Learning Objectives Endergonic reactions can also be pushed by coupling them to another reaction, which is strongly exergonic, often through shared intermediates. Many chemicals' reactions are endergonic (i.e., not spontaneous (\(\Delta G > 0\))) and require energy to be externally applied to occur. However, these reaction can be coupled to a separate, exergonic (thermodynamically favorable \(\Delta G <0\)) reactions that 'drive' the thermodynamically unfavorable one by coupling or 'mechanistically joining' the two reactions often via a share intermediate. Since Gibbs Energy is a state function, the \(\Delta G\) values for each half-reaction may be summed, to yield the combined \(\Delta G\) of the coupled reaction. One simple example of the coupling of reaction is the decomposition of calcium carbonate: \[CaCO_{3(s)} \rightleftharpoons CaO_{(s)} + CO_{2(g)} \;\;\;\;\;\;\; \Delta G^o = 130.40 \;kJ/mol \label{1}\] The strongly positive \(\Delta G\) for this reaction is reactant-favored. If the temperature is raised above 837 ºC, this reaction becomes spontaneous and favors the products. Now, let's consider a second and completely different reaction that can be coupled ot this reaction. The combustion of coal released by burning the coal \(\Delta G^o = -394.36 \;kJ/mol\) is greater than the energy required to decompose calcium carbonate (\(\Delta G^o = 130.40 \;kJ/mo\)). \[C_{(s)} + O_2 \rightleftharpoons CO_{2(g)} \;\;\;\;\;\;\; \Delta G^o = -394.36 \;kJ/mol \label{2}\] If reactions \(\ref{1}\) and \(\ref{2}\) were added \[CaCO_{3(s)} + C_{(s)} + O_2 \rightleftharpoons CaO_{(s)} + 2CO_[{2(g)} \;\;\;\; \Delta G^o = -263.96 \;kJ/mol \label{3}\] and then Hess's Law were applied, the combined reaction (Equation \(\ref{3}\)) is product-favored with \(\Delta G^o = -263.96 \;kJ/mol\). This is because the reactant-favored reaction (Equation \(\ref{2}\)) is linked to a strong spontaneous reaction so that both reactions yield products. Notice that the \(\Delta G\) for the coupled reaction is the sum of the constituent reactions; this is a consequence of Gibbs energy being a state function: \[\Delta G^o = 130.40 \;kJ/mol+ -394.36 \;kJ/mol = -263.96 \;kJ/mol\]
This is a common feature in biological systems where some enzyme-catalyzed reactions are interpretable as two coupled half-reactions, one spontaneous and the other non-spontaneous. Organisms often the hydrolysis of ATP (adenosine triphosphate) to generate ADP (adenosine diphosphate) as the spontaneous coupling reaction (Figure \(\PageIndex{1}\)). \[ATP + H_2O \rightleftharpoons ADP + P_i \label{4}\]
The phosphoanhydride bonds formed by ejecting water between two phosphate group of ATP exhibit a large negative \(-\Delta G\) of hydrolysis and are thus often termed "high energy" bonds. However, as with all bonds, energy is requires to break these bonds, but the thermodynamic Gibbs energy difference is strongly "energy releasing" when including the solvation thermodynamics of the phosphate ions; \(\Delta G \) for this reaction is - 31 kJ/mol.  ATP is the major 'energy' molecule produced by metabolism, and it serves as a sort of 'energy source' in cell: ATP is dispatched to wherever a non-spontaneous reaction needs to occurs so that the two reactions are coupled so that the overall reaction is thermodynamically favored.
