Biochemistry Enzyme Functions Quiz

Enzymes

• Enzymes are protein catalysts that speed up chemical reactions in biological systems without being changed themselves.
• All enzymes are proteins, except some types of RNA molecules, called ribozymes, which act catalytically, primarily in the cleavage and synthesis of phosphodiester bonds.
• The substrate is the substance upon which an enzyme acts.
• Enzymes can be intracellular, acting within cells (e.g., metabolic enzymes ALT & AST), or extracellular, acting outside the cell (e.g., digestive enzymes pepsin & amylase).
• Enzymes are mainly synthesized in the cytosol or endoplasmic reticulum (ER), although some are synthesized and function in mitochondria.
• Enzymes are found in all body cells and are located in extracellular fluids (e.g., plasma, CSF) and intracellular compartments (e.g., cell membrane, cytosol, mitochondria, lysosomes, microsomes, nucleus).

Enzyme Activity Measurement

• International unit (IU): The amount of enzyme that converts one micromole (µmol) of substrate to product per minute. (1 IU = 1 µmol/min)
• Katal (kat): The amount of enzyme that converts one mole (mol) of substrate to product per second (1 kat = 1 mol/s).

Nature of Enzymes

• Most enzymes are proteins, existing either as simple or holoenzymes.
• Simple enzymes consist solely of a protein molecule.
• Holoenzymes are composed of a protein part (apoenzyme) and a non-protein part (cofactor).
• Cofactors can be organic molecules (like vitamins) or inorganic metal ions (e.g., Fe²⁺, Zn²⁺, Mg²⁺).

Organic Molecules in Enzymes

• Prosthetic groups are small organic molecules covalently bonded to the apoenzyme.
• Coenzymes are small organic molecules noncovalently bound to the apoenzyme.
• Coenzymes are often derivatives of vitamins and can accept or donate specific groups during a reaction.

Enzyme Classification

• Enzymes are divided into classes based on the type of reaction they catalyze.
• Codehydrogenases, group-transferring coenzymes (e.g., NAD+, NADP+, FAD, FMN), and other coenzymes are important enzyme components.

Enzyme Specificity

• Enzymes are highly specific for their substrates.
• Absolute specificity: An enzyme acts upon a single substrate or very closely related substrates.
• Group specificity: An enzyme acts on molecules with specific functional groups (e.g. amino, phosphate).

Enzyme Action

• Active sites: Specialized pockets or clefts on enzymes where substrates bind and reactions occur. The binding site is specific for a given substrate. The catalytic site is where the actual reaction occurs.
• Binding site: Portion of the active site where the substrate binds.
• Catalytic site: Portion of the active site where the reaction with the substrate is catalyzed.
• Enzyme-substrate complex: A temporary complex formed when the enzyme interacts with its substrate at the active site.
• Catalytic efficiency: The enzyme significantly increases the rate of a reaction compared to similar uncatalyzed reactions
• Turnover number: Measures the number of substrate molecules one enzyme molecule converts into product per unit time.

Enzyme Kinetics

• Initial velocity (V0): The rate of a reaction at the start of the process, when little product has been formed.
• Michaelis-Menten equation: Describes the relationship between substrate concentration ([S]) and reaction velocity.
• Km (Michaelis constant): This constant relates to the substrate concentration [S] when the initial velocity (V0) half of Vmax. It reflects the affinity between the enzyme and the substrate.

Enzyme Regulation

• Enzyme activity can be regulated through various mechanisms, such as changes in enzyme amount, allosteric regulation, covalent modification, and presence of inhibitors.
• Covalent modification: Reversible activation/inactivation of enzymes by the addition or removal of chemical groups (e.g., phosphorylation).
• Allosteric control: Binding of molecules to sites other than the active site affects enzyme activity.
• Feedback inhibition: The end product of a metabolic pathway inhibits an enzyme earlier in the pathway.

Inhibitors

• Inhibitors are substances that decrease enzyme activity.
• Competitive inhibition: Inhibitor and substrate compete for the same binding site on the enzyme.
• Non-competitive inhibition: Inhibitor and substrate bind to separate, distinct sites on the enzyme, altering its conformation and reducing catalytic ability.
• Irreversible inhibition: Inhibitor permanently binds to the enzyme, rendering it inactive.

Enzyme Nomenclature and Classification

• Trivial names: Provide little insight into the enzyme’s function, source, or reaction.
• Recommended names: Include the substrate and the type of reaction catalyzed (e.g., sucrase, carbohydrase).
• Enzyme commission numbers: Systematic numerical codes assigning enzymes to specific classes of reactions they catalyze.

Isoenzymes

• Isoenzymes: Multiple forms of an enzyme that catalyze the same reaction but have different amino acid sequences, physical properties, or organ distribution.
• Isoenzymes are useful for diagnostic purposes; changes in levels can indicate particular diseases or tissue damage.

Factors Affecting Enzyme Activity

• Temperature, pH, substrate concentration, enzyme concentration, enzyme inhibitors, coenzymes, and activators are factors influencing enzyme activity rates and optimum conditions.

Plasma Enzymes

• Functional and non-functional plasma enzymes are important indicators for tissue and organ damage.