Answered: Sulfur compounds give onions their…

Biochemistry

9th Edition

ISBN:9781319114671

Author:Lubert Stryer, Jeremy M. Berg, John L. Tymoczko, Gregory J. Gatto Jr.

Publisher:Lubert Stryer, Jeremy M. Berg, John L. Tymoczko, Gregory J. Gatto Jr.

Chapter1: Biochemistry: An Evolving Science

Section: Chapter Questions

Problem 1P

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Transcribed Image Text:Sulfur compounds give onions their unique flavor and properties. Compound 1 is the starting material for the majority of these flavor components and is converted to Compound 2 under the action of the enzyme alliinase (Reaction 1, unbalanced). +NH3 Compound 1. Reaction 1 Alliinase is a pyridoxal phosphate (PLP, vitamin B6) dependent enzyme. PLP is a planar molecule with a 6-membered ring, a hydroxyl group, an aldehyde group, and a phosphate. PLP is covalently attached to alliinase by forming an aldimine with the side chain of the amino acid at position 251. During catalysis the aldimine is replaced by one formed with the substrate, which facilitates Reaction 1. OH Compound 2 Compound 2 is a very reactive substance known as a sulfenic acid, with an estimated pK, of 12.5. Compound 2 spontaneously and rapidly condenses to form Compound 3, which is one of the main flavor components of onions. 'S OH Compound 2 alliinase Reaction 2 When an onion is cut, the cell components are allowed to mix and Compound 2 encounters an enzyme known as lachrymatory factor synthase (LFS). LFS catalyzes Reaction 3, which produces Compound 4. Compound 4 is a potent lachrymator (tearing agent). ОН Compound 2 LFS O A. Asp and Tyr O B. Glu and Ser O c. Arg and Tyr O D. Lys and Ser Compound 3 Reaction 3 Researchers hope to use this knowledge to devise a strategy for making flavorful onions that do not cause tearing when they are cut. Adapted from S. Imai et al., "An Onion Enzyme that Makes Eyes Water." Nature. A©2002 Nature Publishing Group.; C. Eady et al., Plant Physiology. A 2008 American Society of Plant Biologists.; L. J. W. Shimon et al., *Two Structures of Allinase from Allium sativum L: Apo form and Ternary Complex with Aminoacrylate Reaction Intermediate Covalently Bound to the PLP Cofactor." Journal of Molecular Biology. ©2006 Elsevier. O A. Reactant H₂O, product = NO₂- OB. Reactant = H₂O, product = NH₁* O C. Reactant = 0₂, product = NO3- O D. Reactant = 0₂, product = NH₂* Compound 4 When the covalent attachment to alliinase is broken, PLP is still held rigidly in the active site by a salt bridge and a Tt-stacking interaction. These interactions are most likely provided by the side chains of which amino acids? (Note: The salt bridging amino acid is listed first.) OA. High E, and AG > 0 OB. High E, and AG < 0 O C. Low E, and AG > 0 OD. Low E, and AG < 0 Which reactant and product (one equivalent each) are necessary to balance Reaction 1? Which energy features characterize the energy profile of Reaction 2? + + H Based on the passage, which type of inhibitor will provide a flavorful onion that does not cause tearing? An inhibitor of: O A. Compound 1 synthesis O B. alliinase O C. Reaction 2 O D. LFS

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