[mi corazon late]

Why can't we synthesize the essential aminino acids? What is missing in our system that we can't synthesize them?

[cswarth]

Why can't we synthesize the essential aminino acids? What is missing in our system that we can't synthesize them?

I haven't take biochemistry yet, so some of this is speculation.   If you are asking about the ultimate cause, I have no idea.   Probably has something to do with the current solution being "good enough" so there is no evolutionary pressure to produce these amino acids ourselves.

But the proximate cause of mammals (not just humans) requiring some amino acids from their diet is that we do not have a gloxylate metabolic pathway that converts two molecules of Acytel-CoA to malate, a precursor to the synthesis of aromatic amino acids.  Plants and bacteria do have this metabolic pathway so they can make amino acids if they are not present in the environment. 

It would be interesting to speculate whether you could transfer the genes necessary for malate synthesis (and other necessary enzymes) and thereby enable an organism to make its own essential amino acids.