Chemical reactions require a specific amount of energy in order to occur; this is known as the activation energy. The way that enzymes function in catalysis is by lowering this activation energy. If you look at the graphs to the right, the dotted red line is the reaction using the enzyme. The lower amount of energy required is what allows enzymes to speed up reactions.

An enzymatic reaction can be exergonic, which releases energy overall, or endergonic which takes energy in. An exergonic reaction is usually catabolic, meaning it breaks molecules apart, while an endergonic reaction is usually anabolic, meaning it builds a larger, more complex molecule.

In the lock and key model of enzyme function, the active site and substrate precisely complement each other. This model explains why enzymes have specificity and will only work with a specific substrate or substrates.

In the induced fit model of enzyme function, the substrate loosely binds, upon which a conformational change (change in protein shape) will occur in the active site, improving binding. This model explains why some enzymes have broad specificity and can work with multiple substrates.

When there are molecules that are structurally and chemically similar enough to a substrate that they can bind to an active site, competitive inhibition can occur. These competitive inhibitors can bind either reversibly or irreversibly to the active site, but either way it prevents the substrate from properly binding to the active site and stops the enzyme-substrate reaction from occurring. Increasing substrate concentration can lessen the effects of the inhibitor as it will make it less likely for the inhibitor to bind to the active site when compared to the substrate.

Noncompetitive inhibitors function by binding to the allosteric site, rather than the active site. This results in a conformational change in the active site and changes the activity of the enzyme. As this type of inhibitor is not in direct competition, increasing the substrate concentration has no effect on the effect of the inhibitor.

If you haven't learned it yet, form fits function. The structure of an enzyme determines its function, by determining which substrates it can interact with. It should make sense, based on this, that if the structure changes, the enzyme will not be able to properly function.

When an enzyme's structure is disrupted due to conditions such as a strong change in temperature or pH, which in particular can change hydrogen bonds, the ability of the enzyme to catalyze reactions is reduced or eliminated.

In some situations, it is possible for the denaturation to be reversed, although first the adverse condition that resulted in the denaturation must be removed. A special protein known as a chaperone protein can aid in the refolding of the enzyme, allowing it to regain its catalytic activity.

Just like how extreme conditions can result in the disruption of enzymatic activity, there are conditions that can improve enzyme activity.

Each enzyme functions best within a specific temperature and pH range, a range which varies depending on the individual enzyme. There are optimal conditions that maximize the ability of the enzyme to bind to substrates.

In addition, the higher the environmental temperature, the faster molecules in a solution are moving. If the temperature increases, but not to the point that it disrupts hydrogen bonds and therefore structure, it will increase the frequency of enzymes and substrates colliding, which will increase the reaction rate.