and DGo
ATP+H2O---->ADP+ Pi
ATP+H2O---->AMP+PPi
PPi+H2O---->2Pi
-7.3Kcal/mol
-10.9Kcal/mol
-4.6Kcal/mol
Review
Energy transduction in cells are via chemical reactions- bond formation/breakage
Covalent bonds share electrons
Homolytic cleavage- each atom leaves the bond with on electron
Heterolytic cleavage-one atom retains both electrons
Nucleophiles-groups rich in and capable of donating electron (attracted to nucleus)
Electrophile- group deficient in electron (attracted to electron)
Non-bonded electrons (dots) are moved
in direction of arrows
Carbonyl
Chemical reaction that occur during metabolism
Carbonyl bonds play a key role in C-C bond formation and breakage
Rearrangements in electrons
Grp transfer- transfer of acyl/phosphoryl from one nucleophile to another
Biological oxidation (loss of electron)-Oxidation releases energy.
Every oxidation is accompanied by a reduction (electron acceptor acquires electrons
removed by oxidation).
Metabolism= Catabolism and Anabolism
Converge and Diverge
Catabolism
• Generate ATP
• Generate building blocks for
biosynthesis
Anabolism
• Utilize energy
• Generate biomolecules
Different enzymes mediate catabolic and anabolic pathways.
Catabolic and anabolic pathways employ different enzymes which are regulated
separately
Some key steps in each pathway are unidirectional
Themes
Allosteric regulation- metabolic intermediate (ATP)
Synthesis/degradation of enzyme Control enzyme levels
Covalent Modification of enzyme- Phosphorylation (integrated via growth
factors/hormones)
Compartmentalization
•One way to allow reciprocal regulation of catabolic and anabolic processes
•Cytosol Vs mitochondria
Specialization of organs
• Regulation in higher eukaryotes
• Organs have different metabolic roles
Liver = gluconeogenesis (glucose)
Muscle = glycolysis
Availability of substrate-(intracellular conc of substrate is often below Km of
enzyme- rate is proportional to substrate conc)
Glucose
Amino acids
(Liver)
Glucose
Gluconeogenesis
(every cell)
Glycolysis
(every cell)
Glycogen
(liver muscle)
Hexose shunt
(every cell)
NADPH
Pyruvate
AcetylCoA
Fatty acids
(liver, adipose)
ATP ATP ATP
Heterotrophic cells obtain free energy from catabolism of nutrients forming ATP
Hydrolysis of ATP has a high negative DG- -30.5kJ/mol. This means that ATP has a strong
tendency to transfer terminal phosphate to water.
ATP hydrolysis in water only produces heat
In cells ATP hydrolysis involves covalent participation of ATP. ATP provides energy by grp
transfer (Substrate Level Phosphorylation).
ATP hydrolysis is exogermic (negative DG). This is coupled with endogermic (positive DG)
reactions in cells allowing these reactions to proceed.
Some processes do involve direct ATP hydrolysis providing energy that changes protein
conformation producing mechanical motion
High energy bond
Why is ATP PO4 bond high energy bond? It is not breaking of bond - it is
difference in free energy between reactant and product.
ATP + H2O <-------> ADP + Pi
DGo’ = -30.5 kJ/mol
Why does ATP have strong tendency to transfer its terminal phosphate?
Electrostatic repulsion
Resonance stabilizationHydration-
Oxidation and Reduction
Oxidation - loss of electrons
Reduction - gain of electrons
Electrons can be transferred from one mol to another
directly as an electron (one electron)
as hydrogen atoms (one proton + one electron)
as hydride ion (:H-) (two electron) (NAD)
direct combination with oxygen
In aerobic organisms
Oxidation of carbon (loss of electrons from carbon) is used to generate ATP
The final acceptor of electrons is oxygen producing CO2
Reduced
High energy
Oxidized
Low energy
Fatty acids are more energy rich
compared to glucose because
carbon in fatty acids is more
reduced
Oxidation
Biological Oxidation Involves loss of electrons from carbon
In cells Carbon (or another atom like nitrogen) exists in a range of oxidation states
because:
Carbon shares electrons with another atom (Oxygen, nitrogen, sulphur, hydrogen)
The more electronegative atom “OWNS” the bonded electrons it shares
C
C
C
O
N
C
C
H
In the C-O bond, the C has partially lost the electron and has undergone oxidation
In the C-N bond, the C has partially lost the electron and has also undergone oxidation even
when no oxygen is involved!
Electron is not Fully transferred
Conjugate redox pairs
AH2 <--------> A + 2e- + 2H+ (redox pair)
B+2e- + 2H+ <-----> BH2 (redox pair)
Two conjugate redox pairs together in soln- electron transfer from electron
donor of one pair to electron acceptor of another pair
AH2 + B <-----> A + BH2
AH2 + NAD+ -----> A + NADH + H+
Electron donating mol is called reducing agent
Electron accepting mol is called oxidising agent
(In a buffer you have proton donor <---------> proton acceptor+ H+)
Overview
Cytosol
Mitochondria
H2O
Glucose
Glycolysis
AcetylCoA
Krebs cycle
O2
CO2
ATP
Electron
Transport
chain
NADH
FADH
ATP
Glycolysis and Anaerobic respiration
4ADP
2NAD
2ATP
Lactate
Muscle
Glucose
2 Pyruvate
Krebs
2ADP
2NADH
4ATP
Yeast
CO2
Acetaldehyde
Ethanol
Krebs cycle
3NAD
3NADH
ADP
Pyruvate
Acetyl
CoA
3 CO2
ATP
FADH2
FAD
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