Spontaneity in Biochemical Reactions

How can we predict what reactions will happen in living cells  

The sign of the change in free energy, AG, indicates the direction of the reaction  

An example of a spontaneous process is the aerobic metabolism of glucose, in which glucose reacts with oxygen to produce carbon dioxide, water, and energy for the organism. 

An example of a nonspontaneous process is the reverse of the reaction that we saw in Section 1.9—namely, the phosphorylation of ADP (adenosine diphosphate) to give ATP (adenosine triphosphate). This reaction takes place in living organisms because metabolic processes supply energy. 

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In biochemical reactions the phosphorylation of amino acids is an important step. Consider the following two reactions and determine whether the phosphorylation of arginine with ATP is spontaneous. 

The thermodynamic principles described in Chapter 2 of this volume can be used to indicate whether or not a reaction can take place spontaneously. They do not, however, provide information about the rate at which a reaction will proceed. Most biochemical reactions proceed so slowly at physiological temperatures that catalysis is essential for the reactions to proceed at a satisfactory rate in the cell. 

AG, is directly associated with the direction in which a particular chemical reaction can proceed. If AG < 0 for a given set of conditions of a particular reaction, then the reaction will proceed spontaneously in the indicated direction until equilibrium is reached. Conversely, if AG is positive, then energy will be needed to shift the reaction further from its equilibrium condition. See Helmholtz Energy Endergonic Exergonic Enthalpy Entropy Thermodynamics Biochemical Thermodynamics... 

Both ways of writing a metabolic reaction have value in biochemistry. Chemical equations are needed when we want to account for all atoms and charges in a reaction, as when we are considering the mechanism of a chemical reaction. Biochemical equations are used to determine in which direction a reaction will proceed spontaneously, given a specified pH and [Mg24], or to calculate the equilibrium constant of such a reaction. 

Reactions in living organisms are no different from reactions in laboratory flasks. Both follow the same laws, and both have the same kinds of energy requirements. For any biochemical reaction to occur spontaneously, AG must be negative. For example, oxidation of 1 mol of glucose, the principal source of energy for animals, has AG° = —2870 kj. 

An enzyme-catalyzed redox reaction can be divided into two half-reactions, one producing electrons and the other consuming electrons. The standard apparent reduction potentials Fr° and E ° for the two half-reactions in an enzyme-catalyzed redox reaction at a specified pH and ionic strength determine E ° for the overall reaction, which is positive for a reaction that can occur spontaneously. A biochemical redox reaction at a specified pH can be represented schematically by... 

Obviously, chemical induction is of great interest, because, on one hand, it allows the induction and acceleration of non-spontaneous reaction and, intrinsically, remains the unique method by which to affect such reactions (except for reactions proceeding under the influence of photochemical and ionizing radiation). On the other hand, chemical induction plays a significant role in biochemical processes. The literal translation from Latin term interference is mutual (inter) collision (ferio), which shows the total situation. 

When the pH is specified, the criterion for spontaneous change and equilibrium is provided by the transformed Gibbs energy and the reactants can be taken to be HgvM, HgvMOa, and O2 that are involved in the following biochemical reaction. 

The relatively large negative value of AG ° indicates that the overall reaction occurs spontaneously under biochemical standard conditions. The biochemical standard state corresponds more nearly to typical conditions in a cellular environment. The first step in the metabolic process is the conversion of glucose into glucose-6-phosphate ... 

The human body is a remarkably efficient organism that is able to reduce the amount of waste and ensure that energy liberated in one part of a biochemical reaction is actually used up, or stored, in another part of such a reaction. This means that we exist more or less at steady state giving out very little surplus energy. In fact, the amount of heat that we lose is equivalent to less than that of an electric light bulb. Less efficient organisms lose a great deal of heat and, hence, one sees spontaneous combustion of farmers haystacks because of bacterial inefficiency en masse. 

Recall from Chapter 4 that the principles of thermodynamics can predict whether a reaction is spontaneous. However, thermodynamic quantities do not provide any information regarding reaction rates. To be useful to an organism, biochemical reactions must occur at reasonable rates. The rate or velocity of a biochemical reaction is defined as the change in the concentration of a reactant or product per unit time. The initial velocity v0 of the reaction A —> P where A = substrate and P = product is... 

Regardless of the AG" for a particular biochemical reaction, it will proceed spontaneously within cells only if AG is negative, given the usual intracellular concentrations of reactants and products. For example, the conversion of glyceraldehyde 3-phosphate (G3P) to dihydroxyacetone phosphate (DHAP), two Intermediates in the breakdown of glucose,... 

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