The mechanism for lysozyme cleavage of its polysaccharide substrate requires Glu35 in its nonionized form, whereas the nearby Asp52 must be ionized (see the figure below). The pk values for the side-chain carboxyl groups on the two amino acids in solution are virtually identical. a) How can one carboxyl group be charged and the other uncharged in the active site of lysozyme? b) The pH optimum for lysozyme is about 5. Why do you suppose that the activity decreases above and below this optimum? Glu35 NAG Asp52 Glu35 -C, Asp52 tri-NAG NAG
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- Chymotrypsin, trypsin, and elastase all share the same catalytic mechanism, but have different specificities. (a) These proteases are considered serine proteases because the active sites contain serine, histidine and aspartate. Describe the roles that each of these amino acid residues play in hydrolyzing protein substrates. (b) The active site pockets of chymotrypsin, trypsin, and elastase are shown in the image below. Based on the image, which side chain of the amino acids would they prefer to cleave respectively, Ala, Arg, or Trp? Explain your answers. Val 216KVal 190 Asp 189 Chymotrypsin Trypsin ElastaseChymotrypsin, trypsin, and elastase all share the same catalytic mechanism, but have different specificities. (a) These proteases are considered serine proteases because the active sites contain serine, histidine and aspartate. Describe the roles that each of these amino acid residues play in hydrolyzing protein substrates. (b) The active site pockets of chymotrypsin, trypsin, and elastase are shown in the image below. Based on the image, which side chain of the amino acids would they prefer to cleave respectively, Ala, Arg, or Trp? Explain your answers. Val 216 Val 190 Asp 189 Chymotrypsin Trypsin ElastaseThe enzymatic activity of lysozyme is optimal at pH 5.2 and decreases above and below this pH value. Lysozyme contains two amino acid residues in the active site essential for catalysis: Glu35 and Asp52. The pK value for the carboxyl side chains of these two residues are 5.9 and 4.5 respectively. What is the ionization state of each residue at the pH optimum of lysozyme? How can the ionization states of these 2 amino acid residues explain the pH-activity profile of lysozyme?
- Parts of the mechanism for lysozyme are shown below. The catalytic lysozyme residue side chains can be identified as being above and below the polysaccharide chain shown at point A. Complete the missing structures (points B and E) and curly arrows (points A, B, C, D and E) in the mechanism, as well as from the knowledge you have gained, label the catalytic amino acids at point A in your mechanism with their correct name and residue number. A OH R OH O-R H₂C- HO NHAC NHAC E R D OH B R C H RO H3C- ° H₂C- NHAC Y R OH NHAC R R. The allosterically regulated enzyme ATCase binds aspartic acid as a substrate and acylates the a-amino group. Succinate acts as a competitive inhibitor of ATCase because it binds the active site but can't be acylated. The dependence of vo on [aspartic acid] for ATCase is shown in panel (a) of the accompanying figure. Panel (b) shows the effect of increasing [succinate] on v, when [Asp] is held at a low concentration (see thick vertical arrow in panel (a)). Note that in panel (b), vo is not zero when [succinate] =0 (see thin horizontal arrow). Explain the shape of the curve in panel (b). Why does v, increase initially, before decreasing at higher [succinate]? Co0- COO CH2 CH, HC -NH, CH, COO COO Asp Succinate [Asp) [Succinate] [Asp] in experiment b (a) (b)Amino transferases catalyze the transfer of amino groups to alpha-keto acids with pyridoxal phosphate as the prosthetic group. Pyridoxal phosphate can accept an amino group, and its aminated form, pyridoxamine phosphate, can donate its amino group to an alpha-keto acid. Illustrate the detailed mechanism of the transamination reaction in the active site of an aminotransferase. Use the glutamate oxaloacetate transaminase as an example.
- An active site of a hypothetical serine protease with a peptide substrate bound is shown below: This serine protease has 3 specificity pockets (S1, S2, S1') as shown in the figure above. S1 pocket has a glutamic acid in the bottom, the S2 pocket is small and hydrophobic, and the S1' pocket is deep and hydrophobic. Based on this information you can conclude that R1 is most likely side chain of a. Asp b. Lys c. Lle d. Gly e. Phe. (a) Explain the biochemical basis for the fact that one can synchro- nize cell populations by treating them with deoxythymidine. (b) Explain the apparent paradox that DATP at low concentrations is an activator of ribonucleotide reductase, whereas at higher concen- trations it becomes inhibitory.Consider the role of Histidine in the Serine protease mechanism and sketch a plot showing the predicted pH profile of chymotrypsin which has a pH optimum of approximately ~8. The pk, for the His in the catalytic triad is 7.3 in free chymotrypsin which increases to greater than 8 with a bound peptide. Be sure to label the plot axes and indicate the pka of His on the plot,
- Amino transferases catalyze the transfer of amino groups to alpha-keto acids with pyridoxal phosphate as the prosthetic group. Pyridoxal phosphate can accept an amino group, and its aminated form, pyridoxamine phosphate, can donate its amino group to an alpha-keto acid. Draw the detailed mechanism of the transamination reaction in the active site of an aminotransferase. Use the glutamate oxaloacetate transaminase as an example.Lysozyme is an enzyme that hydrolyzes bacterial cell wall polysaccharides. When this reaction is carried out in the presence of H218O, it is observed that there is retention of configuration at the C1 carbon of the D site sugar as shown below: Would this result suggest that the enzyme mechanism involves the direct nucleophilic attack of water at C1 or that the enzyme mechanism involves attack by a nucleophilic amino acid side chain of the enzyme that generates a transient covalent intermediate?(c) On the right is a diagram of the ac tive site of E. coli aspartate aminotrans- ferase illustrating the cofactor pyridoxal phosphate (labeled PLP) with the dicar- boxylic acid maleate (labeled MAL) bound in the active site. The structural formula of maleate is shown on the right. Am 194 MAL Arg292 Arg386 Ilx17 Lauf 'coo- H get H Coo- Maleate (c1) Draw the structure of L-aspartate and draw a border around the atoms in the amino acid that maleate simulates. (c2) Identify the active site residues that make hydrogen bonds and electrostatic interac- tions with the oxygen atoms of the carboxylate groups of maleate in the diagram above. Identify the carboxylate groups according to the numbering in the diagram of maleate above. Indicate the hydrogen donor groups of the active site residues. (C3) Compare and draw the structures of L-Arg and L-Lys. On the basis of the diagram why does replacement of an arginine for a lysine have an effect on substrate binding to AspAT? (c4) Of the mutant…