Question 21.5: What substance or substances are the substrate(s) for the ci...
What substance or substances are the substrate(s) for the citric acid cycle? What are the products of the citric acid cycle?
ANALYSIS Study Figure 21.9. Note that acetyl-SCoA feeds into the cycle, but does not come out anywhere. Can you see that all of the other reaction substrates are integral to the cycle and are always present, being continuously synthesized and degraded? Note also that the coenzymes NAD^+ and FAD are reduced and the reduced versions are considered energy-carrying products of the cycle. Also, CO_2 is produced at two different steps in the cycle. Finally, GDP is converted to GTP in Step 5 of the cycle.
Acetyl-SCoA is the substrate for the cycle. Along with GDP and CoA, the oxidized coenzymes NAD^+ and FAD might also be considered substrates despite their status as coenzymes because these substances cycle between the reduced and oxidized states. The products of the cycle are CO_2 and the energy-rich reduced coenzymes NADH/H^+ and FADH_2 as well as GTP.