BIOLOGY – ENZYMES |12 QUESTIONS AND ANSWERS.

Define Enzymes Enzyme is a biological catalyst, which is protein in nature, and can speed up the rate of a chemical reaction, without it being chemically changed at the end of the reaction. How do enzymes work? Enzymes work by lowering the activation energy of a reaction. Activation energy is the minimum amount of energy that reactants must absorb to become activated to start a chemical reaction Properties of an enzyme (7) - Most enzymes are made up of protein - Enzymes alter or speed up chemical reaction without themselves being chemically changed - small amount of enzyme can bring about a large amount of chemical reaction - Enzymes are highly specific in their action (ea enzyme works on specific substrate) - Enzymes are sensitive to temperature - Enzymes are sensitive to pH of the solution - Many enzymes cannot catalyse a metabolic reaction without non-protein helpers called cofactors (either inorganic substances or organic compounds) Lock and Key Hypothesis Each enzyme molecule is a globular protein which has a specific 3-dimensional shape including its active site The active site of an enzyme only binds to a substrate molecule with a complementary shape, resulting in enzymes being very specific Only one substrate (key) can fit exactly onto the active site of the enzyme (lock) to form an enzyme-substrate complex Induced-Fit Hypothesis When a substrate molecule binds to the enzyme, it may causes small changes of the shape of the active site of the enzyme (proteins are wobbly, flexible molecules) Initially the substrate does not fit perfectly into the active site of the enzyme. When it binds to the active site, it changes the shape of the active site Effect of temperature on enzyme activity Enzymes are inactive at 0° Every 10° increase in temperature will double the rate of enzyme activity until the temperature reaches 40° As the temperature increases, the substrate and enzyme molecules gain kinetic energy, increasing the chance of successful collision between the enzyme and substrate, thus increasing the formation of enzyme-substrate complex At 40°, the enzymes are most active and the rate of enzyme activity is at its maximum Enzymes begin to denature at temperatures beyond 40° Denaturation During denaturation, polypeptide chains of enzyme molecules are unfolded, and hence the active site loses or distorts its 3-dimensional specific shape, resulting in substrates being unable to bind to the active sites (enzyme loses its catalytic function) Effect of pH on Enzyme Activity Each enzyme works best at a particular pH (optimum pH) EG Amylase - pH 7, Pepsin - pH 2 Enzymes are denatured at extreme pH (EG Salivary Amylase are denatured in the stomach due to the extreme acidic environment) During denaturation, drastic changes in pH can alter the ionic charges of acidic and basic groups of the enzyme (changing the 3-D shape of the active site) Define limiting factors The factor solely responsible for increase in rate of reaction Competitive Inhibitors Structurally similar to the substrate of the enzyme Bind to the active site (prevents the substrate from binding to the active site) The effects of a competitive inhibitor can be reduced by increasing the substrate concentration, so that more substrate would successfully bind to the active site than inhibitor Non-Competitive Inhibitors Not similar to the substrate Do not bind to the active site of the enzyme Bind to a different site on the enzyme and change the conformation of the active site The substrate may still be able to bind to the active site, but the enzyme is unable to catalyse the reaction or can only do so at a slower rate Increasing the substrate concentration cannot prevent the inhibitor from binding to the enzyme, as the two bind to different sites No matter how high the concentration of the substrate is, some of the enzymes will still be inhibited and the maximum rate of reaction will always be lower