Oxidation Pathways and the Enzymes That Mediate Them

Oxidation Pathways and the Enzymes That Mediate Them 

For an organism to eliminate a lipophilic, chemically inert xenobiotic, it is usually hrst necessary to oxidize it to a more polar form. In addition, many biosynthetic pathways that produce steroid hormones, prostaglandins, leukotrienes, etc. involve oxidative steps. Organisms have evolved many enzymes to carry out these oxidations. Oxidation can occur by addition of oxygen (without addition of hydrogen which would represent hydration), removal of hydrogen atoms (without removal of oxygen which would represent dehydration), or simply removal of electrons. 

It is difficult to understand the oxidative pathways without an understanding of the enzymes that mediate them therefore, we will start with a discussion of the oxidative enzymes with examples of specific oxidations that each enzyme meditates. This will be followed by a discussion of metabolic pathways organized according to functional groups. 

FIGURE 4.1 Other molecules required for P450 function. 

Cytochrome P450s work by activating molecular oxygen (O2). They are all classified as mono-oxygenases because in the overall catalytic process, O2 is split into two oxygen atoms but only one atom is utilized in oxidizing the substrate (RH) while the second atom is reduced by two electrons to form water [Eq. (4.1)]. 

---