Single enzymatic reactions bonds

All steroids are biosynthesized from the triterpene squalene, which is converted to an epoxide by the enzyme squalene 2,3-epoxidase. In mammals, this is converted to lanosterol by 2,3-oxidosqualene-lanosterol cyclase. You should find the first step of the process fairly easy to follow, as it is simply a cascade of electrophilic additions to double bonds. The next step is a cascade of rearrangements— come back and look at that again, once you have studied rearrangements in Chapter 18, as at this point, it may seem a bit mysterious (Figure 16.9). There are 7 stereocenters in lanosterol, which gives a potential for 128 stereoisomers, but a single stereoisomer is produced by the enzymatic reaction. 

Hyal-1, an acid-active lysosomal enzyme, was the first somatic hyaluronidase to be isolated and characterized.191,192 It is a 57 kDa single polypeptide glycoprotein that also occurs in a processed 45 kDa form, the result of two endoprotease reactions. The resulting two chains are bound by disulfide bonds. This is not a zymogen-active enzyme relationship, since the two isoforms have similar specific activities. Why two forms should occur is unknown. Only the larger form is present in the circulation, while both isoforms occur in urine,193 in tissue extracts, and in cultured cells. Why an acid-active hyaluronidase should occur in plasma is not clear. Some species do not have detectable enzymatic activity in their circulation,194 but an inactive 70 kDa precursor form of the enzyme is present in such sera, detectable by Western blot (L. Shifrin, M. Neeman, and R. Stern, unpubl. data). Hyal-1 is able to utilize HA of any size as substrate, and generates predominantly tetrasaccharides. 

The next step is simple—the epoxidation of one of the terminal double bonds—but it leads to two of the most remarkable reactions in all of biological chemistry. Squalene is not chiral, but enzymatic epoxidation of one of the enantiotopic alkenes gives a single enantiomer of the epoxide with just one stereogenic centre. 

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