Structure and Biosynthesis
Pre-pro-lactase, the primary translation product, has a single polypeptide primary structure consisting of 1927 amino acids. It can be divided into five domains: (i) a 19 amino acid cleaved signal sequence; (ii) a large prosequence domain that is not present in mature lactase; (iii) the mature lactase segment; (iv) a membrane spanning hydrophobic anchor; and (v) a short hydrophilic carboxyl terminus. The signal sequence is cleaved in the endoplasmic reticulum, and the resulting 215 kDa pro-LPH is sent to the Golgi, where it is heavily glycosylated and proteolytically processed to its mature form. The prodomain has been shown to act as an intramolecular chaperone in the ER, preventing trypsin cleavage and allowing LPH to adopt the necessary 3-D structure to be transported to the Golgi apparatus.
Mature human lactase consists of a single 160 kDa polypeptide chain that localizes to the brush border membrane of intestinal epithelial cells. It is oriented with the N-terminus outside the cell and the C-terminus in the cytosol. LPH contains two catalytic glutamic acid sites. In the human enzyme, the lactase activity has been connected to Glu-1749 while Glu-1273 is the site of phlorizin hydrolase function.
Read more about this topic: Lactase
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