Bioenergetics and exergonic and endergonic reaction in biochemistry

Content

• Introduction
• Concept of Free Energy
• Principle of Bioenergetics
• Important State functions for study of Biochemical Reaction
• Relationship between change in Free energy, Enthalpy and Entropy
• Redox Potential
• Types of Bioenergetic Reactions
• High Energy Compounds
• Classification of High Energy Compounds
• Adenosine Triphosphate (ATP) - Structure and Functions
• Conclusion
• Reference 

Introduction

•  Bioenergetics means study of the transformation of energy in living organisms.
• 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 is 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.
•  Living organisms produce ATP from energy sources via oxidative phosphorylation. The terminal phosphate bonds of ATP are relatively weak compared with the stronger bonds formed when ATP is hydrolyzed (broken down by water) to adenosine diphosphate and inorganic phosphate. Here it is the thermodynamically favorable free energy of hydrolysis that results in energy release; the phosphoanhydride bond between the terminal phosphate group and the rest of the ATP molecule does not itself contain this energy.
• The term "Bioenergetics" is made up of two words: Bio means Life or Living Energetics means study of energy.
• So, basically Bioenergetics is "the study of energy changes in biological reactions".
• Bioenergetics is the field of biochemistry concerned with the transformation and use of energy by living cells.
• The goal of bioenergetics is to describe how living organisms acquire, transform and utilize energy in order to perform biological work. The study of metabolic pathways is thus essential to bioenergetics.
• The chemical reactions performed by an organism make up its metabolism.
• Catabolic reactions involve the breakdown of chemical molecules.
• Anabolic reactions involve the synthesis of compounds.

• 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,

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.
• The reactions can occur spontaneously if they are accompanied by decrease in free energy.
•  During a chemical reaction, heat may be released or absorbed.
•  Cells require a source of free energy.

Cells are isothermal systems, meaning they function at a constant temperature & pressure.

• Photosynthetic cells acquire free energy from absorbed solar radiation.
• Heterotrophic cells acquire free energy from nutrient molecules.
• Cells transform this free energy into ATP & other energy-rich compounds to provide energy for biological work.

PRINCIPLE OF BIOENERGETIC

From the word itself, it seems like it is related to energy. It obeys the laws of thermodynamics. The first law of thermodynamics and the second law of thermodynamics.

The first law of Thermodynamics:

In simple words, it is the law of conservation of energy. It states that the total energy of any system is always constant, it can be changed from one form to another but can never be created nor destroyed.

• ΔU=Q – W
• Where,
• ΔU = Change in the Energy
•  Q = Heat Added
•  W = Work done by the system

For example, when we do some work with our hands, our muscles are activated, and energy utilization and transformations occur there. The chemical energy in muscles gets transformed into mechanical energy. And how does that chemical energy in muscles come/generate Simple, it is provided by the food we eat, which gets converted in the form of energy stored in our body and then utilized by cells. This way, bioenergetics follow the first law of thermodynamics.

The second law of Thermodynamics:

The second rule of thermodynamics states that energy flows from a higher to a lower energy state, or heat travels from a warmer to a cooler body, not the other way around. The entropy of the system always increases or remains the same.

For example, consider a person who does a workout in the gym. When his/her cells work, it will generate lots and lots of heat, and those heat will dissipate into surroundings. Thus, increasing the entropy (here entropy can be said as the measure of the energy that is not available to allow cells to work) and there will come a time when there is no energy left in the system to allow cells to work until more energy is generated. This is the relation of the second law of thermodynamics with bioenergetics.

The first and second laws state the conservation and transfer of energy from one form to another, respectively. And the concepts of both the laws are used in an expression as

•  ΔH= ΔG + TΔS
• Rearranging as
• ΔG=ΔH – TΔS
•  Where,
• ΔG = Gibbs free Energy
• ΔH= Change in Enthalpy
• ΔS = Change in Entropy
•  T = Temperature in kelvin

Here, Gibbs free energy can be said as the amount of energy capable of doing work. This equation can tell us the direction of a bioenergetic reaction in biochemistry and the spontaneity of a reaction as

• If   is negative, the process is spontaneous and the reaction is an exergonic reaction.
• If   is positive, the process is nonspontaneous and the reaction is endergonic.
• And if is zero, the process or the reaction has reached equilibrium.
• If we speak in terms of bioenergetic reactions in biochemistry,

Both spontaneous and non-spontaneous reactions are possible. The relative free-energy levels of the reactants and products define the ratio of reactants to products present at equilibrium. G has a negative value as long as the total free energy of the reactants is larger than the total free energy of the products, and the reaction progresses in the direction of product production. The bigger the G, the farther the reaction is from equilibrium, and the more work the system can accomplish following the principles of bioenergetics.

