Enzymes are typically categorized according to the type of reaction they facilitate.
Examples:
They lower the activation energy by stabilizing transition states, aligning substrates properly, or providing catalytic residues that facilitate electron flow. Through such general catalysis, enzymes increase the reaction rate constant without altering equilibrium constants or overall free energy changes.
Substrates and enzyme specificity
Enzymes exhibit high specificity toward their substrates based on intricate three-dimensional complementarity. This selectivity arises from well-defined binding pockets that recognize particular shapes, charges, and functional groups. The enzyme’s specificity ensures that only the correct substrates undergo reaction, reducing unwanted side reactions.
Active site model
The active site is a specialized region on the enzyme where substrate binding and catalysis occur. In the lock-and-key representation, the substrate fits into a rigid cavity that is pre-formed to complement its structure.
Induced-fit model
Refining the above concept, the induced-fit model proposes that the enzyme adjusts its shape upon substrate contact. This conformational shift more precisely orients catalytic residues and the substrate, thereby enhancing transition state stabilization and facilitating the chemical transformation.
Cofactors, coenzymes, and vitamins
Some enzymes depend on additional non-protein components known as cofactors to function properly. These can be metal ions (e.g., Fe²⁺, Zn²⁺) or coenzymes, which are organic cofactors often derived from vitamins (e.g., B-complex vitamins). They enable reaction types the enzyme’s amino acids cannot perform on their own, such as redox transfers (e.g., NAD⁺ ⇌ NADH).
Enzyme kinetics explores how factors like substrate concentration or enzyme concentration influence reaction rates. The Michaelis–Menten equation models this behavior, describing how velocity depends on substrate binding and turnover until reaching a maximum rate (Vmax).
Enzymes accelerate reactions by lowering the activation energy. They do not alter the equilibrium position or the reaction’s overall thermodynamics. Both forward and reverse reactions become faster, but the net free energy change (ΔG) remains unchanged.
Under the Michaelis–Menten framework, the reaction velocity (v) increases with substrate concentration [S] and approaches Vmax at saturating levels of substrate. Two key parameters emerge: Km (the substrate concentration at half Vmax) and Vmax (the maximum rate).
Certain multimeric enzymes display cooperativity, in which substrate binding to one subunit changes the affinity of other subunits. Positive cooperativity means binding enhances additional substrate uptake, generating a sigmoidal dependence of velocity on [S].
Environmental factors—pH, temperature, and ionic strength—profoundly influence enzyme structure and reactivity. Each enzyme has an optimal pH and temperature, and deviations can denature or inactivate it. Cells often maintain microenvironments or localization strategies to keep enzymes at their ideal conditions.
Enzymes can be inhibited in several ways:
Many pathways rely on regulatory enzymes that coordinate metabolic flow. Modulation can be:
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