BIOENERGETICS

BIOENERGETICS

The normal activities of living organisms demand constant input of energy. Even at rest, organisms devote considerable portion of their biochemical apparatus to the acquisition and utilization of energy.

Bioenergetics, also known as biochemical thermodynamics is the study of energy changes accompanying biochemical reactions.

As the biological systems are isothermic in nature, they use chemical energy for the energy requirements of living system.

Basic terms:

Free energy (Δ G): ( Also known as Gibb's free energy)

Free energy is the useful energy in a system.

It is the portion of the total energy change in a system which is available for doing work. It is also known as chemical potential.

Entropy (S):

Entropy is the degree of randomness or disorder of a system. It varies with temperature. 

For example incerease in temperature causes increased disorderness.

Units of S are J.K^-1.

Enthalpy:

Enthalpy is the heat content of a system

standard free energy change (ΔG^0'):

It is the free energy change under standard conditions.

Standard conditions: pH is 7.0 ans reactant concentration is 1.0 mol/L

Formula for standard free energy change is

ΔG⁰′= −RT ln K ′eq

                                                                                      where R is gas constant

                                                                                                 T is absolute temparature

 

Biological systems obey laws of thermodynamics.

First law of thermodynamics: Energy is conserved

System: The part of the Universe chosen for observation

Surroundings: The rest of the universe

The first law of thermodynamics states that the total energy of a system, including its surroundings, remains constant. It can be neither created nor destroyed. It can be converted from one form to another.

In living systems, chemical energy is converted to heat, radiant or mechanical energy.

ΔE= Q-W

                                                                            where Q is heat absorbed by the system

                                                        W is work done

Second law of thermodynamics:

It states that the total entropy of a system must increase if a process is to occur spontaneously.

Under conditions of constant temperature and pressure, the relationship between the free energy change (ΔG⁰) of a reacting system and the change in entropy(ΔS) is expressed in following equation, which combines two laws of thermodynamics.

ΔG= ΔH-TΔS

                                                                                 where ΔH = enthalpy

                                                                                                                    T= absolute temperature

In biological systems, ΔH is equal to ΔE

ΔG= ΔE-TΔS

where ΔE= change in internal energy

From the above equation, one can determine the type of reaction .i.e. whether it is exergonic, endergonic or at equilibium.

• If ΔG is negative, the reaction proceeds spontaneously, and it is exergonic
• if ΔG is positive, the reaction requires free energy to proceed and it is endergonic
• If ΔG is zero, the system is at equilibrium and no net change takes place.

In living systems, endergonic processes are coupled with exergonic processes the vital processes like synthetic reactions, muscular contraction, active tansport etc obtain enegy by chemical linkage or coupling to oxidative reactions.

This type of coupling can be represented as

In this, conversion of metabolite A to B occurs with release of energy. This is coupled to another reaction which requires free energy to convert C to D.

In practice, endergonic process must be a component of coupled exergonic-endergonic system.

Usually, the exergonic reactions are catabolic reactions ( breakdown or degradation of molecules) where as endergonic reactions are synthetic or anabolic reactions. Metabolism constitutes anabolic and catabolic reactions in a well coordinated manner.

Mechanism of coupling:.

Mechanism of coupling of exergonic and endergonic reactions may occur in two ways.

1. By formation of a common obligatory intermediate
2. Synthesis of high energy compound

1.Formation of a common intermediate:

In this mechanism, a common intermediate is formed, which takes part in both reactions. The intermediate should be structurally related to products and reactants.

A+C ---> I -----> B+D

Examples for this kind of coupling are dehydrogenation reactions, which are coupled to hydrogenations by intermediate carrier.

2. Synthesis of high energy compound:

In this mechanism, a compound of high energy potential is formed during exergonic reaction and it is incorporated into endergonic reaction, thus free energy is transferred from exergonic to endergonic pathways.

As the compound of high potential energy need not to be sructurally related to reactants and products, free energy can be transformed from vide range of exergonic reactions to vide range of endergonic reactions.

The high energy intermediates are usually phosphates and sulfur compounds. These compounds when hydrolyzed release energy in a large quantity. The high energy bond is designated by the symbol ~.

High energy phosphates- ATP:

In living cells, the principal high energy intermediate or carrier is adenosine triphosphate. It is also known as universal currency of energy. ATP is a nucleoside triphosphate containing adenine, ribose and three phosphate groups. It is usually complexed with Mg²⁺.

The hydrolysis of ATP releases -30.5KJ/mol or -7.3 KCal/mol of energy.

ATP releases energy by donating a phosphate group, and forming ADP. Likewise, ADP accepts a high energy phosphate group to form ATP. (High energy phosphate is designated as

Interconversion of ATP and ADP is catalyzed by the  enzyme Adenylyl kinase.

ATP is continually hydrolyzed and regenerated. An average person at rest consumes and regenerates ATP at a rate of approximately 3 molecules/sec.

Depending on the energy produced by different phosphates they can be classified as

1. Low energy phosphates They have ΔG⁰′ values are smaller than that of ATP.
2. High energy phosphates The ΔG⁰′ value is higher than that of ATP.

standard free energy of some biologically important phosphates

Other high energy compounds are Coenzyme A (eg. Acetyl CoA), acyl carrier protein, aminoacid esters, S-adenosyl methionine, UDP Glucose, PRPP (5^' phospho ribosyl-1-pyrophosphate).