[keetner]

Hello,

I'm a bit confused with essential amino acids. I've always been told that they must be supplied in diet since we can't make it...but when I look further into these pathways, do we not have means to make them? Take Met for example. Image is pulled from Wiki -- not the best, but it works.



While we have this pathway to regenerate Met, I'm guessing we can't make it de novo? For example, one of the precursors is homocysteine (at about step 3), but I guess to even get homocysteine, we would need Met? Is this correct? Or is it because one of the co-enzymes involved is N5-methyl-THF...and if what I've read is correct, it's a vitamin. And I assume humans can't quite make vitamins...

What confuses me more, however, is that in my notes it's said that Met can be derived from an intermediate (oxaloacetate)...well, it actually says all 20 amino acids can be derived from intermediates of glycolysis, TCA, PPP. I don't really know what this means. Because it sounds like we can make all 20.
Unless it was strictly speaking about bacteria...but my notes don't say anything about this.

Any clarification on this would be greatly appreciated. I'm sorry if my question is poorly worded -- I'm not even sure what I'm reading here.

Thanks!

[Yggdrasil]

While we have this pathway to regenerate Met, I'm guessing we can't make it de novo? For example, one of the precursors is homocysteine (at about step 3), but I guess to even get homocysteine, we would need Met? Is this correct? Or is it because one of the co-enzymes involved is N5-methyl-THF...and if what I've read is correct, it's a vitamin. And I assume humans can't quite make vitamins...

Because the only source of homocysteine in humans is from methionine, you cannot use the methylation of homocysteine to synthesize new methionine.

What confuses me more, however, is that in my notes it's said that Met can be derived from an intermediate (oxaloacetate)...well, it actually says all 20 amino acids can be derived from intermediates of glycolysis, TCA, PPP. I don't really know what this means. Because it sounds like we can make all 20.
Unless it was strictly speaking about bacteria...but my notes don't say anything about this.

Your notes are probably referring to plants and bacteria which can synthesize methionine from intermediates of the central metabolic pathways.  Humans lack the necessary enzymes for these transformations.

[keetner]

^Ah, thanks for the clarification! Makes a lot more sense.

Cheers.

[Babcock_Hall]

You are correct about vitamins, but the are turned into catalysts (and are needed in small amounts for that reason).  This next comment might be a little bit tangential.  A few amino acids can be made from so-called salvage pathways, but not de novo from simple precursors.  Tyrosine can be made from phenylalanine, for example.  Cysteine can be made from methionine via homocysteine.  Tyr and Cys are typically not classified as essential for that reason.  As was pointed out, Met can be regenerated, but only from something derived from Met; therefore, this is a little different from the cases of tyrosine and cysteine.