Enzymes

A. Are Proteins (usually) that speed up metabolic reactions by lowering the activation energy. A. Some chemical reactions will occur spontaneously, but it may occur too slowly to be effective in living cells. Example: ___________________ B. Biochemical reactions require enzymes to speed up and control reaction rates. (Catalyst)

A. A catalyst is a substance that speeds up the rate of a chemical reaction without being changed or used up. B. An enzyme may accelerate a reaction by a factor of – that is, making it happen 10,000,000,000 times faster. A reaction that might normally take 1500 years would only take 5 seconds with an enzyme.

Before a reaction can occur, the reactants must absorb energy to break chemical bonds. This initial energy investment is the activation energy (Ea). (the graph on the right)

This is how enzymes speed up the rate of a reaction

B. Substrate specific 1. Enzymes are specific for a particular substrate. a. Substrate – The substance an enzyme acts on and makes more reactive. b. The enzyme is released in original form after thereaction.

Maltose is the Substrate

2. Active Site – Restricted region of an enzyme molecule which binds to the substrate. a. Is usually a pocket or groove on the protein’s surface. b. Determines enzymes specificity. c. Changes its shape in response to the substrate. d. Induced fit – change in the shape of an enzyme’s active site, which is induced by the substrate.

Induced Fit

C. Active Site 1. The enzymatic cycle is very fast. Step 1 – substrate binds to the active site forming an enzyme-substrate complex. (Held together by weak interactions) Step 2 – Induced fit of the active site around the substrate(Conversion of substrate to product) Step 3 – Product departs active site and the enzyme emerges in its original form.

1. Enzymes lower activation energy and speed up reactions. a. Active site can hold two or more reactants in the proper position so they may react. b. Induced fit of the enzyme’s active site may distort the substrate’s chemical bonds so less energy is needed to break them during the reaction. c. Active site might provide a micro-environment conducive to a particular type of reaction.

2. Initial substrate concentration partly determines the rate of an enzyme controlled reaction. a. The higher the substrate concentration, the faster the reaction – up to a limit. b. If high enough (substrate concentration), the enzyme becomes saturated with substrate. c. If saturated, the reaction rate depends upon how fast the active sites can convert substrate to product. d. Add more enzymes to increase reaction rate.

D. Physical and Chemical Environments 1. Each enzyme has optimal environmental conditions that favor the most active enzyme conformation. 2. Temperature and pH a. Optimal temperature allows the greatest number of molecular collisions without denaturing enzymes. 35 – 40 degrees Celsius b. Optimal pH range for most enzymes is pH 6 – 8

3. Cofactors – Small non-protein molecules that are required for proper enzyme catalysis. a. May bind tightly to active site. b. May bind loosely to both active site and substrate. c. Some or inorganic (Zn, Fe, or Cu) d. Some are organic and are called coenzymes (vitamins)

4. Enzyme Inhibitors a. Certain chemicals can selectively inhibit enzyme activity. 1. Irreversible – if the inhibitor attaches by covalent bonds 2. Reversible – if the inhibitor attaches by weak bonds. b. Competitive inhibitors – chemicals that resemble an enzyme’s normal substrate and compete with it for the active site. 1. Blocks active site from the substrate.

c. Noncompetitive inhibitors – Enzyme inhibitors that do not enter the enzyme’s active site, but bind to another part of the enzyme molecule. 1. Cause enzyme to change its shape so the active site cannot bind substrate. 2. May act as a metabolic poison (DDT and antibiotics) 3. Selective inhibition is an essential mechanism in the cell for regulating metabolic reactions.