Unit 7 Part I: Introductions to Biochemistry Chemical Reactions, Enzymes and ATP 19 March 2014 Averett 1
Reaction Graphs Every chemical reaction involves bond breaking and bond forming. In order for bonds to break they must first be stressed the addition of energy can do this. The reactants are then unstable, the bonds break and then all of the individual atoms enter the transition state. The atoms rearrange and then new bonds form creating products. 19 March 2014 Averett 2
At the top of the hill the reactants are in an unstable condition known as the transition state. 19 March 2014 Averett 3
The initial investment of energy for starting a reaction the energy required to stress the reactant molecules so the bonds can break is known as activation energy (E A ) You can think of activation energy as the energy needed to push the reactants over an energy barrier, or hill, so that the down hill part of the reaction can begin. 19 March 2014 Averett 4
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Activation energy is often supplied in the form of heat that the reactant molecules absorb from the surroundings. The bonds of the reactants break only when the molecules have absorbed enough energy to become unstable to enter the transition state. The heat stresses the bonds and also makes the reactants move around more therefore increasing the chance of, and the strength of, the collisions. 19 March 2014 Averett 6
For some reactions the energy in the surroundings at room temperature, or body temperature, is enough to provide the needed activation energy. For the majority of reactions the surroundings do not supply the needed energy. Without help these reactions would not occur fast enough and the organism would die! 19 March 2014 Averett 7
Adding heat would add enough energy to the reactants but what would happen if we added a lot of heat to our cells.. Denaturation! Therefore there needs to be another solution. Something else needs to stress the reactants so that they will enter the transition state This something is called an enzyme. 19 March 2014 Averett 8
Enzymes An enzyme is a protein that acts as a catalyst. It speeds up the rate of the chemical reaction without being used up, or changed by the chemical reaction. It is not a reactant or a product. Without enzymes an organisms metabolism would be too slow to maintain life! 19 March 2014 Averett 9
An enzyme catalyzes a reaction by lowering the activation energy barrier. An enzyme stresses the reactants physically so that less heat energy needs to be added to break the bonds The energy needed to break the bonds of the reactants will be less if the bonds are already stressed. Therefore the activation energy has been lowered. The result is that reactions will occur faster and more frequently than they would occur without an enzyme. This keeps organisms alive! 19 March 2014 Averett 10
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Substrate Specificity in Enzymes Enzymes are very specific for the reactions they catalyze Most enzyme names end in ase Example: Sucrase will catalyze the hydrolysis of sucrose The reactant an enzyme acts on is referred to as the enzyme s substrate So sucrose is the substrate for sucrase 19 March 2014 Averett 12
The region of the enzyme where the substrate binds is called the active site. 19 March 2014 Averett 13
The enzyme binds to its substrate (or substrates, when there are two or more reactants), forming an enzymesubstrate complex. 19 March 2014 Averett 14
The reaction catalyzed by each enzyme is very specific; an enzyme can recognize its specific substrate even among closely related compounds. STRUCTURE AND FUNCTION Remember that most enzymes are proteins and proteins have a unique shape based on its amino acid sequence and the four levels of structure. 19 March 2014 Averett 15
The shape of the active site must fit the shape of the substrate (Like a lock fits a key) As the substrate enters the active site, interactions between its chemical groups and those on the R groups of the amino acids that form the active site of the protein cause the enzyme to change its shape slightly so that the active site fits even more snugly around the substrate. This induced fit is like a clasping handshake Induced fit brings chemical groups of the active site into positions that enhance their ability to catalyze the chemical reaction 19 March 2014 Averett 16
Catalysis occurs in the active site. The substrate bonds to the active site Conversion of substrate to product Product leaves active site Enzyme is free to take another substrate molecule into its active site and repeat The entire cycle happens so fast that a single enzyme molecule typically acts on about a thousand substrate molecules per second. 19 March 2014 Averett 17
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Enzymes use a variety of ways to lower the activation energy and speed up a reaction. The active site provides a template on which the substrates can tome together in proper orientation for a reaction to occur between them. The active site of an enzyme clutches the substrates and puts physical pressure on the bonds stressing and bending the bonds that need to break. This weakens the bonds and lowers the energy needed to break them. Active site provides a microenvironment that is more conducive to the chemical reaction (right ph) Active site actually participates in the chemical reaction but returns to normal when the reaction is over 19 March 2014 Averett 19
Inhibition Certain chemicals inhibit the action of enzymes Competitive inhibition Bind to the enzyme at the active site preventing the substrate from binding. Compete for admission into the active site This kind of inhibition can be overcome by increasing the concentration of substrate so that as the active site does become available, substrate molecules have a better chance of getting there first! Example: Sarin nerve gas 19 March 2014 Averett 20
Allosteric inhibition Noncompetitive inhibitors Do not directly compete with the substrate to bind to the active site of the enzyme They bind to the enzyme at another site. This interaction causes the enzyme to change its shape in such a way that the active site can no longer convert substrate into product Example: Digitalis (substance extracted from foxglove plants) is a non-competitive inhibitor. It binds with the enzyme ATPase, which results in an increase in the contraction of heart muscle. 19 March 2014 Averett 21
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Feedback the output, or product, or a process regulates that very process. Negative feedback Accumulation of an end product of a process slows that process. Body temperature Positive feedback Accumulation of an end product speeds up its production. Blood clotting Platelets attract more 19 platelets March 2014 Averett 23
The activity of an enzyme how efficiently the enzyme functions is affected by general environmental factors such as temperature and ph. Optimal conditions The temperature or ph that favors the most active shape for the enzyme molecule 19 March 2014 Averett 24
Temperature Up to a point, the rate of an enzymatic reaction increases with increasing temperature. This is due to the substrates colliding with active sites more frequently when the molecules are moving faster. Above a given temperature, however, the speed of the enzymatic reaction drops sharply. To much of an increase in temperature will cause the enzyme to denature! The temperature at which an enzyme works best is the optimal temperature 19 March 2014 Averett 25
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ph Just as each enzyme has an optimal temperature, it also has a ph at which it is most active. The optimal ph values for most enzymes fall in the range of ph 6-8, but there are exceptions. Examples: pepsin - digestive enzyme in stomach optimal ph 2 Trypsin digestive enzyme in intestine optimal ph 8 19 March 2014 Averett 27
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