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Three important factors which impact the rate at which enzymes catalyze reactions are substrate concentration, temperature, and pH. 1. Substrate concentration is important because at low substrate concentrations, many enzymes are just waiting around for a substrate. At high concentrations, all the enzymes are hard at work, and the extra substrates are waiting for a free enzyme. So, as substrate concentration increases, the amount of enzyme activity saturates, as shown in this graph. 2. In addition, the rate of activity of an enzyme is highest within its optimal temperature range. Enzyme activity occurs randomly when substrate and enzyme bump together and bind. At low temperatures, molecules have low kinetic energy and are unlikely to randomly bump into each other. As temperature increases, the random interactions happen more frequently. As the temperature increases past a certain point, however, the enzymes start to denature, or fall apart, resulting in a decrease in enzyme activity. 3. Enzymes typically work best in a narrow pH range. pH affects enzyme activity by affecting the structure of the enzymes themselves. Enzymes have many ionizable side chains and prosthetic groups which affect the intermolecular bonds which hold together their secondary and tertiary structure (pop-up: if you need a review of these concepts, see my video on protein folding – link in the description). Maintaining the correct secondary and tertiary structure is critical for the proper alignment and functioning of the active site. Outside of an optimal pH range, changes to ionizable side chains can break bonds holding together secondary and tertiary structures, causing the active site to fall apart, and decreasing enzymatic activity. In addition, the substrate binding to the enzyme might have its own ionizable side chains, which can also be affected by pH. Substrate concentration, temperature, and pH are not the only factors affecting enzyme activity. Cells also have some control over enzymatic activity. For example, they can assert control through the use of post-translational mechanisms such as phosphorylation and glycosylation, each of which can increase or decrease enzymatic activity. In addition, cells in different organs produce different enzymes, and each cell has control over how much of each enzyme it produces as well as which enzymes go to which organelles. For example, digestive enzymes are found in the stomach, but not in the brain. There are also substances that increase enzymatic activity, called activators, and substances that decrease enzymatic activity, called inhibitors. Inhibitors can interact either reversibly or irreversibly with enzymes. Reversible inhibitors interact non-covalently with enzymes. Irreversible inhibitors usually associate with the enzyme through covalent interactions, and so decrease the concentration of active enzyme. There are three major categories of reversible inhibitors: competitive, non-competitive, and uncompetitive. Normally, a substrate binds to an enzyme at the active site, after which the enzyme catalyzes a reaction and releases the products. Competitive inhibitors compete with the substrate for access to the active site, where they bind the enzyme. Hence, they can only bind to free enzymes, which are not bound to a substrate. Non-competitive inhibitors not only bind to free enzymes, but also to those bound to substrates. This means that they bind not to the active site, but to the so-called allosteric site. In doing so, they cause a conformational change that increases or decreases activity of the enzyme. Uncompetitive inhibitors only bind to enzymes when the enzymes are bound to substrates, and they bind in such a way that the enzyme cannot release its products. PROTEIN FOLDING VIDEO - • PROTEIN FOLDING Backgrounds from Nebula from RocketStock