Enthalpy, H

• Enthalpy, H, is the heat content of the reacting system.
•  It reflects the number and kinds of chemical bonds (covalent and noncovalent) in the reactants and products.
• When a chemical reaction releases heat, it is said to be exothermic; the heat content of the products is less than that of the reactants, and the change in enthalpy, ΔH, has a negative value.
• Reacting systems that take up heat from their surroundings are endothermic and have positive values of ΔH.

Entropy, S

• Entropy, S, is a quantitative expression for the randomness or disorder in a system.
• When the products of a reacting system are less complex and more disordered than the reactants, the reaction is said to proceed with a gain in entropy.
• ΔS has a positive sign when entropy increases.

RELATIONSHIP BETWEEN THE CHANGE FREE ENERGY, ENTHALPY AND ENTROPY

• 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.
• ΔG = ΔΗ – TΔS
• [ΔG = Gibbs Free Energy; ΔH = Change in Enthalpy; T = Temperature in K; ΔS = Change in Entropy]
• T represents the absolute temperature in Kelvin (K=273+°C).

REDOX POTENTIAL

• The oxidation-reduction potential may be defined as a quantitative expression of the tendency that a compound has to give or receive electrons.
• The redox potential of a system may be calculated from the following equation.
•   In Bioenergetics Redox Potential is the ratio of NAD+ to NADH+ + H+.
• It describes the availability of NAD+ for metabolism.

 

Types of the Bioenergetics Reaction

1.Exergonic Reaction: -

• Exergonic implies the release of energy from a spontaneous chemical reaction without any concomitant utilization of energy.
• 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.
• The release of free energy, G, in temperature) is denoted as an exergonic reaction (at const. pressure and temperature) is denoted as
•  ΔG= Gproducts-Greactants <0       
•  [i.e., ΔG = negative]
•  In exergonic reactions, energy is released to the surrounding. Due to that reason, the change in enthalpy is a negative value for exergonic reactions.
• The entropy is increased due to the disorder of the system.
•  Exergonic reactions include exothermic reactions.

Fig.- Graph of exergonic reaction

2. Endergonic Reactions: -

• Endergonic in turn is the opposite of exergonic in being non-spontaneous and requires an input of free energy.
• Most of the anabolic reactions like photosynthesis and DNA and protein synthesis are endergonic in nature.
• The release of free energy, G, in an endergonic reaction (at const. pressure & temperature) is denoted as 
• ΔG=Gproducts-Greactants >0           
• [i.e., ΔG = positive]
•  In a non-spontaneous reaction, energy should be provided from outside for the progression of the reaction.
•   Since new products are formed, the entropy of the system is decreased.
• Then, according to the above equation, the ΔG is a positive value.
• Endergonic reactions include endothermic reactions.

 

Fig.- Graph of endergonic reaction

Difference between Endergonic & Exergonic

Exergonic reaction	Endergonic reaction
The exergonic reaction is a type of reaction in which free energy is released	Endergonic reactions are the type of reaction in which free energy is absorbed.
Here Gibbs free energy is negative	Here Gibbs free energy is positive
Exergonic reactions indicate that the energy is released in the system	Endergonic reactions indicate that the energy is absorbed by the system.
All the exothermic reactions are exergonic.	All endothermic reactions are endothermic.
Exergonic reactions do not require energy to begin	Endothermic reactions always require energy to begin.
It is a downhill reaction.	It is an uphill reaction
Fatty Acid Catabolism, Glycolysis, cellular respiration	DNA/RNA Synthesis, Protein synthesis, Fatty acid synthesis.

 

 

HIGH ENERGY COMPOUNDS

High energy compounds are also called energy-rich compounds. Compounds present in the biological system that when hydrolysed, produce free energy that is greater or equal to that of ATP (△G is -7.3 kcal/mol) are termed high energy compounds.

Low-energy compounds have an energy yield of less than -7.3 kcal/mol. High-energy bonds are found in the majority of high-energy compounds that produce energy upon hydrolysis. Most of the high energy compounds contain phosphate groups and thus they are also termed high-energy phosphates. These high energy compounds are mainly classified into five groups:

•  Pyrophosphates
• Acyl phosphate
•  Enol phosphate
• Thiol phosphate
•  Phosphagens or guanido phosphates

Types of High Energy Compounds

The energy released during catabolism is captured in the form of a group of compounds called high energy phosphates. There are also high energy compounds belonging to the Sulphur group like acetyl CoA, succinyl CoA and fatty acyl CoA.

