6.7 Regulation of the respiratory chain

Most of the time and in most cells, the respiratory chain runs at rates that are substantially below the maximal rate. How is the flow through the respiratory chain controlled? In a healthy and not maximally exerted cell, there is much more ATP than ADP or phosphate, so that these become limiting for the flow. If ATP synthase is short of substrates, the proton-motive force will not be dissipated, so that the proton pumps will have a harder time to extrude more protons and will eventually stall. In addition, the flow through the respiratory chain is also coupled to the flow through preceding pathways such as glycolysis and the TCA. Such coupling occurs by negative feedback at various levels:

A low flux through the respiratory chain will lead to the accumulation of NADH, which slows down glyceraldehyde-3-phosphate dehydrogenase, pyruvate dehydrogenase, and NAD⁺-dependent isocitrate dehydrogenase.
A low consumption of ATP will result in its accumulation to higher levels. Many enzymes, including phosphofructokinase, are inhibited by ATP.

These regulatory mechanisms are reasonably straightforward. There is, however, one remaining mystery. We have already noted that there are two forms of isocitrate dehydrogenase, one using NAD⁺ and the other NADP⁺ as the cosubstrate. While the NAD⁺-dependent form is inhibited by NADH and ATP, the NADP⁺-dependent form, which actually has the higher activity, is not subject to such inhibition, which would suggest that might go at full blast even when the demand for ATP is low and NADH is high. How, then, is this enzyme prevented from uncontrolled consumption of isocitrate? It appears that, at least during times of low or moderate demand for ATP, NADP⁺-dependent isocitrate dehydrogenase is close to equilibrium. This equilibrium is sustained by high intramitochondrial levels of NADPH, which in turn are maintained by NAD⁺/NADPH transhydrogenase. This remarkable protein, which is both and enzyme and a transporter, reduces NADP⁺ to NADPH at the expense of NADH. It is located in the inner mitochondrial membrane and, like ATP synthase, is coupled to the translocation of protons:

NADH + NADP⁺ + H⁺_out→ NAD⁺ + NADPH + H⁺_in

Figure 6.7-1 shows how the function of transhydrogenase is integrated with the function and regulation of the TCA and the respiratory chain¹:

When the demand for ATP is low, NADH and the proton-motive force will both be at high levels, which will cause the transhydrogenase to reduce NADP⁺ at the expense of NADH, until near-equilibrium conditions will be reached. At the equilibrium, NADPH will have a high concentration, which will translate into near-equilibrium conditions for the NADP⁺-dependent isocitrate dehydrogenase also. There may, however, be a low net flux within an interesting futile cycle, which involves the two isocitrate dehydrogenases and the transhydrogenase, the net effect of which is the influx of one proton per cycle (see Figure 6.7-1a).
When demand for ATP is very high, the proton-motive force and the level of NADH will be lower. Under these conditions, the transhydrogenase will switch direction, now consuming NADPH to produce more NADH, which will be consumed at high speed in the respiratory chain. At the same time, the enzyme will work as an auxiliary proton pump, thus directly contributing to the proton-motive force. This will of course lower the level of NADPH, which in turn will topple the NADP⁺-dependent dehydrogenase equilibrium in favour of supplying more NADPH, which will helpt to keep the entire process going.

Now that is a marvelous piece of engineering by Nature, isn't it—you might even feel tempted to call it intelligent design.

1: This is my take on the subject—there is, however, considerable variety of opinion on the role of this fascinating enzyme. Another opportunity for you to contribute to these notes an earn an honorable mention.