EnzymesEdexcel A-Level Biology A: Revision notes

Enzymes are globular proteins that act as biological catalysts. They speed up metabolic reactions without being used up. Almost all reactions need energy to get started, called the activation energy. Enzymes provide an alternative reaction pathway with a lower activation energy, so more molecules have enough energy to react and the reaction is faster at body temperature.

An enzyme does not change the products or the final amount of product. It only changes how quickly the reaction happens.

The active site is a region of the enzyme with a specific three-dimensional shape, formed by the folding of the polypeptide (its tertiary structure). The molecule that binds is the substrate, and the two form an enzyme–substrate complex.

Enzymes are specific: only a substrate with a shape complementary to the active site can bind. If the tertiary structure changes, the active site changes shape and the enzyme no longer works.

In the induced-fit model, the active site changes shape slightly as the substrate binds, which strains the bonds in the substrate and so lowers the activation energy. This replaces the older, rigid lock-and-key model.

Intracellular enzymes are made in a cell and work inside it, for example catalase in liver cells breaking down hydrogen peroxide, or the enzymes of respiration.

Extracellular enzymes are made by cells but released to work outside them, for example amylase from the salivary glands and pancreas and trypsin from the pancreas. Digestive enzymes are extracellular because food molecules are too large to enter cells, so they are hydrolysed into small soluble molecules outside the cells and then absorbed.

The rate of an enzyme-controlled reaction is measured as the amount of product formed, or substrate used, per unit time. Rates are compared as initial rates, measured at the start, because later the substrate concentration falls and product builds up, which slows the reaction.

To find an initial rate, plot product (or substrate) against time and draw a tangent at time zero. The gradient of the tangent is the initial rate, for example in cm³ s⁻¹. Where only a quantity over a short time is available, the mean rate is the change divided by the time.

To investigate the effect of enzyme concentration, keep the substrate in excess and change only the enzyme concentration.

• Make different enzyme concentrations by diluting a stock solution.
• Control temperature with a water bath and pH with a buffer; use the same volume and concentration of substrate each time.
• Measure the product, for example oxygen from catalase and hydrogen peroxide collected in a gas syringe, over time.
• Calculate the initial rate for each concentration and repeat to find a mean.

The initial rate is directly proportional to enzyme concentration, because doubling the enzyme doubles the number of active sites.

To investigate substrate concentration, use a fixed amount of enzyme and a dilution series of the substrate, for example amylase with starch measured with iodine solution and a colorimeter, using a calibration curve.

As substrate concentration rises, the initial rate rises because collisions between substrate molecules and active sites become more frequent. At high concentration all active sites are occupied (saturated), so the rate levels off at its maximum. The enzyme concentration is then the limiting factor.

If more enzyme is added at this stage, there are more active sites and the rate rises again.