BIOENERGETICS IN BIOCHEMISTRY, Lecture notes of Medical Biochemistry

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BIOCHEMISTRY

BIOENERGETICS

by

PROF. (Dr.) ANJU GOYAL

AND

MS. PRINCY AGARWAL

BIOENERGETICS UNIT-I BIOENERGETICS: INTRODUCTION  Bioenergetics isthe field ofbiochemistryconcerned with thetransformation anduse of energy byliving cells.  Bioenergetics is “the study of energy changes in biological reactions”.  Thechemical reactionsoccurring in livingbeings(orbiochemical reactions) are associated with the liberation ofenergy, as the reacting system moves from a higher to alower energy level. Most often, the energy is liberated inthe form of heat.  The goal of bioenergetics is to describe how living organisms acquire and transform energy in order to perform biological work. The study of metabolic pathways is thus essential to bioenergetics.  In a living organism, chemical bonds are broken and made as part of the exchange and transformation of energy.  Energy is available for work (such as mechanical work) or for other processes (such as chemical synthesis and anabolic processes in growth), when weak bonds are broken and stronger bonds are made. The production of stronger bonds allows release of usable energy.  Adenosine triphosphate (ATP) is the main "energy currency" for organisms; the goal of metabolic and catabolic processes are :

• To synthesize ATP from available starting materials (from the environment), and
• To break-down ATP (into adenosine diphosphate (ADP) and inorganic phosphate) by utilizing it in biological processes.  In a cell, the ratio of ATP to ADP concentrations is known as the " energy charge " of the cell.  A cell can use this energy charge to relay information about cellular needs;
• If there is more ATP than ADP available, the cell can use ATP to do work, but
• If there is more ADP than ATP available, the cell must synthesize ATP via oxidative phosphorylation.  The chemical reactions performed by an organism make up its metabolism.
• Catabolic reactions involve the breakdown of chemical molecules, while
• Anabolic reactions involve the synthesis of compounds. CONCEPT OF FREE ENERGY:  Every living cell and organism must perform work to stay alive, to grow and to reproduce.  The energy processes in living organisms are defined by the basic laws of thermodynamics.  The energy actually available to do work (utilizable) is known as free energy. Changes in the free energy (ΔG) are valuable in predicting the feasibility of chemical reactions. G) are valuable in predicting the feasibility of chemical reactions. The reactions

can occur spontaneously if they are accompanied by decrease in free energy.

BIOCHEMISTRY LECTURE NOTES—

BIOENERGETICS UNIT-I  The conditions of biological systems are constant temperature and pressure. Under such conditions the relationships between the change in free energy, enthalpy and entropy can be described by the expression where T is the temperature of the system in Kelvin. ∆G = ∆H − T∆S [∆G = Gibbs Free Energy; ∆H = Change in Enthalpy; T = Temperature in K; ∆S = Change in Entropy] T represents the absolute temperature in Kelvin (K=273+ºC).  The term standard free energy represented by ∆G ºis often used. It indicates the free energy change when the reactants or products are at a concentration of 1 mol/l at pH 7.0.  At a constant temperature and pressure, ∆G is dependent on the actual concentration of reactants and products. For the conversion of reactant A to product B (A →B), the following mathematical relation can be derived ∆ G = ∆ G ° + RT ln [ B ] [ A ] Where, ∆ G ° = Standard free energy change R = Gas constant (1.987 Cal/mol) T = Absolutetemperature (273 + ºC) ln = Natural logarithm [B] = Concentration of product [A] = Concentration of reactant.  ∆G° is related toequilibrium constant (Keq): When a reaction A↔B is at equilibrium(eq), the free energy change is zero. The aboveequation may be written as ∆ G = ∆ G ° + RT ln

[ B ] eq.

[ A ] eq.

Since, ∆^ G^ = 0 Hence,∆G° = - RT InKeq. REDOX POTENTIAL  It is also known as Oxidation Reduction Potential.  It quantitatively expresses the free energy changes of a redox system which is proportionate to the tendency of reactants to donate or accept electrons.  Increase in the negative value of the system indicates increased tendency to lose electrons.  Increase in the positive value of the system indicates increased tendency to accept electrons.  The redox potential of a system may be calculated from the following equation. BIOCHEMISTRY LECTURE NOTES—

BIOENERGETICS UNIT-I E = E 0 +

n log Conc. of Reducing agent Conc. of Oxidising agent  Since electrons have a tendency to move in such a direction that free energy of the reacting system decreases, therefore they tend to move from electronegative system towards electropositive system.  In Bioenergetics Redox Potential is the ratio of NAD+^ to NADH+^ + H+.  It describes the availability of NAD+^ for metabolism. TYPES OF BIOENERGETICS REACTIONS

