|Human Physiology 6/e Fox|
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Chapter 4: Enzymes and Energy
Enzymes as Catalysts
I. Enzymes are biological catalysts.
A. Catalysts increase the rate of chemical reactions.
1. A catalyst is not altered by the reaction.
2. A catalyst does not change the final result of a reaction.
B. Catalysts lower the activation energy of chemical reactions.
1. The activation energy is the amount of energy needed by the reactant molecules to participate in a reaction.
2. In the absence of a catalyst, only a small proportion of the reactants possess the activation energy to participate in the reaction.
3. By lowering the activation energy, enzymes allow a larger proportion of the reactants to participate in the reaction, thus increasing the reaction rate.
II. Most enzymes are proteins.
A. Protein enzymes have specific three-dimensional shapes, which are determined by the amino acid sequence and, ultimately, by the genes.
B. The reactants in an enzyme-catalyzed reaction, called the substrates of the enzyme, fit into a specific pocket in the enzyme called the active site.
C. By forming an enzyme-substrate complex, substrate molecules are brought into proper orientation and existing bonds are weakened. This allows new bonds to be formed more easily.
Control of Enzyme Activity
I. The activity of an enzyme is affected by a variety of factors.
A. The rate of enzyme-catalyzed reactions increases with increasing temperature, up to a maximum.
1. This is because increasing the temperature increases the energy in the total population of reactant molecules, thus increasing the proportion of reactants that have the activation energy.
2. At a few degrees above body temperature, however, most enzymes start to denature, which decreases the rate of the reactions which they catalyze.
B. Each enzyme has optimal activity at a characteristic pH, called the pH optimum for that enzyme.
1. Deviations from the pH optimum will decrease the reaction rate because the pH affects the shape of the enzyme and charges within the active site.
2. The pH optima of different enzymes can be quite different, pepsin has a pH optimum of 2, for example, while trypsin is most active at a pH of 9.
C. Many enzymes require metal ions in order to be active. These ions are therefore said to be cofactors for the enzymes.
D. Many enzymes require smaller organic molecules for activity. These smaller organic molecules are called coenzymes.
1. Many coenzymes are derived from water-soluble vitamins.
2. Coenzymes transport hydrogen atoms and small substrate molecules from one enzyme to another.
E. The rate of enzymatic reactions increases when either the substrate concentration or the enzyme concentration is increased.
1. If the enzyme concentration remains constant, the rate of the reaction increases as the substrate concentration is raised, up to a maximum rate.
2. When the rate of the reaction does not increase upon further addition of substrate, the enzyme is said to be saturated.
II. Metabolic pathways involve a number of enzyme-catalyzed reactions.
A. A number of enzymes usually cooperate to convert an initial substrate to a final product by way of several intermediates.
B. Metabolic pathways are produced by multienzyme systems in which the product of one enzyme becomes the substrate of the next.
C. If an enzyme is defective due to an abnormal gene, the intermediates formed after the step catalyzed by the defective enzymes will decrease, and the intermediates formed prior to the defective step will accumulate.
1. Diseases that result from defective enzymes are called inborn errors of metabolism.
2. Accumulation of intermediates often results in damage to the organ that contains the defective enzyme.
D. Many metabolic pathways are branched so that one intermediate can serve as the substrate for two different enzymes.
E. The activity of a particular pathway can be regulated by end-product inhibition.
1. In end-product inhibition, one of the products of the pathway inhibits the activity of a key enzyme.
2. This is an example of allosteric inhibition, in which the product combines with its specific site on the enzyme, changing the conformation of the active site.
I. The flow of energy in the cell is called bioenergetics.
A. According to the first law of thermodynamics, energy can neither by created nor destroyed but only transformed from one form to another.
B. According to the second law of thermodynamics, all energy transformation reactions result in an increase in entropy (disorder).
1. As a result of the increase in entropy, there is a decrease in free (usable) energy.
2. Atoms that are organized into large organic molecules thus contain more free energy than more disorganized, smaller molecules.
C. In order to produce glucose from carbon dioxide and water, energy must be added as sunlight.
1. Plants use energy from the sun for this conversion, in a process called photosynthesis.
2. Reactions that require the input of energy to produce molecules with higher free energy than the reactants are called endergonic reactions.
D. The combustion of glucose to carbon dioxide and water releases energy in the form of heat.
1. A reaction that releases energy and thus forms products that contain less free energy than the reactants is called an exergonic reaction.
2. The same total amount of energy is released when glucose is converted into carbon dioxide and water within cells, even though this process occurs in many small steps.
E. The exergonic reactions that convert food molecules into carbon dioxide and water in cells are coupled to endergonic reactions that form adenosine triphosphate (ATP).
1. Some of the chemical-bond energy in glucose is, therefore, transferred to the "high energy" bonds of ATP.
2. The breakdown of ATP into adenosine diphosphate (ADP) and inorganic phosphate results in the liberation of energy.
3. The energy liberated by the breakdown of ATP is used to power all of the energy-requiring processes of the cell. ATP is thus the "universal energy carrier" of the cell.
II. Oxidation-reduction reactions are coupled and usually involved the transfer of hydrogen atoms.
A. A molecule is said to be oxidized when it loses electrons, it is said to be reduced when it gains electrons.
B. A reducing agent is thus an electron donor, and an oxidizing agent is an electron acceptor.
C. Although oxygen is the final electron acceptor in the cell, other molecules can act as oxidizing agents.
D. A single molecule can be an electron acceptor in one reaction and an electron donor in another.
1. NAD and FAD can become reduced by accepting electrons from hydrogen atoms removed from other molecules.
2. NADH + H+, and FADH2, in turn, donate these electrons to other molecules in other locations within the cells.
3. Oxygen is the final electron acceptor (oxidizing agent) in a chain of oxidation-reduction reactions that provide energy for ATP production.
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