Enzymes and Bioenergetics

Correct Option: (b) decrease the activation energy

Justification: Enzymes are biological catalysts. They function by lowering the activation energy (energy barrier) required for a reaction to proceed, thereby increasing the rate of the reaction. Enzymes do not alter the equilibrium constant of a reaction, nor do they need to be saturated with substrate to catalyse a reaction. They certainly do not increase the activation energy.

Given: Pepsin is a gastric (stomach) enzyme.

Optimum pH of Pepsin:
Pepsin has an acidic optimum pH, approximately pH 1.5–2.5. This is consistent with the highly acidic environment of the stomach (due to HCl secreted by parietal cells), where pepsin is most active in digesting proteins.

What happens when pepsin enters the duodenum:
The duodenum receives bicarbonate secretions from the pancreas, which neutralise the acidic chyme coming from the stomach. The pH in the duodenum rises to approximately pH 7–8 (alkaline/neutral).

Since pepsin's optimum pH is strongly acidic, the rise in pH in the duodenum causes a conformational change in the enzyme's active site, leading to denaturation or inactivation of pepsin. As a result, pepsin loses its catalytic activity and can no longer digest proteins in the duodenum. Protein digestion in the duodenum is then carried out by pancreatic proteases such as trypsin and chymotrypsin, which have alkaline optimum pH values.

Concept: Vitamins as Precursors of Coenzymes (Co-factors)

Many enzymes require non-protein organic molecules called coenzymes for their catalytic activity. These coenzymes are often derived from vitamins, particularly the B-group vitamins.

Relationship:
- Vitamins themselves are not directly catalytically active, but they serve as precursors or building blocks for coenzymes.
- When vitamins are metabolised in the body, they are converted into coenzymes that associate with apoenzymes (the protein part) to form active holoenzymes.

Examples:

| Vitamin | Coenzyme Derived |
|---|---|
| Vitamin B₁ (Thiamine) | Thiamine pyrophosphate (TPP) |
| Vitamin B₂ (Riboflavin) | FAD (Flavin Adenine Dinucleotide) |
| Vitamin B₃ (Niacin) | NAD⁺ / NADP⁺ |
| Vitamin B₅ (Pantothenic acid) | Coenzyme A (CoA) |
| Vitamin B₆ (Pyridoxine) | Pyridoxal phosphate (PLP) |
| Vitamin B₁₂ (Cobalamin) | Cobalamin coenzymes |

Conclusion: Vitamins are essential dietary components because they cannot be synthesised by the body in sufficient amounts, yet they are indispensable for the formation of coenzymes that are required for enzymatic reactions. A deficiency of vitamins therefore leads to deficiency of the corresponding coenzymes, impairing enzyme function and causing metabolic disorders.

Effect of Various Factors on Enzyme Activity:

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(i) Effect of Temperature:
- Enzyme activity increases with increasing temperature up to an optimum temperature (usually around 37°C for human enzymes).
- Beyond the optimum temperature, the rate of reaction decreases sharply because the high thermal energy disrupts the weak bonds (hydrogen bonds, ionic bonds, van der Waals forces) maintaining the three-dimensional structure of the enzyme, leading to denaturation.
- The active site loses its specific shape and can no longer bind the substrate.
- The relationship between temperature and enzyme activity follows a bell-shaped curve.
- The temperature coefficient $Q_{10}$ (ratio of reaction rate at $T+10°C$ to rate at $T°C$) is approximately 2 for most enzymes in the physiological range.

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(ii) Effect of pH:
- Each enzyme has a characteristic optimum pH at which its activity is maximum.
- Example: Pepsin — pH 1.5–2.5 (acidic); Salivary amylase — pH 6.8–7.0 (neutral); Trypsin — pH 7.8–8.0 (alkaline).
- At pH values above or below the optimum, the ionisation state of amino acid residues in the active site changes, altering the enzyme's shape and its ability to bind substrate.
- Extreme pH values cause denaturation of the enzyme.
- The activity vs. pH graph is also bell-shaped.

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(iii) Effect of Substrate Concentration:
- At low substrate concentrations, the reaction rate increases proportionally with increasing substrate concentration (first-order kinetics), as more substrate molecules are available to bind to free active sites.
- As substrate concentration increases further, the rate of increase slows down.
- At very high substrate concentrations, all active sites of the enzyme are occupied (enzyme is saturated), and the reaction rate reaches a maximum velocity ($V_{max}$). Further increase in substrate concentration does not increase the rate (zero-order kinetics).
- This relationship is described by the Michaelis-Menten equation:
$$v = \frac{V_{max}[S]}{K_m + [S]}$$
where $[S]$ is the substrate concentration and $K_m$ is the Michaelis constant.

Correct Option: (c) the product formation

Justification: In Michaelis-Menten kinetics, the overall reaction is represented as:
$$E + S \underset{k_{-1}}{\overset{k_1}{\rightleftharpoons}} ES \overset{k_2}{\longrightarrow} E + P$$
The formation of the enzyme-substrate (ES) complex is a rapid equilibrium step. The rate-determining (slowest) step is the conversion of the ES complex into product (P) and free enzyme (E), governed by the rate constant $k_2$ (also called $k_{cat}$). This step determines the overall rate of the reaction, making product formation the rate-limiting step.

Definition of $K_m$ (Michaelis Constant):

$K_m$ is defined as the substrate concentration at which the reaction velocity is half of the maximum velocity ($V_{max}/2$).

Mathematically, from the Michaelis-Menten equation:
$$v = \frac{V_{max}[S]}{K_m + [S]}$$
When $v = \dfrac{V_{max}}{2}$:
$$\frac{V_{max}}{2} = \frac{V_{max}[S]}{K_m + [S]}$$
$$K_m + [S] = 2[S]$$
$$\boxed{K_m = [S]}$$

Thus, $K_m$ equals the substrate concentration at half-maximal velocity. Its units are mol/L (M) or mM.

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Significance of $K_m$:

1. Measure of enzyme–substrate affinity: $K_m$ is inversely related to the affinity of the enzyme for its substrate. A low $K_m$ means the enzyme has high affinity for the substrate (reaches half-maximal velocity at low substrate concentration). A high $K_m$ means low affinity.

2. Identification of natural substrate: When an enzyme can act on multiple substrates, the substrate with the lowest $K_m$ is considered the enzyme's natural (preferred) substrate.

3. Comparison of enzymes: $K_m$ allows comparison of different enzymes or the same enzyme under different conditions.

4. Diagnostic tool: Abnormal $K_m$ values can indicate mutations in enzyme structure or the presence of inhibitors.

5. Determination of enzyme kinetics: $K_m$ is used in the Lineweaver-Burk (double reciprocal) plot to determine kinetic parameters and the type of inhibition.

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:

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(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.

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(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).

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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:

| Condition | Meaning |
|---|---|
| \Delta G < 0 (negative) | Reaction is spontaneous (exergonic); free energy is released |
| \Delta G > 0 (positive) | Reaction is non-spontaneous (endergonic); free energy is required |
| $\Delta G = 0$ | System is at equilibrium |

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.

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Structure of ATP:
ATP consists of:
- Adenine (nitrogenous base)
- Ribose (5-carbon sugar)
- Three phosphate groups linked by high-energy phosphoanhydride bonds ($\sim$P)

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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.