16.1Mechanism of ATP Synthesis (Kinases and ATP Synthase)

As discussed in See Chapter 4, adenosine 5′-triphosphate (ATP) is the most important energy currency in cells in all living organisms. There are 2 ways of synthesizing ATP: one is mediated by kinases and the other by ATP synthases (Figure 16-1). Kinase is the general name for enzymes that catalyze the transfer of 5′-terminal phosphate groups of ATP to substrates or vice versa. Although most of the known kinases catalyze the former reaction, two enzymes*1 of the glycolytic system transport the phosphate group bound to the substrate with high-energy bonds to adenosine 5′-diphosphate (ADP) to synthesize ATP. This process is called substrate-level phosphorylation. Although the process is able to extract energy efficiently because reactions progress at a substrate-to-ATP ratio of 1:1, it requires different kinases that specifically act on different target substrates. On the other hand, ATP synthase*2 is an enzyme that synthesizes ATP by a mechanism that is coupled to H+ transport, thus requiring a closed membrane vesicle for storing H+.

*1 Phosphoglycerate and pyruvate kinases (Figure 16-2)

Figure 16-1 Three mechanisms of ATP synthesis

As the process involving ATP synthase functions in the mediation of aerobic respiration and photosynthesis, it is also called oxidative phosphorylation or photophosphorylation. The driving force for ATP synthesis is the H+ gradient between the two sides of a membrane, which is generated by H+ transport across the membrane using the energy from electron transport reactions in aerobic respiration and photosynthesis. Because H+ has an electrical charge, the transport of H+ across the biomembrane produces a membrane potential together with the concentration gradient. The combination of the two high-energy states is called a H+ electrochemical gradient. This process is advantageous in that ATP can be synthesized by a common mechanism by using high-energy electrons extracted from a range of redox (reduction-oxidation) reactions.

*2 Although ATP synthase has been called F-ATPase (hydrolase), it should normally be called F-type ATP synthase (discussed later).

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