Melanin determine our skin tone color. Typically, melanin exist in two types: eumelanin & pheomelanin. Eumelanin is dark brown, bigger spot and distribute evenly; Pheomelanin are yellowish red, smaller spot and centralized. Black men have more eumelanin while white men tend to have more pheomelanin, Asians are in between two of them. Gene will determine the amount and type of melanins, thus decide your skin tone whether look fair or dark.
The first step of the biosynthetic pathway for melanin is catalysed by tyrosinase:
Cu2+ → Tyrosinase → L-tyrosine → DOPA → dopaquinone → Melanin
Dopaquinone can combine with cysteine to pheomelanins. Alternatively, dopaquinone can be converted to leucodopachrome and follow two more pathways to the eumelanins.
Dopaquinone + cysteine → 5-S-cysteinyldopa → benzothiazine intermediate → pheomelanin
Dopaquinone → leucodopachrome → dopachrome → 5,6-dihydroxyindole → quinone → eumelanin
The degree of melanin formation is depends on
1) relative concentrations of copper and of the copper-containing enzyme tyrosinase
2) UV rays exposure
4) hormone changes during pregnancy
5) lack of sleep