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Microbiology, 4/e Prescott, Harley, Klein | ||||||
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Microbiology, 4/e Prescott, Harley, Klein | ||||||
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8 Metabolism: Energy and Enzymes
CHAPTER OVERVIEW
This chapter discusses energy and the laws of thermodynamics. The participation of energy in cellular metabolic processes and the role of adenosine-5¢-triphosphate (ATP) as the energy currency of cells is examined. The chapter concludes with a discussion of enzymes as biological catalysts and the ways in which enzymes work and are affected by their environment.
CHAPTER OBJECTIVES
After reading this chapter you should be able to:
o discuss the first and second laws of thermodynamics and show how they apply to biological systems
o discuss enthalpy, entropy, and free energy and their application to biological reactions
o discuss the use of ATP as the energy currency of the cell and show how it is used to couple energy-yielding exergonic reactions with energy-requiring endergonic reactions
o discuss reduction potential and its relationship to exergonic and endergonic processes
o describe the role of enzymes in the catalysis of biological reactions, and discuss the ways in which enzymes are influenced by their environment
CHAPTER OUTLINE
I. Energy and Work
A. Energy cycle
1. Cells must efficiently transfer energy from their energy-trapping systems to the systems that actually carry out work
2. Cells must also use various metabolic processes to replace the energy used in doing work
3. Energy currency is adenosine 5¢-triphosphate-ATP
B. Cellular work
1. Chemical work-synthesis of complex molecules
2. Transport work-nutrient uptake, waste elimination, ion balance
3. Mechanical work-internal and external movement
C. The ultimate source of all biological energy is visible sunlight through photosynthesis
D. Complex molecules manufactured by photosynthetic organisms serve as a carbon source and energy source (through aerobic respiration) for chemoheterotrophs
E. The major energy currency in a living cell is adenosine-5'-triphosphate (ATP)
II. The Laws of Thermodynamics
A. First law-energy can be neither created nor destroyed
1. The total energy in the universe remains constant
2. Energy may be redistributed either within a collection of matter called a system, or between the system and its surroundings
3. Energy is measured in calories where 1 calorie is the amount of heat energy needed to raise 1 gram of water from 14.5 to 15.5°C
B. Second law-physical and chemical processes proceed in such a way that the disorder of the universe increases to the maximum possible
III. Free Energy and Reactions
A. Free energy change (DG) is the amount of energy in a system that is available to do work
1. A negative DG indicates that the reaction is favorable and will proceed spontaneously (i.e., the reaction is exergonic)
2. A positive DG indicates that the reaction is unfavorable and will only proceed if energy is supplied (i.e., the reaction is endergonic)
IV. The Role of ATP in Metabolism
A. The reaction in which the terminal phosphate of ATP is removed goes to completion with a large negative standard free energy change (i.e., the reaction is strongly exergonic)
B. Exergonic breakdown of ATP can be coupled with various endergonic reactions to facilitate their completion
C. Metabolic-energy-trapping processes are used to catalyze the formation of ATP from ADP and Pi, and thus to restore the energy balance of the cell
V. Oxidation-Reduction Reactions and Electron Carriers
A. Oxidation-reduction (redox) reactions involve the transfer of electrons from a donor (reducing agent or reductant) to an acceptor (oxidizing agent or oxidant)
B. The equilibrium constant for the reaction is called the standard reduction potential (E0) and is a measure of the tendency of the reducing agent to lose electrons
C. Biological cells use electron carriers to transfer electrons from a reductant to an acceptor with a greater, more positive reduction potential, and they thereby allow the release of free energy, which is often used in the formation of ATP
D. Biological cells have a variety of electron carriers, and each is used in particular types of redox reactions; the particular carrier used in any given reaction will depend on the nature and location of the reaction
VI. Enzymes
A. Structure and classification of enzymes
1. Enzymes are protein catalysts with great specificity for the reaction catalyzed and the molecules acted upon
2. A catalyst is a substance that increases the rate of a reaction without being permanently altered itself
3. The reacting molecules are called substrates and the substances formed are the products
4. An enzyme may be a pure protein, or it may be a holoenzyme, which consists of a protein component (apoenzyme) and a nonprotein component (cofactor)
a. Prosthetic group-a cofactor that is firmly attached to the apoenzyme
b. Coenzyme-a cofactor that is loosely attached to the apoenzyme; it may dissociate from the apoenzyme and carry one or more of the products of the reaction to another enzyme
B. The mechanism of enzyme reactions
1. Enzymes increase the rate of a reaction, but do not alter its equilibrium constant (or its standard free energy change)
2. Enzymes lower the activation energy required to bring the reacting molecules together correctly to form the transition-state complex, which resembles both the substrates and the products; once the transition state has been reached the reaction can proceed rapidly
3. Enzymes lower activation energy in several ways:
a. Local concentrations of the substrates are increased at the active (catalytic) site of the enzyme
b. Molecules at the active site are oriented properly for the reaction to take place
C. The effect of environment on enzyme activity
1. The amount of substrate present affects the reaction rate, which increases as the substrate concentration increases until all available enzyme molecules are binding substrate and converting it to products as rapidly as possible; no further increase in rate occurs with subsequent increases in substrate concentration-the reaction is then said to be proceeding at maximal velocity (Vmax)
2. The Michaelis constant (Km) of an enzyme is the substrate concentration required for the reaction to reach half maximal velocity and is used as a measure for the apparent affinity of an enzyme for its substrate
3. Enzyme activity is affected by alterations in pH and temperature; each enzyme has specific pH and temperature optima
4. If the temperature rises too much above the optima, an enzyme's structure will be disrupted and its activity lost; this phenomena, known as denaturation, may be caused by extremes of pH or temperature extremes or by other factors
D. Enzyme inhibition
1. Competitive inhibition occurs when the inhibitor binds at the active site and thereby competes with the substrate (if the inhibitor binds, then the substrate cannot, and no reaction occurs); this type of inhibition can be overcome by adding excess substrate
2. Noncompetitive inhibition occurs when the inhibitor binds to the enzyme at some location other than the active site, and changes the enzyme's shape so that it is inactive or less active; this type of inhibition cannot be overcome by the addition of excess substrate