Here is a link to the KEGG Encyclopedia map of tyrosine metabolism. The only enzymes leading from tyrosine to pheomelanin are tyrosinase and polyphenol oxidase, catalyzing the conversion of tyrosine to dopaquinone. The step from dopaquinone to cysteinyl-DOPA seems to be uncatalyzed. The step from cysteinyl-DOPA to pheomelanin seems to be uncharacterized.

http://www.kegg.jp/kegg-bin/show_pathway?map00350

Adam 

Then my question comes..

I had incorporated cysteine in the medium for bacteria.It still produced eumelanin not pheomelanin.

Then what is the regulation occur there to shift between eumelanin and pheomelanin pathway without a specific enzyme

There is no an specific enzyme, but cysteine should be in the reduced state (free thiol group) to react with dopaquinone. Cystine do not react, and as you know cysteine is easily oxidized to cystine. On the other hand, black melanin does not mean pure eumelanin. You can have cysteine addition to dopaquinone and mixed polymerization with the final appearance of eumelanin. High tyrosinase activities lead to this situation. pH and other factors also affect. It should be taken into account that cysteine addition to dopaquinone competes with dopaquinone cyclization to leukodopachrome or even other chemical reactions. 

Yes Solano

The pH and other factors can affect.I will work on it.

and I have not interpreted pheomelanin based on its color.There may be mix ups.But I had gone for elemental analysis.Where the sulfur content still remains between 1-2%.By this only i had confirmed the melanin to be eumelanin.May be there the formed pheomelanin be less than when compare to eumelanin.It can be a possibility.

Thanks for igniting by minds 

Hi All,

Loved the tyrosine metabolism pathway link - it's really useful!!

Just to chip in on this, I've read papers which say, "wherever free thiol compounds are present in sufficient concentration, pheomelanogenesis is favoured" over eumelanogenesis (Solano 2007; Land & Riley 2000). Indeed, Land & Riley (2000) go so far as to say that the "balance between the formation of eumelanin and pheomelanin is regulated principally by the availability of cysteine at the site of melanogenesis".

Not knowing about your work Kurian, may be you could try adding excess L-cysteine so you have a surplus to react with the dopaquinone formed?