My go-to website for enzyme questions is the BRENDA database (brenda-enzymes.org).  Below are the URLs for 2 searches I did using "4-hydroxybenzoate" as "product".  First search (results for any organism) shows that indeed, there are no entries for homo sapiens, which at least seems to confirm that there is no direct evidence of the pathway in humans.  Second search refined for mammals, with only 4 unique entries.  Perhaps these enzymes might point you in the right direction to define/reconstruct the pathway, if it exists?

First search (any organism):  http://www.brenda-enzymes.org/search_result.php?a=25&W%5B2%5D=4-hydroxybenzoate&T%5B2%5D=2&l=100&RN=&T%5B0%5D=2&W%5B1%5D=&T%5B1%5D=2&W%5B3%5D=&T%5B3%5D=2&W%5B4%5D=&T%5B4%5D=2&Search=Search&orderByHTMLField=Organism

Second search (show mammals only):  http://www.brenda-enzymes.org/search_result.php?a=25&W%5B2%5D=4-hydroxybenzoate&T%5B2%5D=1&l=100&tt=1&RN=&T%5B0%5D=2&W%5B1%5D=&T%5B1%5D=2&W%5B3%5D=&T%5B3%5D=2&W%5B4%5D=mammalia&T%5B4%5D=2&Search=Search

@Craig, I'm also a big fan of BRENDA, but I didn't think of doing the search with 4-hydroxybenzoate as product. Thanks for posting these searches. 

hi

try humancyc (https://biocyc.org/HUMAN/NEW-IMAGE?type=NIL&object=PWY-5754-1) and you'll get what you want (see picture).

fred

Back are the times when I worked in metabolism and indeed I was not aware of this conversion of Tyr to 4HB. I just want to point that 4HB would not be a vitamin as it is possible to produce it in humans (either by human or microorganism enzymes). Eventually the vitamin could be the precursor (as thiamine that is converted into thiamine pyrophosphate), but in this case the precursor is Tyr (a nonessential amino acid). 

@Fred, Thanks for posting the link to HumanCyc's diagram of the putative pathway from tyrosine to 4-hydroxybenzoate.  You're correct that this is the pathway in the textbooks and it replicates the pathway as seen in KEGG, but if we look closely we see that some of the required enzymes don't yet have EC numbers and of the others only TAT has a known human gene. So the question remains, are there human genes encoding enzymes for the 5 remaining steps in this pathway? I suppose some of the steps may be spontaneous, but presumably the redox steps are not. To the best of my understanding this pathway was inferred from the presence of its intermediates in human blood plasma, which is certainly not conclusive evidence that the human genome encodes the required enzymes. So the evidence so far strongly suggests the reactions in the diagram are catalyzed by enzymes that exist in the biosphere. But I do not yet see the evidence that there are human enzymes capable of the transformation of L-tyrosine to 4-hydroxybenzoate.

@Rafael, You make a good point. Certainly, if it were possible for humans to produce 4-hydroxybenzoate from a non-essential amino acid, 4-HB would not be a human vitamin. I may have an incomplete or even incorrect definition of a vitamin, but to me it seems misleading to deny vitamin status to essential molecules that cannot be produced by human cells but can be produced by the gut microbiome. Indeed, such a definition might, I suppose, eliminate vitamin B12 from the list of human vitamins since B12 is synthesized exclusively by microorganisms (see https://www.ncbi.nlm.nih.gov/pubmed/11935176). 

So I think the original question remains open. Pending identification of the human enzymes that convert L-tyrosine to 4-HB, 4-hydroxybenzoate IS a human vitamin. Do you agree?

Hi Robert

I agree!. Indeed some definitions are quite clear for the classical examples that served to make definitions. IN this case I woudl agree with you in that the "vitamin" definition if  there is "today" a consensus one (I am not sure on this) should be open to include compounds such as 4-HB.

A colleague at Stanford pointed out that at least in mice, germ-free animals live for months/years on a sterile diet tht includes the presently identified vitamins. This suggests that 4-HB cannot be a human vitamin. We still, however, do not know the identities of the human genes that catalyze the conversion from, say, tyrosine to 4-HB.