ENZYMES

Enzymes Enzymes are biological catalysts, they increase the rate of over 4000 reactions in the body. The name of an enzyme often ends in –ase. Ex. amylase (breaks down starch), maltase (breaks down maltose).

Most Enzymes are Proteins Most enzymes are proteins. Thus their shape is important to their function. Each enzyme works only with a specific reaction. The reactants that fit in the enzyme are called the substrate. An enzyme is not changed when it performs its function, it can be reused over and over.

Enzyme Structure The 3D structure of the enzyme has grooves or pockets. The active site is the groove where the substrate(s) bind. There is only one active site on an enzyme. Enzyme-substrate complex: an enzyme with its substrate attached to the active site.

Enzyme Structure The enzyme may also have an allosteric site where inhibitors or activators can bind. The allosteric site is far away from the active site. When substances bind there they affect the shape of the enzyme and the active site.

How an Enzyme increases Rate All reactions require energy. The Activation energy (E A ) is the amount of energy needed to ensure a reaction proceeds (ex. Striking the flint over the bunsen burner).

How an Enzyme increases Rate At the peak of the activation energy the reaction reaches the transition state, where the reactants bonds are breaking and product bonds are starting to form.

How an Enzyme increases Rate Enzymes increase the rate of reaction by lowering the activation energy. They do this by: straining or weakening the bonds in the substrate molecule helping to position the molecules in the correct position. DEMO

Lock and Key Model In 1894, Emil Fischer suggested that the active site and substrate have complimentary geometric shapes and fit into each other like a key into a lock. The model explained how specific enzymes were but did not give the enzyme flexibility to stabilize the transition state.

Induced-Fit Model In 1958, Daniel Koshland modified the lock and key model. His Induced-fit model states that the substrate causes the enzyme to change its shape to better hold the substrate.

Induced-Fit Model As the substrate approaches, its functional groups interact with the enzyme’s functional groups and cause a slight shape change. Active sites were seen as ‘flexible’ and not rigid structures like before. The active site returns to its original shape when the products leave (the enzyme remains unchanged and is resusable ).

Allosteric Activation Some enzymes require a molecule known as an activator to attach to their allosteric site in order for the active site to be receptive to substrate.

Allosteric Inhibition Noncompetitive Inhibitors: substances that attach to the allosteric site and change the shape of the active site to prevent the substrate from binding to it.

Allosteric Inhibition Feedback Inhibition Many enzyme-catalyzed reactions in the body occur in a sequence. Feedback inhibition is when the product(s) of one reaction allosterically inhibit(s) an enzyme of a previous reaction. This is a means of homeostasis/regulation.

Competitive Inhibition Competitive Inhibitors: an inhibitor competes with the substrate for access to the active site of the enzyme.

Inhibition by Location Restricting enzymes to certain locations within the cell helps to regulate activity. These locations include: incorporation into specific organelle membranes or fluid- filled spaces (cytoplasm / inside organelles) – we will see more of this in unit 2.

Factors affecting the Rate of Enzyme Activity Since most enzymes are proteins they work best under an optimal range of pH and temperature. Outside of this range they will denature.

Temperature temp = rate of reaction The same is true for enzyme controlled reactions. However, if the temperature reaches beyond a critical point, denaturation of the enzyme may occur and since the enzyme is no longer functioning the reaction will slow down. (Body enzymes : 37  C)

pH Most enzyme also have an optimum pH, since their 3D structure is dependent on interactions of side chains, which only form in the proper environmental conditions. Above and below the optimum pH, the reaction will slow down. Ex. digestive enzymes

Concentration / Limiting Factor Number of enzymes: When the amount of substrate is increased, the enzymes become saturated, the reaction rate will plateau. Adding more enzymes will speed up a reaction until the concentration of the substrate is the limiting factor.

Other factors Some enzymes need other components before they work properly: Cofactors: non-protein components that bind to the enzyme active site or the substrate to help the enzyme (ex. Zn) Coenzymes: these compounds often shuttle molecules from one enzyme to another (ex. NAD +, FAD).

Industrial Uses of Enzymes