1.In human,phenylalanine can be metabolized to tyrosine,and methionine
can be metabolized to cysteine.So we call tyrosine and cysteine
"semiessential amino acids" in nutrition.
2.Threonine can synthesize isoleucine,and isoleucine which is produced
can feedback threonine.

My question is:
Since isoleucine can be produced , why we didn't call it
"semiessential" (just like tyrosine and cysteine) but "essential" ??

Hi -

The reaction of which you speak, the threonine-isoleucine
transformation is the major means of branched chain amino acid
synthesis (eg valine, leucine, isoleucine) in bacteria and other
organisms capable of synthesizing all of their amino acids.  Humans
(and other animals) do not possess this synthetic pathway, thus
isolecuine and threonine are not inter-convertable and are both
essential amino acids.  Humans do possess the other pathways which you
mentioned.


A nice, though not very detailed page describing amino acid
biochemistry
http://www.indstate.edu/thcme/mwking/amino-acid-metabolism.html

A technically better, but much harder to navigate page:
http://www.cm.utexas.edu/academic/courses/Spring1999/CH369/LEC22/Lec22.htm