Example \(\PageIndex{1}\): Phosphorylating Carboxylic Acids Aldehydes \(RCHO\) are organic compounds that can be oxidized to generate carboxylic acids and nicotinamide adenine dinucleotide (NAD) is a coenzyme found in all living cells and in the reduced form, \(NAD^+\), it acts as an oxidizing agent that can accept electrons from other molecules. The NAD+-linked oxidation of an aldehyde is practically irreversible with an equilibrium that strongly favors the products (\(\Delta G >> 0\): \[RCHO + NAD^+ + H_2O \rightleftharpoons RCOOH + NADH + H^+ \label{Spontaneous}\] The position of equilibrium for phosphorylating carboxylic acids lies very much to the left: \[RCOOH + P_i \rightleftharpoons RC(=O)(O-P_i) + H_2O \label{Nonspontaneous}\]
The non-spontaneous formation of a phosphorylated carboxylic acid can be driven by coupling it to the (spontaneous) NAD+-linked oxidation of an aldehyde? Similarly, ATP hydrolysis can be used to combine amino acids together to generate polypeptides (and proteins) as graphically illustrated by Figure \(\PageIndex{2}\). In this case, the reverse of Equation \(\ref{4}\) is initially coupled to the oxidizing glucose by oxygen \[C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O \label{5}\] Reaction \(\ref{5}\) is strongly spontaneous with \(\Delta G = −2880 \;kJ/mol \) or close to 100x greater energy capability than the hydrolysis of ATP in Equation \(\ref{4}\). Hence, the equilibrium for this reaction so strongly favors the products that a single arrow is typically used in the chemical equation as it is essential irreversible. It may not be surpising that glucose and all sugars are very energetic moleculess since they are the primary energy source for life. References
Two (or more) reactions may be combined such that a spontaneous reaction may be made 'drive' an nonspontaneous one. Such reactions may be considered coupled. Changes in Gibbs energy of the coupled reactions are additive. Contributors and AttributionsThe correct hypothesis is the number (2) hypothesis: the hydrolysis of ATP has a negative standard free energy change (∆G0) Observations about this reaction. - The chemical adenosine is linked to three phosphate groups in adenosine triphosphate (ATP). Adenosine is a nucleoside made up of adenine, a nitrogenous base, and ribose, a five-carbon sugar. The three phosphate groups are labeled alpha, beta, and gamma, in order of proximity to the ribose sugar. These chemical groups work together to create a powerful energy source. The two phosphate bonds (phosphoanhydride bonds) are equal high-energy bonds that, when broken, release enough energy to fuel a number of biological reactions and processes. Because the products [adenosine diphosphate (ADP) and one inorganic phosphate group (Pi)] have lower free energy than the reactants, the link between the beta and gamma phosphate is called "high-energy" (ATP and a water molecule). Hydrolysis is the breakdown of ATP into ADP and Pi that consumes a water molecule (hydro-, meaning "water," and lysis, meaning "separation"). ATP Hydrolysis and Synthesis In the following reaction, ATP is hydrolyzed into ADP: ATP+H2O→ADP+Pi+free energy The hydrolysis of ATP to ADP, like other chemical processes, is reversible. ADP + Pi are combined in the reverse reaction to regenerate ATP from ADP. Because ATP hydrolysis releases energy, ATP synthesis requires free energy input. In the following reaction, ADP is coupled with phosphate to generate ATP: ADP+Pi+free energy→ATP+H2O ATP and Energy Coupling When ATP is hydrolyzed, how much free energy (G) is released, and how is that free energy employed to perform cellular work? For the hydrolysis of one mole of ATP into ADP and Pi, the predicted delta G is -7.3 kcal/mole (-30.5 kJ/mol). However, this is only true under ideal conditions, as the delta G for the hydrolysis of one mole of ATP in a living cell is almost twice as high: 14 kcal/mol (-57 kJ/mol). Adenosine triphosphate (ATP) is a highly unstable chemical. ATP spontaneously dissociates into ADP + Pi unless it is employed to perform work quickly, and the free energy released during this process is lost as heat. Energy coupling is a mechanism used by cells to utilize the energy contained within the ATP bonds.
Step-by-step explanation ATP: Adenosine Triphosphate The energy currency for cellular operations is adenosine triphosphate (ATP). Both energy-consuming endergonic processes and energy-releasing exergonic reactions, which require a minimal amount of activation energy, are powered by ATP. Energy is produced when the chemical bonds inside ATP are broken, and it can be used for cellular activities. A molecule's potential energy increases as the number of bonds increases. Because the ATP connection is so easily broken and reformed, it functions as a rechargeable battery that supports cellular processes such as DNA replication and protein synthesis. Energy Coupling in Sodium-Potassium Pumps The exergonic reaction of ATP hydrolysis is coupled to the endergonic reactions of cellular activities in cells. Transmembrane ion pumps, for example, employ ATP energy to pump ions across the cell membrane and generate an action potential in nerve cells. The sodium-potassium pump (Na+/K+ pump) transports sodium out of the cell while bringing potassium in. Phosphorylation occurs when ATP is hydrolyzed and its gamma phosphate is transferred to the pump protein. The free energy is gained by the Na+/K+ pump, which then experiences a conformational shift, allowing it to release three Na+ to the outside of the cell. Two K+ ions from outside the cell bond to the protein, causing it to alter the shape and release the phosphate. Phosphorylation drives the endergonic reaction by contributing free energy to the Na+/K+ pump. Energy Coupling in Metabolism Certain molecules must be slightly altered in conformation during cellular metabolic reactions, such as the synthesis and breakdown of nutrients, in order to become substrates for the next step in the reaction series. The process of glycolysis is used to break down glucose in the early stages of cellular respiration. The phosphorylation of glucose requires ATP, resulting in a high-energy but unstable intermediate. This phosphorylation event induces a conformational change in the phosphorylated glucose molecule, allowing enzymes to convert it to the phosphorylated sugar fructose. Fructose is a necessary intermediary in the progression of glycolysis. The exergonic process of ATP hydrolysis is connected with the endergonic reaction of glucose conversion for utilization in the metabolic pathway in this example. Reference: https://courses.lumenlearning.com/boundless-biology/chapter/atp-adenosine-triphosphate/ |
How would you determine whether a metabolic reaction might require coupling to atp hydrolysis?
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