Do desulfhydrase-catalysed reactions take place in animal cells?

Question

It isn't a homework question. I'm just stuck with desulfhydrase reactions and am unable to find enough information in the usual places. Any external source on this topic would be helpful.

Answer 1

A desulfhydrase catalyzes the removal of hydrogen sulfide from a compound. So the answer is yes, but it's not the most-commonly annotated reaction for the enzyme cystathionine gamma-lyase.

I didn't know off-hand. Here's how I answered the question:

I searched Entrez Protein (or NCBI...whatever the thing's called) with the following terms:

(desulfhydrase) AND "Homo sapiens"[porgn:__txid9606] 

And got three hits for isoforms of cystathionine gamma-lyase. I clicked on one of them and started reading. Key things I noted:

EC_number="4.4.1.1"

...

Cystathionine gamma-synthase is a PLP dependent enzyme and catalyzes the committed step of methionine biosynthesis. This pathway is unique to microorganisms and plants, rendering the enzyme an attractive target for the development of antimicrobials and herbicides.

So it's a good question because, if people normally characterize this enzyme in terms of Met biosynthesis, then we don't have a desulfhydrase. But I trusted the thing was annotated properly and that the comment above was just too medicine-oriented (as much biochemical literature is).

So to really understand the enzyme itself I searched MetaCyc for the Enzyme Commission identifier 4.4.1.1.

This let's me easily visualize the catalyzed reaction. And the annotation spells it out clearly:

Enzyme Commission Summary: A multifunctional pyridoxal-phosphate protein. The enzyme cleaves a carbon-sulfur bond, releasing L-cysteine and an unstable enamine product that tautomerizes to an imine form, which undergoes a hydrolytic deamination to form 2-oxobutanoate and ammonia. The latter reaction, which can occur spontaneously, can also be catalysed by EC 3.5.99.10, 2-iminobutanoate/2-iminopropanoate deaminase.

Also catalyses elimination reactions of L-homoserine to 2-oxobutanoate and ammonia, of L-cystine, producing thiocysteine, pyruvate and ammonia, and of L-cysteine, producing pyruvate, hydrogen sulfide and ammonia.

Answer 2

In addition to the reaction described in Ryan's answer, cystathionine beta-synthase (CBS) also catalyses a reaction that yields hydrogen sulfide. These two reactions are thought to be the major sources of hydrogen sulfide in mammals.