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Section I – Proteins: Structure & Function

  1. B.

  2. D.

  3. That fermentation required intact cells was disproved by the discovery that a cell-free yeast extract could convert sugar to ethanol and carbon dioxide. This discovery led to the identification of the intermediates, enzymes, and cofactors of fermentation and glycolysis.

  4. Fermentation ceased over time, but resumed when inorganic orthophosphate was added. This led to the isolation of phosphorylated intermediates. Other experiments using heated yeast extract led to the discovery of ATP, ADP, and NAD.

  5. Preparations used to identify metabolites and enzymes included perfused liver, liver slices, and tissue homogenates fractionated by centrifugation.

  6. Radioactive14C, 3H, and 32P facilitated the isolation of intermediates of carbohydrate, lipid, nucleotide, and amino acid metabolism and enabled precursor product relationships between intermediates to be tracked.

  7. Garrod proposal that alkaptonuria, albinism, cystinuria, and pentosuria resulted from “inborn errors of metabolism” led to the field of biochemical genetics.

  8. Regulation of cholesterol biosynthesis illustrates the link between biochemistry and genetics. Cell surface receptors internalize plasma cholesterol, which then regulates cholesterol biosynthesis. Defective receptors result in extreme hypercholesterolemia.

  9. Key model organisms include yeast, slime mold, fruit fly, and a small round worm, each with a short generation time and readily mutated.

  10. D. Hydrocarbons are water insoluble.

  11. A. Of the protein amino acids, only phenylalanine, tyrosine, and tryptophan absorb light at 280 nm.

  12. D. When present in solution at a pH equal to their pKa only half of the molecules of a monofunctional weak acid (eg, ammonium ion or acetic acid) are in the charged state. Maximal mobility will occur either at a pH 3 or more pH units below the pKa for ammonium ion, or at a pH 3 or more pH units above the pKa for acetic acid.

  13. C. At its pI an amino acid has an equal number of positive and negative charges, but has no net overall charge.

  14. C. The Edman technique involves successive derivatization and removal of N-terminal residues.

  15. Self-association in an aqueous environment as a large droplet minimizes the surface area in contact with water, and hence the number of water molecules whose degrees of rotational freedom are restricted.

  16. Strong bases and acids dissociate essentially completely in water, NaOH as Na+ and OH-. By contrast, a weak acid such as pyruvic acid dissociates only partially in solution.

  17. E. Tandem mass spectrometry can separate complex mixtures of peptides.

  18. E. Many proteins undergo posttranslational processing, for example, insulin, which is synthesized as a single polypeptide which subsequent proteolysis converts to two polypeptide chains linked by disulfide bonds.

  19. pI is the pH at which a molecule bears no net charge. In this example, the pI is a pH midway between the third and fourth pKa values: pI = (6.3 + 7.7)/2 = 7.0. As pH is adjusted from acidic to basic, net charge will change successively as follows: +3, +2, +1, 0, –1, –2, –3.

  20. All of the protein amino acids are essential since ...

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