Cellular bioenergetics and concept of free energy Cellular Biochemistry and metabolism 1 CLS 331 Dr. Samah Kotb Lecturer of Biochemistry 2015

Bioenergetics & ATP Dr Samah Kotb Lecturer of Biochemistry

Bioenergetics & ATP Bioenergetics is basically how living systems make use of free energy

Bioenergetics & ATP There are 2 types of energy that can be used by systems to do work:- Free Energy Heat Energy

Free energy is the kind of energy that can be used to do work under conditions of constant temperature & pressure.

Heat energy can be used to do work only through a change of temperature.

Bioenergetics & ATP: Heat is not a significant source of energy for living cells because heat can only do work as it passes from a zone at one temperature to another at a lower temperature.

Since living cells have the same temperature throughout, they cannot make use of heat energy.

Cells use free energy (G) which can work at constant temperature and pressure. Free energy is obtained by animal cells from the catabolism of energy rich nutrient molecules whereas plant cells obtain it from solar radiant.

What do you know about Anabolic Pathways And Catabolic Pathways

Anabolic Pathways Reactions that result in the synthesis of biomolecules using basic unit components and require an input of energy to take place .

Catabolic Pathways Reactions through which energy rich nutrient molecules are broken down by chemical reactions into simple end products. As a result of catabolic pathways energy is produced and released to the cell.

Standard free energy change (G) of a chemical reaction: G is the difference between the free energy content of the reactants and that of the products under standard conditions of temperature and pressure (298k & 1 atmospheric pressure).

When a reaction results in release of energy

It means that the products contain less free energy than the reactants It means that the products contain less free energy than the reactants. Here G for the reaction will have a negative value and the reaction will be catabolic in nature.

A reaction is anabolic and will have a positive G value if the products contain more free energy than the reactants. Energy has to be put into the reaction for it to proceed.

The free-energy change of a reaction (ΔG) divided into 3 types:

1. A reaction can occur only if ΔG is negative 1. A reaction can occur only if ΔG is negative. An output of free energy is required to drive such a reaction, Such reactions are said to be exergonic.

2. A system is at equilibrium and no net change can take place if ΔG is zero.

3. A reaction can occur if ΔG is positive 3. A reaction can occur if ΔG is positive. An input of free energy is required to drive such a reaction. These reactions are termed endergonic.

It means that the products contain less free energy than the reactants It means that the products contain less free energy than the reactants. Here G for the reaction will have a negative value and the reaction will be catabolic in nature.

A reaction is anabolic and will have a positive G value if the products contain more free energy than the reactants. Energy has to be put into the reaction for it to proceed.

Standard free energy change (G) of a chemical reaction: For every reaction G can be calculated using:- G = - 2.303 RT log Keq While: R = Gas constant T = Absolute Temp. Keq = Equilibrium constant Note: G indicates constant temperature & pressure and physiological pH 7.2 for cells. Unites of free energy = calorie (cal) or kilocalorie (kcal) /mole

Units of energy A calorie (cal) is equivalent to the amount of heat required to raise the temperature of 1 gram of water from 14.5°C to 15.5°C. A kilocalorie (kcal) is equal to 1000 cal. A joule (J) is the amount of energy needed to apply a 1-newton force over a distance of 1 meter. A kilojoule (kJ) is equal to 1000 J. 1 kcal = 4.184 kJ. The kilocalorie (abbreviated kcal) and the kilojoule (kJ) will be used as the units of energy. One kilocalorie is equivalent to 4.184 kilojoules.

The ΔG of this reaction is given by Consider the reaction The ΔG of this reaction is given by In which ΔG° is the standard free-energy change, R is the gas constant, T is the absolute temperature, and [A], [B], [C], and [D] are the molar concentrations (more precisely, the activities) of the reactants.

ΔG° is the free energy change for this reaction under standard conditions that is, when each of the reactants A, B, C, and D is present at a concentration of 1.0 M (for a gas, the standard state is usually chosen to be 1 atmosphere). Thus, the ΔG of a reaction depends on the nature of the reactants (expressed in the ΔG° term of equation 1) and on their concentrations (expressed in the logarithmic term of equation 1).

The ΔG of a reaction depends only on the free energy of the products (the final state) minus the free energy of the reactants (the initial state). The ΔG of a reaction is independent of the path (or molecular mechanism) of the transformation. The mechanism of a reaction has no effect on ΔG. The ΔG provides no information about the rate of a reaction.

G values of pathways can be calculated The G value of an overall pathway can be calculated as the algebraic sum of the G values of the individual reactions making the pathway:- Gpathway = G1 + G2 + G3 + G4 + G5

Chemistry of ATP (Adenosine – tri – phosphate): ATP is a nucleotide type molecule made of the following components:- The nitrogenous base adenine The pentose sugar ribose Three phosphate groups

Chemistry of ATP (Adenosine – tri – phosphate) Thus:- ATP ADP + Pi G = -7.3 kcal/mole ADP AMP + Pi G = -7.2 kcal/mole AMP Adenosine + Pi G = -3.2 kcal/mole ATP, ADP and AMP are present in all forms of life. They occur not only in the cytosol of cells but also in the mitochondria & nucleus. In normal respiring cells ATP makes up 80% of the three ribonucleotides. ADP & AMP account for 20%.

Chemistry of ATP (Adenosine – tri – phosphate): At pH 7, ATP occurs as the multiply charged anion ATP4- whereas ADP occurs as ADP3-. This is because their phosphate groups are completely ionized at the intracellular PH. ATP and ADP occur inside cells as magnesium complexes:- ATP4- + Mg2+ (ATP-Mg)2- ADP3- + Mg2+ (ADP-Mg)ــ

Chemistry of ATP (Adenosine – tri – phosphate): Inside cells the concentration of ATP remains normally relatively constantly high. It’s rate of formation equals it’s rate of hydrolysis. Thus the terminal phosphate group of ATP undergoes continuous removal & replacement from the pool of inorganic phosphate during cell metabolism.

G values for some characteristic reactions Super high energy compounds are compounds generated during catabolism. They are phosphorylated compounds. Once formed along a catabolic pathway, they undergo immediate hydrolysis (dephosphorylation). As a result a large amount of energy is released this is used by the cell to synthesize ATP from ADP and the hydrolyzed inorganic phosphate.

G values for some characteristic reactions

The Bioenergetics of Muscle Contraction The contraction of muscle requires a large amount of energy that cannot be fulfilled by the ATP stored inside muscle tissue. In addition to ATP there is a super-high energy compound stored in muscle cells that plays a major role in the energetics of muscle. This super-high energy compound is also present in large concentrations in other contractile tissues such as brain & nerve tissue.

The Bioenergetics of Muscle Contraction This compound is PHOSPHOCREATINE. It serves as a storage form of high energy phosphate groups. The G value for the hydrolytic reaction of phosphocreatine is highly negative (-10.3 kcal/mole). This is greater than that of ATP. The energy released is sufficient to allow coupled synthesis of ATP from ADP:-

The Bioenergetics of Muscle Contraction Phosphocreatine thus functions to keep the ATP concentration in muscle cells at constantly high level whenever some of the ATP of muscle cells is used for contraction, ADP is formed. Through the action of creatine kinase phosphocreatine is quickly hydrolyzed and donates its phosphate group to ADP to form ATP.

The Bioenergetics of Muscle Contraction The phosphocreatine level inside muscle is 3-4 times greater than that of ATP and thus stores enough high energy phosphate groups to keep the ATP level constantly high during short periods of intense muscular contraction.

The Bioenergetics of Muscle Contraction: