Question

Some bacteria can excrete a toxin that breaks down sphingomyelin in mammalian erythrocytes, causing those red blood cells to lyse. Examination of the toxin showed it to be an enzyme that could break the phosphoester bond between sphingosine and phosphocholine (ie. the products are ceramide and phosphocholine). The mechanism of the reaction is as follows: The active site of the enzyme contains three critical residues—histidine 296, glutamate 53 and histidine 151. The glu positions a Mg+2 ion near the phosphate. First, histidine 296 acts as a general base and converts an active site water into a hydroxide ion—this species is stabilized by the presence of the metal ion. Next, the substrate binds and the hydroxide ion carries out an in-line attack on the phosphate. The leaving group is protonated by the general acid histidine 151 (creating ceramide). Draw the active site and show how the mechanism works. You will need to draw 3 or 4 different active site “snap-shots” to show the entire process. Be sure to clearly show the transition state and how metal ion catalysis could encourage its formation.

Answer 1