The pathway used to convert squalene into steroids appears to be universal to all life. A pathway similar to the mammali

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The pathway used to convert squalene into steroids appears to be universal to all life. A pathway similar to the mammalian one exists in archaea. Bacterial have a reaction that converts squalene into the triterpenoid hopene, which has a function in membranes similar to cholesterol. The enzymes that perform these cyclizations in mammals and bacteria have the same fold and are believed to have a common ancestor. Here is a comparison of mammalian and bacterial reactions starting with squalene. Lanosterol synthesis in mammals Hopene synthesis in bacteria co undefined The active sites of these enzymes are similar in that they use the same mechanism to form the carbocation and remove it. They have diverged slightly, resulting in a change in how squalene folds in each active site (highlighted), resulting in different products. This is a spectacular example of subtle changes in the enzymes' active sites direct the migration pattern of the carbocations, independent of how squalene folds. the imperfections of enzyme catalysis. Some enzymes catalyzing complex reactions cannot fully control the structure of the product (highlighted). the concept that atomic sequence is not the sole determinant of how a biomolecule folds (highlighted). O evolution finding an improvement to an already effective enzyme. catalysis by approximation. Each active site places different atoms next to each other in the highlighted portion of squalene.where the five-membered ring will form.

The precursor to cholesterol and steroids is lanosterol. The enzyme squalene epoxidase converts squalene te an epoxide called oxidosqualene. Then oxidosqualene cyclase (OSC) transforms the epoxide to lanosterol. OS is remarkable it creates six stereocenters with 100% stereospecificity. The reaction mechanism uses an oxidosqualene binding site and just two catalytic residues. It is shown here: Squalene Lanosterol HO of shery Squalene cpoxidase His232 तो Ap455 Ko HO HO Key enzyme residues of OSC, aspartic acid 455 and histidine 232, are shown in green. This question is limited their roles in steps 1 and 2 (circled numbers). Use the word well after the last blank to fill in the blanks. Asp455 initiates a carbocation cation migration by acting as His232 removes the carbocation by acting as A/ to initiate a series of hydride and methyl migrations. The structure in square brackets is an intermediate rather than a transition state. This means the two reactions are not In order to complete the catalysis, the enzyme must reset by Asp455, and His 232 Word well a base a nucleophile a radical initiator a radical quencher an acid an electrophile breaking hydrogen bonds to hydrolyzing the ester with concerted deprotonating oxidizing protonating reducing sequential

Here is a step in the mechanism of an enzyme that acetylglucosamine (NAG) Glu35 NAG -NAG NAG -NAG Asps2 The substrate is black, the middle six-membered ring is a shorthand for the middle NAG. The enzyme residues are green. What are the roles of Glut35 and Asp52 respectively? (Order is important) a base and an electrophile an acid and a nucleophile O an acid and a base a base and an acid a base and a nucleophile O an acid and an electrophile Question 17 (1.25 points Here is a step from the citric acid cycle enzyme citrate synthase. His274 's-COA SCOA Aap 375 o Substrate is black and the residues are on What are the roles of 274 and A375 respectively Order is important)

His274 OH- oo H S-COA SCOA H 00 Asp375 Substrate is black and enzyme residues are green. What are the roles of His274 and Asp375 respectively? (Order is important.) O A base and a base An electrophile and an acid An acid and a base O An electrophile and a base O Positioning substrate and transition state and an acid Positioning substrate and transition state and a base.