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 How are the reducing agents (NADH and FADH2) able to generate the free-energy currency molecule (ATP)?
As discussed in an earlier section about coupling reactions, ATP is used as free-energy currency by coupling its (spontaneous) dephosphorylation (Equation 3) with a (nonspontaneous) biochemical reaction to give a net release of free energy (i.e., a net spontaneous reaction). Coupled reactions are also used to generate ATP by phosphorylating ADP. The nonspontaneous reaction of joining ADP to inorganic phosphate to make ATP (Equation 8, below, and Figure 2, above) is coupled to the oxidation reaction of NADH or FADH2 (Equation 9, below). (Recall, NADH and FADH2 are produced in glycolysis and the citric-acid cycle as described in the blue box). For simplicity, we shall henceforth discuss only the oxidation of NADH; FADH2 follows a very similar oxidation pathway.
The oxidation reaction for NADH has a larger, but negative, ΔG than the positive ΔG required for the formation of ATP from ADP and phosphate. This set of coupled reactions is so important that it has been given a special name: oxidative phosphorylation. This name emphasizes the fact that an oxidation (of NADH) reaction (Equation 9 and Figure 5, below) is being coupled to a phosphorylation (of ADP) reaction (Equation 8, below, and Figure 2, above). In addition, we must consider the reduction reaction (gaining of electrons) that accompanies the oxidation of NADH. (Oxidation reactions are always accompanied by reduction reactions, because an electron given up by one group must be accepted by another group.) In this case, molecular oxygen (O2) is the electron acceptor, and the oxygen is reduced to water (Equation 10, below) .
The individual reactions of interest for oxidative phosphorylation are:
phosphorylation |
ADP3- + HPO42- + H+ →
ATP4- + H2O
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ΔGo= +30.5 kJ
(nonspontaneous)
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(8) |
oxidation |
NADH → NAD+ + H+ + 2e-
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ΔGo= -158.2 kJ
(spontaneous)
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(9) |
reduction |
1/2 O2 + 2H+ + 2e- → H2O
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ΔGo= -61.9 kJ
(spontaneous)
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(10) |
The net reaction is obtained by summing the coupled reactions, as shown in Equation 11, below.
ADP3- + HPO42- + NADH + 1/2 O2 + 2H+ →
ATP4- + NAD+ + 2 H2O
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ΔGo= -189.6 kJ
(spontaneous)
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(11) |
The molecular changes that occur upon oxidation of NADH are shown in Figure 5, below.
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Figure 5
This is a two-dimensional (ChemDraw) representation showing the change that occurs when NADH is oxidized to NAD+. "R" represents the part of the structure that is shown in black in the drawing of NADH in Table 1, and does not change during the oxidation half-reaction. The molecular changes that occur upon oxidation are shown in red. |
In this tutorial, we have seen that nonspontaneous reactions in the body occur by coupling them with a very spontaneous reaction (usually the ATP reaction shown in Equation 3). We have just seen that ATP is produced by coupling the phosphorylation reaction with NADH oxidation (a very spontaneous reaction). But we have not yet answered the question: by what mechanism are these reactions coupled?
Related Practice Problems
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