1. Exergonic Reaction

• Exergonic implies the release of energy from a spontaneous chemical reaction without any concomitant utilization of energy.
• The reactions are significant in terms of biology as these reactions have an ability to perform work and include most of the catabolic reactions in cellular respiration.
• Most of these reactions involve the breaking of bonds during the formation of reaction intermediates as is evidently observed during respiratory pathways. The bonds that are created during the formation of metabolites are stronger than the cleaved bonds of the substrate.
• The release of free energy, G, in an exergonic reaction (at const. pressure and temperature) is denoted as ΔG = GG = Gproducts – Greactants< 0 [i.e. ΔG = GG= negative] - Exergonic reactions include exothermic reactions. - In exergonic reactions, energy is released to the surrounding. Therefore, the products have a lower energy than that of the reactants. - Due to that reason, the change in enthalpy is a negative value for exergonic reactions. [i.e. ΔG = GH = negative] - The entropy is increased due to the disorder of the system. [i.e. ΔG = GS= positive] - According to the above relationship, the Gibbs free energy is a negative value. [i.e. ΔG = GG = **negative]

2. Endergonic Reactions** BIOCHEMISTRY LECTURE NOTES—

BIOENERGETICS UNIT-I BIOCHEMISTRY LECTURE NOTES—

BIOENERGETICS UNIT-I HIGH ENERGY COMPOUNDS:  Certain compounds are encountered in the biological system which, on hydrolysis, yields energy.  The term high-energy compounds orenergy rich compounds are usually applied tosubstances which possess sufficient freeenergy to liberate at least 7.0 kcal/mol at pH 7.0.  Certain other compounds which liberate less than 7.0kcal/mol (lower than ATPhydrolysis to ADP + Pi) are referred to as lowenergycompounds.  All the high energy compounds when hydrolysed liberate more energy than that of ATP. Most of high energy compounds contain phosphate group (exception acetyl CoA) hence they are also called high energy phosphates. Classification of high energy compounds There are at least 5 groups of high energy compounds (table-1): Table-1 High Energy Compounds Class Bond Example (s) Pyrophosphates – C – P – P ATP, pyrophosphate Acyl phosphates O ║

• C – O ~ P 1,3- Bisphosphoglycerate, Carbamoyl phosphate, Acetyl phosphate. Enol phosphates – CH ║
• C – O ~ P Phosphoenol pyruvate Thiol esters (thioesters) C ║
• C – O ~ S – Acetyl CoA, Acyl CoA Guanido phosphates (phosphagens)

– N~ P

Phosphocreatine, Phosphoarginine  High–energy bonds: The high energy compounds possess Acid anhydride bonds(mostly phosphor-anhydride bonds) which are formed by the condensation of two acidic groups or related compounds. These bonds are referred to as high energy bonds, since the free energy is liberated when these bonds are hydrolysed. Lipmann suggested use of the symbol ~ to represent high energy bond. For instance, ATP is written as AMP ~ P ~ P. ADENOSINE TRIPHOSPHATE (ATP)  Adenosine-5'-triphosphate (ATP) is a multifunctional nucleotide used in cells as a coenzyme. BIOCHEMISTRY LECTURE NOTES—

BIOENERGETICS UNIT-I  These values can be used to calculate the change in energy under physiological conditions and the cellular ATP/ADP ratio (also known as the Energy Charge). CYCLIC ADENOSINE MONOPHOSPHATE (cAMP, cyclic AMP or 3'-5'-cyclic adenosine monophosphate)  It is a second messenger important in many biological processes.  cAMP is derived from adenosine triphosphate (ATP) and used for intracellular signal transduction in many different organisms, conveying the cAMP-dependent pathway.  cAMP is synthesised from ATP by adenylyl cyclase located on the inner side of the plasma membrane.  Adenylyl cyclase is activated by a range of signaling molecules through the activation of adenylyl cyclase stimulatory G (Gs)-protein-coupled receptors and inhibited by agonists of adenylyl cyclase inhibitory G (Gi)-protein-coupled receptors.  Liver adenylyl cyclase responds more strongly to glucagon , and muscle adenylyl cyclase responds more strongly to adrenaline.  cAMP decomposition into AMP is catalyzed by the enzyme phosphodiesterase.  Function:

1. cAMP is a second messenger, used for intracellular signal transduction, such as transferring the effects of hormones like glucagon and adrenaline, which cannot pass through the cell membrane.
2. It is involved in the activation of protein kinases and regulates the effects of adrenaline and glucagon.
3. It also regulates the passage of Ca2+^ through ion channels.
4. cAMP and its associated kinases function in several biochemical processes, including the regulation of glycogen, sugar, and lipid metabolism by activating protein kinase. BIOCHEMISTRY LECTURE NOTES—