In an evolutionary sense, why is it that only amino acids from protein can be turned into glucose for fuel through gluconeogenesis? Why couldn't the body have evolved to be COMPLETELY protein-sparing and just turn fat into glucose for energy?
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Because a common pool of chemical-group carriers and associated enzymes evolved a pretty efficient (not perfectly efficient) system of interconverting proteins, carbs, and fats right around the crux between glycolysis and the TCA cycle. As important as it was in archaic lifeforms, it was conserved almost as is from bacteria right on up (bad choice of word?) to humans with a few minor alterations because it worked pretty well from the start. Asking why something didn't evolve a different way is kind of a question without an answer since evolution is a force of order emerging through chaos. You could always ask the great designer in the sky why he designed this system with some inefficiencies. Now that's an answer I'd like to hear myself. |
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One reason is that they're so similar, it's easy to interconvert amino acids to carbohyrates (and hard/impossible?) with fats. First, here's a list of the glucogenic amino acids in humans. Go to each page and look at their structures: http://en.wikipedia.org/wiki/Glucogenic_amino_acid Next, here's what glucose looks like http://en.wikipedia.org/wiki/Glucose Notice that they're similar, chains of carbons, lots of OH bonds Now here's a fatty acid, I chose Oleic acid, but they all look the same just different lengths http://en.wikipedia.org/wiki/Oleic_acid Note how it's completely different in structure. Long chain of C's and H's, no OH groups to be found except at the hydrophilic end. It would take too much energy, even if it is possible, to convert fats to carbohydrates. That's probably why we never evolved to that. |
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Good answers thus far. Just another point on the evolutionary selection pressure:- The human body needs bugger all glucose to operate. We can make what we need from protein (gluconeogenesis) and everything else runs perfectly well on fat & ketones. If for whatever reason we needed a lot more glucose chances are the selection pressure would of seen us evolve with more mechanisms to generate it from other macro nutrients (or become extinct, of course). But we don't and we didn't. |
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