Bioenergetics and Biological Oxidation

This lecture explains the basics of bioenergetics and biological oxidation along with related aspects.

Outline of Lecture

Bioenergetics and Biologial Oxidation

Bioenergetics

High energy phosphate compounds

Role of ATP

Free energy → chemical energy

Biological oxidation

Individual components of respiratory chain

Respiratory chain and

Oxidative phosphorylation

Inhibitors of respiration

Enzymes in outer mitochondrial membrane

Monoamine oxidase

AcylCoA synthetase

Glycerophosphate  acyltransferase

Monoacyl glycerophosphate acyl transferase

Phospholipase – A2

Enzymes in intermembrane space

Adenylate kinase

Creatine kinase

Enzymes in inner mitochondrial membrane

Soluble enzyme of the TCA

Enzymes of B oxidation

Succinate dehydrogenase

3 – hydroxy butyrate dehydrogenase

Glycerol 3 – phosphate dehydrogenase

BIOENERGETICS
The Role of ATP

Energy changes occurring in biochemical reactions are known as bioenergetics

First law of thermodynamics

Total energy of the system or universe remains constant

Enthalpy: It is a measure of HEAT Contents of Reactants and Products

Entropy : It is a measure of randomness or disorder of reactants and products for example: ICE : has high degree of order than water Entropy of ice is less than water

(Energy is either transferred from one part to other or transformed to other form like chemical energy to heat energy)

Second law of thermodynamics

Physical and chemical processes proceed in such a way direction that the total entropy of the universe must increases to the maximum and then equilibrium is established

Free energy (r G) (Chemical potential)

It is that part of the total energy of the reaction components available to do work at constant pressure and temperatures

r G =  H- T  S or  G=  E – T  S

r G = Change in fee energy

r H  = Change in entropy

r T = Absolute temperature in degree Kelvin (K)

K = C + 273

r S = Change in entropy

r H =  E (total Change in Internal energy of reaction)

 

At equilibrium

Entropy is maximum

Free energy is minimum

r G -ve i.e. (-) these will be net loss of energy and reaction will proceed spontaneously. (exergonic reaction)

r G + ve i.e. (=) – there will be not gain of energy. Reaction needs input of energy to proceed (endergonic reaction)

r G = 0 , the reaction is at equilibrium & no net change takes place

r G⁰ = Standard free energy change at PH-7 : reactants are at 1 mole / litre

 

THIOL ESTERS (involving Coenzyme A)

Acyl carrier protein – ACP

Amino acid esters (protein synthesis)

S. adinosylmethionine (active methionine)

UDPGLc (Uridine diphosphate glucose)

PRPP – ( 5 – phosphoribosyl -1- pyrophosphate)

 

High energy phosphates are designated by symbol ~(P)

High energy phosphate act as the energy currency (ATP) energy currency of the cell

The ATP is continuously consumed and regenerated by following three sources

Oxidative phosphorylation (enough)

Glycolysis : 2 ATP mole (anaerobic)

Substrate level (TCA) 1 ATP mole

Citric acid cycle

 

Other nucleoside triphosphates participate

In the transfer of high energy phosphate

UTP uridine triphosphate

GTP guanidine triphosphate

CTP cytidine triphosphate

ATP + UDP            Nuclioside                   ADP + UTP

ATP + GDP            diphosphate                 ADP + GTP

ATP + CDP            Kinase             ADP + CTP

All these triplhosphates are used for phosphorylation in cell

Similarly nucleoside monophosphate kinases catalyze the formation of nucleoside diphosphate

ATP + Nucleoside – P Specific nucleoside

Redox Potential

Tendency of reactants to donate or accept electron

Enzymes involved in oxidation and reduction are oxidoreductases

Oxidases

Cytochrome oxidase (a a3)

Flavoproteins (FMN & FAD)

L-amino acid oxidase – FMN linked

Xanthene oxidase – Molybdenum

Aldehyde dehydrogenase – FAD e.g. glucose oxidase

Dehydorgenases (can not use O² as H-acceptor)

Nicotinamide Co-enzyme dehydrogenases

NADH + → Oxidative pathways e.g. glycolysis, TCA Cycle & Resp Chain

NADPH + → reductive synthesis – FAs, Steroid  synthesis, HMP

NAD+ &  Zn – alcohol dehydrogenase

Glyceraldehyde – 3P- dehydrogenase

Riboflavin Co enzyme dehydrogenases

Respiratory chain

Lipoate dehydrogenase

Redox Reactions

Conjugate redox pair:

In any redox half reaction one species will function as an e^– donor and the other as an e^– acceptor.  Example:  Fe³⁺ (acceptor) and Fe²⁺ (donor)

Reducing Equivalent:

used to designate a single electron equivalent taking part in a reaction, in biological systems we usually observe 2 equivalents in enzyme catalyzed reactions, so we usually refer to a unit of biological oxidation as two reducing equivalents

Electron Carriers in the cell

NAD – Nicotinamide adenine dinucleotide

NAD⁺ + 2e^– + 2H⁺  →  NADH + H⁺

Addition of :H^–

 

NADP – Nicotinamide adenine dinucleotide phosphate

NADP⁺ + 2e^– + 2H⁺ → NADPH + H⁺

Addition of :H^–

 

Electrons From Cytosolic NADH Enter Mitochondria Via Shuttles

The Glycerol Phosphate Shuttle

The Malate-Aspartate Shuttle

Flavoproteins

Enzymes that catalyze oxidation-reduction reactions using as coenzymes either:

FMN – Flavin mononucleotide, or

FAD – Flavin adenine dinucleotide

 

Both undergo reduction by accepting one or two hydrogen atoms

This allows the Flavin coenzymes greater flexibility and they can participate in a greater variety of reactions than their NAD(P) counterparts

B.   complex Ⅱ    ( succinic-Q reductase )

D.   complexⅣ
(CytC oxidase )

C. The mechanism  of Oxidative phosphorylation

inhibitors of oxidative phosphorylation
B.   Succinate  oxidation
respiratory chain

Redox Reactions, we will consider gain or loss of electrons, 4 ways that can happen

Directly as electrons

Fe²⁺ + Cu²⁺  ↔ Fe³⁺ + Cu⁺

As a hydrogen atom

AH₂ ↔ A + 2H → A + 2e^– + 2H⁺

As a hydride ion (:H^–)

as in NAD-linked dehydrogenases

Through combination with oxygen

R-C=O   +  Cu²⁺ ↔  R-C=O  +Cu₂O

|                                |

H                             OH

FMN and FAD

The Flavin coenzymes are more tightly bound than their NAD(P) counterparts

They tend to remain bound to a particular enzyme, transferring their reducing capabilities as part of that enzyme’s activity

 

Both undergo changes in their absorption spectrum upon reduction.

The changes can detect single equivalent reduction and double equivalent reduction

Phosphagens

Storage forms of high energy phosphates

Creatine phosphate : muscles and brain

Maintain ATP conc, during it utilization

Build up itself when ATP / ADP ratio is high and act as store house of ~P

Act as Creatine phosphate shuttle ~P  from mitochondria to sarcolemma. Acts as immediate buffer against effects of Myocardial infarction

Arginine phosphates (in invertebrate muscle)

 

ATP allows coupling of thermodynamically unfavorable reactions to favourable ones

Glycolysis first reaction

GLc + Pi →  GLc – 6- P + H₂O (r G⁰ = + 13.8 KJ mol)

ATP → ADP + Pi ((r G⁰ = – 30.5 KJ mol)

Glucose + ATP + Pi →  GLc – 6P + ADP (r G^0’ = – 16.7 KJ mol)