 Hi, I'm Sally, I'm a third year PhD student at the Australian National University and I work on an enzyme called Rubisco. So enzymes are biological catalysts. They work to allow reactions to happen under biological conditions and a lot of different things affect how enzyme functions and how well it functions. Enzymes are biological catalysts, usually made of proteins that allow reactions to occur under biological conditions. A number of factors will affect the rate at which and how well enzymes work. And most enzymes are specifically adapted to work in a very limited range of conditions. Temperatures can have massive effects on enzymes. If you consider warm-blooded organisms like people, the cellular environment in which enzymes operate is constantly maintained at 37 degrees Celsius. For many enzymes, if they get too hot, the bonds that hold the proteins together in their secondary and tertiary structures break and the enzymes lose their shape. This is called denaturing and when that happens they can no longer perform chemical reactions. Chemical reactions in biology are still chemical reactions and this means that energy in the form of heat makes them go faster. At low temperatures there isn't much energy and things move slowly, reactions go more slowly. This means that for enzymes the fastest rate is usually an optimization between a temperature at which it's warm enough for the substrates and the enzyme to be moving about but cool enough that the enzyme still maintains its tertiary structure. And because most enzymes are made of proteins, they're sensitive to the same things that proteins are sensitive to. So proteins are made up of amino acids and each of the 20 amino acids differs by the side chains that those amino acids have and some of these side chains are sensitive to pH. They might change their shape or their charge at different pHs. This has a flow on effect to the rest of the protein and can result in differences in the way that protein functions. Enzyme activity can also be altered by the presence of other compounds in the solution it's working in. Competitive inhibition occurs when a substrate similar to substrate of the enzyme is added. This inhibitor also fits nicely into the active side of the protein. When the non-substrate substance is bound it prevents the substrate from binding and this reduces the rate of reaction. Non-competitive inhibition occurs when the inhibitor doesn't bind to the active site but to another part of the enzyme. When the inhibitor is bound the shape of the active site changes and the substrate can no longer bind. This reduces the rate of reaction. Inhibition doesn't only occur by the action of non-substrate substances though. In some cases the presence of large concentrations of the substrate or the product can result in inhibition of the enzyme. High concentrations of the product means that the product is more likely to re-bind to the active site than the substrate is. This means that the reactions occur less frequently and the rate of reaction slows. Processes of enzyme inhibition are important biological processes that are closely regulated within organisms. Rubiscoe is a really important enzyme in photosynthesis. It's the enzyme in plants that fixes carbon dioxide from the atmosphere beginning the process of converting carbon dioxide into sugars. It does this by fixing the CO2 to a sugar phosphate called RUBP. Because photosynthesis uses light from the sun it only occurs during the day. At night CA1P another sugar phosphate that binds the active site but doesn't bind CO2 binds to rubiscoe and this switches off the enzyme by competitive inhibition. It's then removed by another enzyme in the morning so that rubiscoe can start its work again. These processes of inhibition are important in regulating and maintaining enzyme activity in organisms.