High Energy Compounds Examples

•  Pyrophosphates

Pyrophosphate energy bonds are nothing but acid anhydride bonds. Condensation of acid groups (primarily phosphoric acid) or their derivatives results in the formation of these bonds. ATP (△G = -7.3 kcal/mol) is an example of a pyrophosphate. It has two phosphoanhydride diphosphate bonds.

• Acyl phosphates

The reaction between the carboxylic acid group and the phosphate group forms a high energy bond in this compound.1,3-bisphosphoglycerate (△G = -11.8 kcal/mol) is an example of acyl phosphate.

• Enol phosphates

The enol phosphate bond is present here. It is formed when a phosphate group binds to a hydroxyl group that is bound to a double-bonded carbon atom. As an example, consider phosphoenolpyruvate (△G = -14.8 kcal/mol).

• Thiol phosphates

There is no high energy phosphate bond here. Instead, a high energy thioester bond is found here. Thioester bonds are formed by the reaction of thiol and carboxylic acid groups. Acetyl CoA (△G = -7.7 kcal/mol) is an example.

• Phosphagens

Guanidine phosphate bonds are present in phosphagens or guanido phosphates. The phosphate group is attached to the guanidine group to form it. Phosphocreatine (△G = -10.3 kcal/mol) is the most important compound with this type of bond.

ATP – Cell’s Energy Currency

The majority of ATP is produced in the mitochondrial matrix during cellular respiration. Thus, mitochondria are termed the powerhouse of the cell. ATP is a nucleotide made up of the molecule adenosine and three phosphate groups. It is water soluble and contains a lot of high energy due to the presence of 2 phosphoanhydride bonds connecting the 3 phosphate groups. As a significant amount of energy is released when they are broken, and they are referred to as high energy bonds. The available energy is stored in the phosphate bonds and is released when they are split into molecules. This is accomplished by the addition of a water molecule (hydrolysis). When the outer phosphate group of ATP is removed to yield energy, ATP is converted into ADP, which is the form of the nucleotide with only two phosphates.

• Hydrolysis of ATP (exergonic)

ATP + H₂O → ADP + Pi

• Dehydration of ADP (endergonic)

ADP + Pi → ATP + H₂O

Hydrolysis of ATP is associated with the release of large amounts of energy (7.3 kcal/mol) which is used for various processes like active transport, muscle contraction, etc.

The hydrolysis of ATP to AMP releases -10.9 kcal/mol of energy

ATP + H2O → AMP + PPi.

ADENOSINE TRIPHOSPHATE (ATP)

• Adenosine-5'-triphosphate (ATP) is a multifunctional nucleotide used in cells as a coenzyme.
•  It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism.
•  It is produced by photophosphorylation and cellular respiration and used by enzymes and structural proteins in many cellular processes.
• One molecule of ATP contains three phosphate groups and it is produced by ATP synthase from Inorganic Phosphate and Adenosine Diphosphate (ADP) or Adenosine Monophosphate (AMP).
• The three main functions of ATP in cellular function are:

1. Transporting organic substances such as sodium, calcium, potassium-through the cell membrane.
2. Synthesizing chemical compounds, such as protein and cholesterol.
3. Supplying energy for mechanical work, such as muscle contraction.

Structure of ATP

• The structure of this molecule consists of a purine base (adenine) attached to the 1' carbon atom of a pentose sugar (ribose).
• Three phosphate groups are attached at the 5' C atom of the pentose sugar
•  It is the addition and removal of these phosphate groups that inter-convert ATP, ADP and AMP.
• The energy released by cleaving either a phosphate (Pi) or pyrophosphate (PPi) unit from ATP at standard state of 1 M are:

              ATP + H2O → ADP + Pi AG° = -30.5 kJ/mol (-7.3 kcal/mol)

              ATP + H2O → AMP + PPI AG° = -45.6 kJ/mol (-10.9 kcal/mol)

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).

Fig.- Structure of ATP

Conclusion

Bioenergetics is the study of processes performed by living organisms, how the energy transformation, collection, and consumption takes place inside the cells, tissues, and organisms. The two principles of bioenergetics governing its processes are the First and the Second Law of thermodynamics used with the concept of Gibbs free energy. Gibbs free energy allows us to understand the spontaneity and the direction of the reaction.

In Summary, the transition of potential to kinetic energy and vice versa is the main focus of bioenergetic processes. Metabolic reactions, which are essential to the operation of every biological system, represent such energy transformation.

References

• https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5851778/
• https://www.sciencedirect.com/book/9780123884251/bioenergetics
•   https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2828274/
• http://aulanni.lecture.ub.ac.id/files/2012/01/15616949-Lehninger-Principles-of-Biochemistry-1-copy.pdf