I. Enzyme Action Figure 1: Activation Energy (Ea) Activation Energy (Ea):

Transcription

1 I. Enzyme Action Figure 1: Activation Energy (Ea) *Activation energy represents an energy barrier that reactant molecules must overcome in order to react & form products. Think of an Olympic track star trying to jump a hurdle they can only overcome this barrier if they possess sufficient energy. Activation Energy (Ea): a) The E a required for a given reaction is usually provided in the form of heat absorbed from the surroundings. As heat is absorbed, the molecules collide more frequently & forcefully (this makes the bonds more likely to break, allowing new ones to form). Figure 1.1: Activation Energy: Differences in Ea As seen in figure 1.1, for some reactions the E a is low enough that there is enough heat energy at room temperature for the reactants to react & form products. As a result, these reactions occur very quickly. For others, E a is so high the 1

2 reactants must be heated in order to posses enough energy in order to react & form products (such reactions occur slowly). Figure 2: Enzymes Enzymes are protein catalysts, substances that increase the rate of a chemical reaction, but are not themselves consumed by it. Enzymes help reactant molecules to overcome the Ea barrier so vital reactions may occur at temperatures that would not harm the cell Figure 2.1: Enzymes & Activation Energy *The enzyme is NEVER permanently altered during the reaction it controls. Only the substrate is altered (enzyme is therefore reusable). Enzymes increase the rate of chemical reactions without an increase in temperature by lowering the amount of E a required for a given reaction (see figure 2.1). 2

3 Enzymes lower Ea for a reaction by first binding to particular substrates within a specialized binding sited called an Active Sites. Once within the active site, the enzyme interacts with the substrate(s) in the following ways: a) Bonds within substrates can be either stressed to the point of breaking. b) Substrates may be brought together in the correct orientation so their functional groups can react. c) Both of these actions decreases the reaction s dependence on heat, thus resulting in a lower Ea requirement. As a result, the reaction can occur at a significantly higher rate, producing enough vital products to keep a cell alive. Enzyme: Enzyme-Substrate Complex: Figure 2.2: Enzyme Binding Models: Lock & Key Model Lock & Key Model: a) A major problem with this model is that some enzymes do not have active sites precisely complementary to their substrates. As a result in was ultimately abandoned as a possible explanation for enzyme-substrate interactions. Figure 2.3: Enzyme Binding Models: Induced Fit Model Induced Fit Model: 3

4 I. Factors Affecting Enzyme Activity Graph 1: Substrate Concentration vs Enzyme Action Reaction rate initially increases as substrates are added to a fixed amount of enzyme, until a Saturation Point is reached. At this point, all enzymes are reacting with a substrate & the maximum reaction rate is achieved. As substrates continue to be added, the rate of the reaction will no longer increase, but rather remain constant. Graph 2: Temperature vs Enzyme Action As temp rises, the rate of enzyme-controlled reaction increases. Once an Optimum Temperature is reached, the enzyme functions at its greatest efficiency human enzymes work best at body temperature (37C). Temperatures greater than the optimum result in the denaturation & the loss of the enzyme s active site, causing reaction rates to crash. 4

5 Graph 3: ph vs Enzyme Action All enzymes work best at an Optimum ph, with those in human cells working best usually at or near a ph of 7. Select digestive enzymes work best at a much lower ph. In most cases, extreme acidic or basic ph s will result in denaturation & the loss of the enzyme s active site, causing the reaction rate to crash. Figure 3: Factors Affecting Enzyme Action: Competitive vs Noncompetitive Inhibition Competitive Enzyme Inhibition Noncompetitive (Allosteric) Inhibition During Competitive Inhibition, an inhibitor molecule that is similar to the normal substrate, which binds to & blocks the enzyme s active site, reducing reaction rates. It can be prevented by increasing the concentration of the normal substrate. During Noncompetitive Inhibition, inhibitor molecules bind tightly to an allosteric site (alternative binding site) on the enzyme to alter the shape of the active site. This reduces its ability to bind to the normal substrate, reducing 5

6 Figure 3.1: Noncompetitive Inhibition & Homeostasis The inhibitor molecules that function to shut down enzyme activity are usually the end-products of a metabolic pathway (synthesis of the amino acid histidine in figure 3.1). By binding to an allosteric site on one of the enzymes (thereby inactivating it) in this pathway, the end product prevents its own over-production (a waste of cell resources & energy). The above is an example of Negative Feedback, in which a change within a system (in this case the production of histidine) is, at some critical point, resisted to prevent it from becoming too large. By contrast, Positive Feedback involves the intensification of a change within a system. 6

7 7