Biochemistry 2280 Topic 10.docx

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Western University
Biochemistry 2280A
Christopher Brandl

Topic 10 Biological forms of energy and reducing power Readings: p.82-84, 86-98, 104-114 By the end of this topic, you should be able to: • Differentiate betweenΔG andΔG° • Predict whether a chemical reaction is spontaneous based on itsΔG • Give examples of energetically favourable reactions the cell uses to drive unfavourable reactions forward, and explain the general mechanism by which this occurs Living organisms carry out a diverse set of tasks, such as building and maintaining physical structures, moving, synthesizing macromolecules, maintaining electrochemical gradients, and maintaining a constant body temperature.All of these processes require energy. One fundamental problem living organisms face is how to obtain energy to carry out these tasks. When considered at their most basic level, all of the activities mentioned above involve chemical reactions. Any chemical reaction can, in principle, proceed in one of two directions, in which the “reactants” are transformed into “products”, or vice versa. Whether or not a chemical reaction requires energy to proceed in a particular direction depends on the change in Gibbs free energy (ΔG, which equals G – G ) that products reactants takes place when the reaction occurs. TheΔG for a given reaction depends in part on the inherent characteristics of the reaction and the molecules involved, and in part on the molecules’concentrations and environmental factors. If ΔG for a reaction equals zero, then the reaction is said to be at equilibrium.At equilibrium, the forward and reverse reactions occur to the same extent and no net reaction takes place. If ΔG is negative, (i.e., <0), the forward reaction is more energetically favourable, or spontaneous (Fig 3-16, p.91). If all molecules begin at equal concentration, there will be a net conversion of reactants to products without addition of energy. This will continue until equilibrium is reached, at which time the concentration of products will be higher than that of reactants (Fig 3-18, p.93). However, if ΔG for a reaction is greater than zero, the forward reaction is energetically unfavourable, and at equilibrium reactants have a higher concentration than products. To take a specific example, the reaction of combining glucose and fructose to make sucrose (p.95, top right panel) has a standard free energy change (ΔG°) of +5.5 kcal/mole. (Note: the small circle inΔG° indicates that the value is calculated using the G for each molecule under “standard” conditions, with defined pressure and concentration. TheΔG° incorporates information about only the natures of the molecules involved, and not about the actual conditions under which the reaction occurs in a particular system. Even though the standard conditions are not normally met in biological systems, they are useful as a point of reference.) If sucrose is to be made in appreciable quantities, the natural tendency of the glucose and fructose molecules to remain apart must be overcome. One strategy to make products even when theΔG° is positive is to change the concentrations of the reactants and products, such that the ratio of reactants to products is high. This is frequently done in living systems, but is not always practical or desired. An alternate strategy to drive an unfavourable reaction forward is to couple that reaction to an energetically favourable reaction (i.e., one with a negativeΔG), such that the total ΔG for both reactions is negative (Fig 3-17, p.92; Fig. 3-30, p.105). If the totalΔG is negative, then both reactions will proceed spontaneously. Knowing this, the question becomes: is there a suitable energetically favourable reaction that organisms can couple to many different unfavourable reactions? If so, where do the reactants for the energetically favourable reaction come from? The vast majority of energy used to sustain life comes ultimately from the sun. Light energy is captured by plants and stored in carbohydrates (this process, photosynthesis, is not covered in this course). The plants can then use cellular respiration to break down the carbohydrates, releasing energy that the plants use to carry out processes necessary for life (Fig 3-9, p.87). Animals likewise use respiration to derive energy from sugars, fats, and proteins obtained by eating plants and other animals. However, the breakdown of (for example) sugar molecules like glucose is not used to directly power chemical reactions in the cell.Amajor reason for this is that a lot more energy is available in a glucose molecule than is needed to drive individual chemical reactions in the cell. Most of the energy would be wasted as heat. It is more desirable to convert the energy available in glucose into a form that can be used in small amounts, a little at a time, as needed (Fig 13-1, p.426). The universal energy carrier in biological systems is adenosine triphosphate (ATP; Fig 2-23, p.57, and Movie 2.3).ATP is a nucleotide and one of the building blocks for RNA; the general structure of nucleotides is covered in Topic 18. Hydrolysis ofATP to formADP and inorganic phosphate (P) is energetically favourable (ΔG° of -7.3 kcal/mol). Combining the hydrolysis ofATP with unfavourable reactions such as synthesis of sucrose from glucose and fructose can result in an overall reaction that is favourable (p.95). Another example of an energetically unfavou
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