Question

8a. Describe the proposed mechanims of the lactose permease symporter.

8b. What are the differences of hydrophobic pockets in chymotrypsin, trypsin and elastase?

Answer 1

8 a. Lactose permease is a membrane protein channel seen on the plasma membrane of the cell. It is for the transport of lactose into the cell. It is seen in the Lac operon in bacterial cell, and is coded by the gene Lac Y. This is an integral membrane protein. The channel is an H+- lactose symporter. Here the sugar (lactose) is co- transported along with H+ , H+being transported down their concentration gradient. The protein consist of 12 transmembrane helices. The protien has sugar binding site exposed to the periplasm. There, the lactose from the periplasm binds to the sugar binding site, along with the proton (H+). When both lactose and sugar bound to the site, there occurs a confirmational change which causes the sugar binding site being open to the cytoplasm of cell.There it releases the proton to cytoplasm, thus occurs the dissociation of bounded sugar( lactose) molecule also to the cytoplsm. The H+ flows down its concentration gradient, that is its higher concentration is at the periplasmic space, and lower concentration at the cytoplasm. But, the lactose is transported against its concentration gradient, and is co- transported along with the H+ ion.

8 b. The hydrophobic site in an enzyme is the one resposible for its subtrate specificity. In chymotrypsin and trypsin the hydrophobic site is known as S1. The difference between the S1 of both enzymes is that, the 189th residue of S1 is aspartate in trypsin but serine in chymotrypsin. Aspartate is a negatively charged aminoacid, and serine is a polar amino acid. In elastase the hydrophobic pocket only contain small hydrophobic side chains.