Differentiate between enzymes and coenzymes and describe their mechanisms of action.

Enzymes: Definition and Characteristics

Enzymes are biological catalysts, typically proteins, that speed up the rate of a specific chemical reaction without being consumed in the process. They possess a unique three-dimensional structure, including an active site where the substrate binds. Enzymes exhibit several key characteristics: specificity, efficiency, regulation, and sensitivity to environmental factors like temperature and pH.

Coenzymes: Definition and Characteristics

Coenzymes are non-protein organic molecules that assist enzymes in catalyzing reactions. They are often derived from vitamins and bind to the enzyme, either loosely or covalently, to facilitate its activity. Unlike enzymes, coenzymes are often modified during the reaction and must be regenerated to continue functioning. They act as carriers of specific atoms or functional groups.

Differentiating Enzymes and Coenzymes

The key differences between enzymes and coenzymes can be summarized in the following table:

Feature	Enzyme	Coenzyme
Nature	Primarily protein	Organic non-protein molecule (often vitamin-derived)
Role	Catalyzes reactions	Assists enzyme catalysis; carries chemical groups
Consumption	Not consumed in the reaction	Often modified during the reaction; requires regeneration
Binding	Forms enzyme-substrate complex	Binds to enzyme (loosely or covalently)
Specificity	High specificity for substrates	Specificity for the type of reaction it participates in

Mechanisms of Action: Enzymes

Lock and Key Model

The earliest model proposed for enzyme action was the ‘lock and key’ model. This suggests that the enzyme's active site has a rigid shape complementary to the substrate, fitting together like a lock and key. While conceptually simple, this model doesn't account for the enzyme's flexibility.

Induced Fit Model

The more accepted ‘induced fit’ model proposes that the enzyme's active site is flexible and changes shape upon substrate binding. This conformational change optimizes the interaction between the enzyme and substrate, leading to catalysis. This model explains the enzyme's specificity and ability to accommodate slightly different substrates.

Enzyme Kinetics

Enzyme kinetics describes the rate of enzyme-catalyzed reactions. Key parameters include:

• Vmax: The maximum rate of reaction when the enzyme is saturated with substrate.
• Km: The Michaelis constant, representing the substrate concentration at which the reaction rate is half of Vmax. It reflects the enzyme's affinity for the substrate.

Mechanisms of Action: Coenzymes

Coenzymes participate in enzymatic reactions in several ways:

• Electron Transfer: Coenzymes like NAD+ and FAD accept or donate electrons in redox reactions. For example, NAD+ accepts electrons during glycolysis, becoming NADH.
• Group Transfer: Coenzymes like Coenzyme A (CoA) carry acyl groups, while tetrahydrofolate (THF) carries one-carbon units. These groups are transferred to substrates during metabolic pathways.
• Stabilization of Intermediates: Some coenzymes stabilize reactive intermediates formed during enzymatic reactions.

For instance, in the citric acid cycle, Coenzyme A plays a vital role in carrying acetyl groups, enabling the cycle to proceed. Without CoA, the cycle would be significantly hindered.