Enzymes and Bioenergetics — NCERT Solutions

Definition of One Unit of Enzyme:

One International Unit (IU) of an enzyme is defined as the amount of enzyme that catalyses the conversion (transformation) of one micromole ($1\ \mu mol$) of substrate per minute under specified (standard) conditions of temperature (usually 25°C or 37°C), pH, and substrate concentration.

$$1\ \text{IU} = 1\ \mu\text{mol of substrate converted per minute}$$

Note: The SI unit of enzyme activity is the katal (kat), defined as the amount of enzyme that converts 1 mole of substrate per second:
$$1\ \text{kat} = 1\ \text{mol}\ \text{s}^{-1} = 6 \times 10^7\ \text{IU}$$

Enzyme units are used to quantify enzyme activity in a preparation and are essential for comparing enzyme preparations and calculating specific activity.

Definition of Specific Activity:

The specific activity of an enzyme is defined as the number of enzyme units (IU) per milligram of total protein in the enzyme preparation.

$$\text{Specific Activity} = \frac{\text{Total enzyme activity (units)}}{\text{Total protein (mg)}}$$

$$\text{Units: } \frac{\mu\text{mol min}^{-1}}{\text{mg protein}} = \text{units/mg protein}$$

Significance:

1. Specific activity is a measure of the purity of an enzyme preparation. As the enzyme is purified (removal of contaminating proteins), the specific activity increases.
2. A pure enzyme has the highest possible specific activity.
3. It is used to monitor the progress of enzyme purification at each step — the fold purification can be calculated by comparing specific activity at each step to that of the crude extract.
4. It allows comparison of enzyme preparations from different sources or batches.

Laws of Thermodynamics:

(i) First Law of Thermodynamics (Law of Conservation of Energy):

Statement: Energy can neither be created nor destroyed; it can only be converted from one form to another.

• The total energy of the universe (system + surroundings) remains constant.
• In biological systems, chemical energy stored in food molecules (e.g., glucose) is converted into other forms such as heat energy, mechanical energy (muscle contraction), electrical energy (nerve impulses), and chemical energy (ATP synthesis).
• Mathematically: $\Delta U = q - w$, where $\Delta U$ = change in internal energy, $q$ = heat absorbed by the system, $w$ = work done by the system.
• Biological relevance: The energy released during oxidation of glucose is not lost but is conserved in the form of ATP, which is then used for cellular work.

(ii) Second Law of Thermodynamics (Law of Entropy):

Statement: Every spontaneous process is accompanied by an increase in the total entropy (disorder) of the universe.

• Entropy (S) is a measure of the randomness or disorder of a system.
• In any spontaneous process, the entropy of the universe always increases: $\Delta S_{universe} > 0$.
• No energy conversion is 100% efficient; some energy is always lost as heat (which increases disorder/entropy of the surroundings).
• Living organisms maintain a high degree of internal order (low entropy) by continuously consuming energy and releasing heat to the surroundings, thereby increasing the entropy of the surroundings.
• Biological relevance: Metabolic processes such as cellular respiration release heat, increasing the entropy of the surroundings, even as the cell maintains its ordered structure.

Definition of Entropy:

Entropy (S) is a thermodynamic quantity that measures the degree of randomness, disorder, or dispersal of energy in a system. Higher entropy means greater disorder.

• Units: $\text{J K}^{-1}\text{mol}^{-1}$ (Joules per Kelvin per mole)
• Entropy increases when a system becomes more disordered (e.g., a solid dissolving into solution, a large molecule being broken into smaller ones).

Relationship between Free Energy and Entropy — Gibbs Free Energy:

The relationship between free energy, entropy, and enthalpy is given by the Gibbs free energy equation:

$$\Delta G = \Delta H - T\Delta S$$

Where:

• $\Delta G$ = Change in Gibbs free energy (energy available to do useful work)
• $\Delta H$ = Change in enthalpy (total heat content of the system)
• $T$ = Absolute temperature (in Kelvin)
• $\Delta S$ = Change in entropy

Interpretation:

Relationship explained:

• A large positive $\Delta S$ (increase in entropy) makes $-T\Delta S$ more negative, thus making $\Delta G$ more negative → favours spontaneity.
• A negative $\Delta S$ (decrease in entropy, increased order) makes $\Delta G$ more positive → opposes spontaneity.
• In biological systems, many biosynthetic reactions (e.g., protein synthesis, DNA replication) have negative $\Delta S$ (increased order) and are endergonic ($\Delta G > 0$), so they are coupled to exergonic reactions (like ATP hydrolysis) to proceed.

ATP as the Universal Energy Currency:

ATP (Adenosine Triphosphate) is called the universal energy currency of the cell because it is the primary molecule used to store, transfer, and supply energy for virtually all biological processes in all living organisms.

Structure of ATP:
ATP consists of:

• Adenine (nitrogenous base)
• Ribose (5-carbon sugar)
• Three phosphate groups linked by high-energy phosphoanhydride bonds ($\sim$P)

Reasons why ATP is called universal energy currency:

1. High-energy phosphate bonds: The bonds between the phosphate groups (especially the terminal $\beta$–$\gamma$ bond) are high-energy bonds. Hydrolysis of ATP releases approximately 30.5 kJ/mol (7.3 kcal/mol) of free energy:

$$\text{ATP} + H_2O \rightarrow \text{ADP} + P_i + \text{Energy} \quad (\Delta G = -30.5\ \text{kJ/mol})$$

2. Energy coupling: ATP acts as an intermediate between energy-releasing (catabolic) reactions and energy-requiring (anabolic) reactions:

• During catabolism (e.g., cellular respiration, breakdown of glucose), energy is released and used to synthesise ATP from ADP and inorganic phosphate ($P_i$).
• During anabolism and other energy-requiring processes (e.g., muscle contraction, active transport, biosynthesis, nerve impulse transmission), ATP is hydrolysed to ADP + $P_i$, releasing energy to drive these processes.

3. Universal use: ATP is used as the energy source in all living organisms — from bacteria to humans — for all types of cellular work.

4. Recyclable: ATP is continuously regenerated from ADP and $P_i$ using energy from food oxidation or photosynthesis, making it a recyclable energy carrier. A single ATP molecule may be recycled hundreds of times per day.

5. Intermediate energy level: ATP has an intermediate position in the scale of phosphate transfer potential, making it suitable to accept energy from high-energy donors and donate energy to low-energy acceptors.

Conclusion: Because ATP is produced in energy-releasing processes and consumed in energy-requiring processes universally across all life forms, it is aptly called the universal currency of free energy in biological systems.