Just for Fun: Why Does the TCA Cycle Produce GTP?
GTP is energetically equivalent to ATP, so why doesn't the TCA cycle just produce ATP in the succinyl CoA synthetase reaction instead of GTP, since ATP is produced in all the other energetic reactions in glycolysis and electron transport? The GTP produced in the TCA cycle may actually be a very ancient molecular "fossil". It is thought by some scientists that the early earth had a reducing atmosphere, lacking molecular oxygen and being rich in CO2. This was when the first cells appeared. Look at the TCA cycle and pyruvate dehydrogenase and run them backwards in your mind instead of forward as happens in our bodies today. We could start with an acetate group, and then pyruvate dehydrogenase would add a CO2 to it while pyruvate carboxylase would add another CO2 (it still does) and you would get oxaloacetate. Now, run the TCA cycle backwards and you will end up with the 6-carbon citrate. It is thought by some that these central metabolism pathways originated as a way to trap carbon and use it to build compounds with larger carbon skeletons by binding CO2. The pyruvate dehydrogenase reaction and the TCA cycle running backward could have been fueled by electrons from the reducing environment and also may have required GTP for energy. At the time of the first cell, protein synthesis, which also requires GTP for energy, may have been getting started, as well as polymerization of certain filaments which even today require GTP. It may be that at the beginning, both GTP and ATP were equally available for energy and that the succinyl CoA synthetase reaction happened to choose GTP and that reaction is still with us today, billions of years later, even though we run the TCA cycle clockwise (forward) instead of backwards.