Enzymes

• Enzymes are biological catalysts, they speed up chemical reactions and catalyse metabolic reactions - both at a cellular level and for the organism as a whole.
• Enzymes can affect structures in an organism as well as functions
• Enzyme action can be intracellular - within cells, or extracellular - outside cells
• Enzymes are globular proteins and they have an active site which has a specific shape, this is where the substrate molecules bind to
• The specific shape of the active site is determined by the enzymes tertiary structure
• For the enzyme to work, the substrate has to fit into the active site otherwise it will not work
• Enzymes reduce activation energy by lowering the activation energy of the reaction, this often makes reactions happen at a lower temperature. This speeds up a reaction
• When a substance binds to an enzymes active site, an enzyme-substrate complex is formed. This formation lowers the activation energy because either attaching to the enzyme holds them closer together, reducing any repulsion between the molecules so they can bond easier or if the enzyme is catalysing a breakdown reaction, fitting into the active site puts a strain on bonds in the substrate. This strain means the substrate molecules breaks up more easily
• The 'lock and Key' hypothesis only works with substrates that fit their active site. This hypothesis is where the substrate fits into the enzyme in the same way that a key fits into a lock.
• The induced fit model is a better theory because it explains why enzymes are so specific and only bond to one particular substrate, the substrate can make the active site change shape in the right way
• Temperature has a big influence on enzyme activity, the rate of an enzyme reaction increases when the temperature increases. More heat means more kinetic energy so molecules move faster, causing more collisions with the substrate molecules. The energy of the collisions also increases but if there is too much heat then the enzymes denature.
• The temperature coefficient or Q10 value for a reaction shows how much the rate of a reaction changes when the temperature is raised by 10 degrees.
• The equation for the temperature coefficient is rate at higher temperature/ rate at lower temperature
• pH also affects enzyme activity. All enzymes have an optimum pH value, most human enzymes work best at pH 7 with some exceptions like Pepsin which works best at acidic pH 2 because it is in the stomach.
• Above and below the optimum pH, the H+ ions and OH- ions found in acids and alkalis can mess up the ionic bonds and hydrogen bonds that hold the enzymes tertiary structure in place. This causes the active site to change and the enzyme will become denatured
• Enzyme concentration affects the rate of reaction, the more enzyme molecules there are in a solution, the more likely a substrate molecule is to collide with one and form an enzyme-substrate complex. Increasing the concentration of the enzyme increases the rate of reaction.
• If the amount of substrate is limited, there comes a point when theres more than enough enzyme molecules so adding more enzyme has no further effect
• Substrate concentration affects the rate of reaction up to a point. The higher the substrate concentration, the faster the reaction. More substrate molecules means a collison is more likely and so more active sites will be used
• This is only true up to the saturation point. After that, there are so many substrate molecules that all the active sites become full and adding more substrate makes no difference to the rate of reaction
• Substrate concentration decreases with time during a reaction so if no variables get changed then the rate of reaction will decrease over time. This means that the initial rate of reaction is the highest rate of reaction
• One way to measure the rate of enzyme controlled reaction is to use the enzyme catalase, it catalyses the breakdown of hydrogen proxide into water and oxygen. You use an upside down measuring cylinder, delivery tube and hydrogen peroxide solution and catalase enzyme
• You can also measure the disappearance of the substrate rather than the appearance of the product and use this to compare the rate of reaction under different conditions.
• Cofactors and coenzymes are essential for enzymes to work, some cofactors are inorganic molecules or ions, they help by helping the enzyme and substrate bind together, they do not directly participate in this reaction so they are not used up or changed in any way
• Some cofactors are organic molecules, they are called coenzymes and participate in the reaction and are changed by it, they often act as carriers, moving chemical groups between different enzymes and are continually recycled during this process.
• If a cofactor is tightly bound to the enzyme it is known as a prosthetic group
• Competitive inhibitor molecules have a similar shape to that of the substrate molecules and they compete with the substrate molecules to bind to the active site but no reaction takes place and instead they block the active site so no substrate molecules can fit in it
• Non-competitve inhibitor molecules bind to the enzyme away from its active site, this site is known as the enzymes allosteric site. This causes the active site to change shape so the substrate molecules can no longer bind to it. They do not compete with the substrate molecules because they are a different shape
• Inhibitors can be reversible or non-reversible
• If the inhibitors are strong, covalent bonds then the inhibitor cannot be removed easily and the inhibition is irreversible
• If the inhibitors are weaker, hydrogen bonds or weak ionic bonds then the inhibitor can be removed and the inhibition is reversible
• Some drugs and metabolic poisons are enzyme inhibitors, medicinal drugs are enzyme inhibitors and metabolic poisons interfere with metabolic reactions causing damage, illness or death
• A metabolic pathway is a series of connected metabolic reactions. Most enzymes are inhibited by the product of the reaction they catalyse (product inhibition)
• End-product inhibition is when the final product in a metabolic pathway inhibits an enzyme that acts earlier on in the pathway, a nifty way of regulating the pathway and controlling the amount of end-product that gets made
• Both product and end-product inhibition are reversible so when the level of product starts to drop, the level of inhibition will start to fall and the enzyme can start to function again, meaning more product can be made
• Enzyme inhibition can help to protect cells, enzymes are sometimes synthesised as inactive precursors in metabolic pathways to prevent them causing damage to cells.
• Part of the precursor molecule inhibits its action as an enzyme. Once it has been removed the enzyme becomes active