Lecture 11. February 14, 2003 R. Jones Chapters 10.6, 2.17, 3.11-3.18 and 5-5.8
Metabolism: Energy, Thermodynamics and Enzymes
1. Using recombinant DNA technology, i.e. genetic engineering, its is possible to introduce specific new genes into aninals and plants that may confer benefits. One way that this is done in plant is to use a plasmid from the bacterium Agrobacterium tumefaciens. The most recent example of beneficial transformation (introduction of a new gene or genes into a plant) was reported in rice. So-called golden rice has been made where a gene encoding vitamin E synthesis has been introduced into rice. Rice grains harboring this gene have higher vitamin E levels. Vitamin E is a major vitamin deficiency in Asia and this strategy is aimed at relieving vitamin deficiency, especially in children. Recombinant DNA technology can also be abused and like all technologies there are overwhelmingly negative and positive aspects.
2. The phenotype of an organism is expressed as proteins: The One Gene-One Enzyme Hypothesis: This is now understood in the context of the DNA--> RNA--> Protein paradigm of molecular biology. Garrod and inherited diseases such as alkaptonuria and Beadle and Tatum and metabolic mutants of Neurospora, a mold (see section 10.6 of text).
3. Almost all proteins are enzymes. Although there are many so-called structural proteins, even some of these are thought to act as enzymes. (Note here that not all enzymes are proteins because some RNA molecules can act as enzymes, known as ribozymes). Enzymes are important components of energy transformations in cells. Enzymes are proteins and are therefore the direct products of genes. Read sections 3.11-.318 for more information on protein composition structure.
4. Living things obey the physical laws of the universe as far as energy transformations are concerned and organisms obey the First and Second Laws of Thermodynamics. The First Law states that energy can neither be created nor destroyed. Energy is transformed by living systems-light to sugar for example in photosynthesis. The efficicency of these energy conversions is generally low, however. Most mechanical machines operate with ~ 25% efficiency. Cells can operate at about 40%. See Chapter 5-5.8.
5. The Second Law of Thermodynamics also says that during energy transformations matter goes from a state of high order to one of a lower state of order. Energy is not only lost as heat or light but energy is also lost as entropy, a category of energy that relates to the state of order in a system.
6. The release of energy by striking a match illustrates the fundamental difference between the reactions of chemistry and those of biochemistry and metabolism.When a match is struck, energy is put into the system to inititate ignition then the match burns dissipating energy very rapidly as heat and light. In biological systems reactions do not occur this way.
7. Enzymes function as catalysts for the biological reactions of metabolism. Their action is coupled to coenzymes such as NAD and NADP that are involved in oxidation/reduction reactions and lead to ATP synthesis. These compounds are important in capturing the energy released in enzyme-catalyzed reactions.
8. Enzymes reduce the activation energy of a reaction and enzymes themselves do not become changed. The reactants and products of enzyme catalyzed reactions are also not changed by the presence of the enzyme and the net energy change (delta G) also remains unchanged.
9. Endergonic reactions energy must be put into the reactants and these types of reactions are synthesis reactions, e.g. photosynthesis, whereas the exergonic reactions such as those of metabolism release energy as in the breakdown of sugar in cells.
10. The breakdown of hydrogen peroxide (H2O2) is exergonic and can be used to demonstrate the effectiveness of catalysts and enzymes. H2O2 can be broken down into H2O and O2 with a net free eneergy change (delta G) of -25 calories. For this reaction to proceed a lot of heat energy must be used equivalent to 18 calories.This is called the energy of activation. If the metal platinum, a common metal catalyst, is put into the H2O2 the amount of energy needed to inititate the reaction is reduced to 12 calories. However, when the enzyme catalase is added the energy of activation is reduced to 2 calories.