Enzymes — OCR A-Level Biology

Enzymes are biological catalysts that lower the activation energy of a reaction, allowing metabolic reactions to proceed at a rate sufficient for life. Teach learners that enzymes act by binding substrate at an active site to form an enzyme–substrate complex, and that the active site is complementary to the substrate. At the cellular level, enzymes catalyse reactions such as those of respiration and photosynthesis, controlling the metabolic pathways inside cells. At the whole organism level, the combined activity of enzymes across tissues and organs determines the organism's overall metabolic rate and its ability to function. A concrete example: intracellular enzymes catalyse the steps of glycolysis, while the summed activity of enzymes throughout the body sets the organism's metabolic rate.

Enzymes catalyse reactions both inside cells and outside them. Intracellular enzymes work within the cytoplasm or organelles; for example, catalase breaks down hydrogen peroxide in peroxisomes, and respiratory enzymes in mitochondria catalyse reactions of the Krebs cycle. Extracellular enzymes are secreted from cells and act outside them; for example, amylase secreted by salivary glands and pancreas digests starch in the gut, and protease secreted into the stomach digests protein. In both cases the enzyme lowers activation energy by forming an enzyme–substrate complex at its active site, and the products are released. The location differs, but the catalytic mechanism is the same. Extracellular digestion allows large food molecules to be broken into smaller soluble molecules that can be absorbed.

Enzymes are globular protein catalysts that lower the activation energy of a reaction. The currently accepted mechanism is the induced-fit model: the active site is not a rigid shape but moulds around the substrate as it binds, so the substrate and active site become complementary. Formation of the enzyme–substrate complex strains bonds in the substrate and positions reacting groups correctly, which speeds up the reaction. The active site is specific because its shape and chemistry (including charged and hydrophobic residues) only fit particular substrates. After the reaction the products leave the active site, which is free to bind more substrate. A common MCQ trap is confusing the older lock-and-key idea with induced fit: lock-and-key treats the active site as a fixed shape, whereas induced fit describes a conformational change on binding.

Enzyme activity is measured as the rate of product formation or substrate disappearance. Increasing substrate concentration raises the rate until all active sites are occupied, giving a plateau (Vmax); further substrate has no effect because the enzyme is saturated. Increasing enzyme concentration raises the rate proportionally, provided substrate is not limiting. Temperature increases rate up to an optimum because molecules have more kinetic energy and more successful collisions; above the optimum, bonds holding the tertiary structure break, the active site loses its shape and the enzyme is denatured, so rate falls sharply. pH affects the ionisation of amino acids in the active site; each enzyme has an optimum pH, and extremes of pH alter charges and disrupt the active site, reducing activity. MCQ questions often test which factor is limiting at a given point on a graph.

Practical work here means planning and carrying out valid investigations, not just describing graphs. For each variable, identify the independent variable, control the others, and choose a measurable dependent variable such as volume of gas evolved, time to clear a substrate, or mass change. Use a buffer to set pH and a water bath or thermometer to hold temperature; vary enzyme concentration by changing the volume or concentration of enzyme solution while keeping substrate constant, and vary substrate concentration while keeping enzyme constant. Take repeat readings, calculate a mean and a rate, and control temperature, pH and volumes. Explain the shape of each result using collision theory and active-site saturation. Assess by how well you justify choices, control variables and interpret data.

Some enzyme-controlled reactions proceed only when a helper substance is present. A cofactor is a non-protein substance required for an enzyme to catalyse a reaction; it may be an inorganic ion, such as Cl⁻ for amylase or Zn²⁺ for some enzymes, bound loosely or tightly to the enzyme. A coenzyme is an organic cofactor, often derived from a vitamin, that binds temporarily and transfers chemical groups or electrons between reactions; NAD and FAD carry hydrogen in respiration, and coenzyme A carries acetyl groups. Prosthetic groups are cofactors bound permanently, as in the haem group of catalase. Without the cofactor the active site may be the wrong shape or the substrate cannot be chemically transformed, so the rate falls. In an MCQ, identify the role of the named substance and distinguish coenzyme from inorganic cofactor.

Inhibitors reduce the rate at which an enzyme-controlled reaction proceeds by interfering with enzyme activity. Competitive inhibitors resemble the substrate and bind the active site, so they compete with substrate molecules; their effect is reduced by raising substrate concentration because more substrate molecules occupy the active sites. Non-competitive inhibitors bind at a site other than the active site, changing the shape of the active site so the substrate can no longer bind; adding more substrate does not overcome this, so the maximum rate falls. Both lower the rate, but the pattern of effect differs. When interpreting a rate graph, compare initial rate and maximum rate: competitive inhibition raises the substrate concentration needed to reach a given rate but can still reach the same maximum, whereas non-competitive inhibition lowers the maximum rate achievable.