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- 5. Suppose an enzyme needed a cysteine thiol (sulfhydryl) group for its catalytic activity, and catalysis required the Cys thiol to act as a nucleophile at the beginning of the catalytic cycle. That would mean the Cys residue had to be in its UNPROTONATED (conjugate base) form. If that particular Cys residue in that protein had a pKą of 7.5, what fraction (percent, or proportion) of the total enzyme molecules would have the Cys R group in its ACTIVE form at pH 7.0?1.The diagram below shows an outline of the aminotransferase mechanism that skips the specific steps that show how electrons flow when a Schiff base is formed or is hydrolyzed. Using the mechanistic details given below A. Draw the mechanism for Enzyme- PLP Schiff base formation using pyridoxal and the lysine amine group using arrow to indicate electron flow. B. Draw the mechanism for hydrolysis of the Schiff base to form the a-keto acid, which is the reverse of the first reaction. Stage 1 Lys residue 2-O,PO- Enz Amino acid Lys residue (CH2)4 (CH₂2)4 H R-C-COO R- NH₂ NH₂ R-C-COO *NH₂ H H- 2-0,PO- 2-0,PO- A. show how this forms Stage 1 Lys residue Enz (CH2)4 HO 2-O,PO Enzyme-PLP Schiff base OH Amino acid-PLP Schiff base (aldimine) LOH 2-0₂PO- OH 20,PO- a-Keto acid COO Ketimine a-Keto acid 2-0,PO- R-C- NH₂ B. show how the a-keto acid forms a-Keto acid H Enzyme-PLP Schiff base الرواية وطري Ketimine Pyridoxine (vitamin B6) Pyridoxal phosphate (PLP) Pyridoxamine phosphate Stage 2 Amino acid…2. When enzyme solutions are heated, there is a progressive loss of catalytic activity over time due to denaturation of the enzyme. A solution of the enzyme incubated at 45 °C lost 50% of its activity in 12 min, but when incubated at 45 °C in the presence of a very large concentration of one of its substrates, it lost only 3% of its activity in 12 min. Suggest why thermal denaturation of hexokinase was retarded in hexokinase the presence of one of its substrates.
- 1. Consider the three-dimensional model of the tertiary structure of an enzyme below. Amino acids involved in binding are shaded blue, and amino acids involved in catalysis are shaded red. A. Suppose research has shown that amino acid 82 in the red shaded region is lysine, an amino acid with a positively-charged side chain. This lysine is critical for catalysis. Other studies have found that amino acids 12 and 62 in the blue region are both phenylalanine, an amino acid with a nonpolar side chain, and are critical for substrate binding. These amino acids are relatively close in the active site but are separated by 20-70 amino acids in the primary structure. Using what you know about protein structure, explain how amino acids separated in the primary structure can come close together in the active site. B. Use this information and figure 4.2 in your book to answer the following questions: Do you think changing amino acid 82, lysine, an amino acid with a positively-charged side…10. Chymotrypsin is a serine protease enzyme. The Km for the reaction of chymotrypsin with N-acetylvaline ethyl ester is 8.8*102, and the Km for the reaction of chymotrypsin with N- acetyltyrosine ethyl ester is 6.6*10“ M. catalytic triad Ser 195 His 57 Gly 193 N-H OH R-N- Ca N-H O-C- Asp 102 N-acetyl valine N-acetyl tyrosine Chymotrypsin Active Site a. What is the nucleophile here and how is it activated? b. Which substrate has an apparent higher affinity for the enzyme. c. Propose a reason for the difference in affinity based on the shape of each of the substrates (see active site figure, cleaves on the C-side of aromatic residues).4.1
- 2. Cofactors, coenzymes, and prosthetic groups are important non-protein substances that are required for enzyme function. This is a table of cofactors/coenzymes/prosthetic group we've discussed. Draw the functional portion of the cofactor/pro and explain the functional role in each reaction. a. Cofactor Name thiamine pyrophosphate (TPP) oxidized Lipoamide/lipoic acid/lipoyl-lysine Coenzyme A (CoASH) flavin adenine dinucleotide (FAD) reduced nicotinamide adenine dinucleotide (NADH + H+) oxidized nicotinamide adenine dinucleotide (NADP+) biotin Reaction Pyruvate -> acetaldehyde Pyruvate -> Acetyl CoA aKetoglutarate -> Succinyl-CoA Succinate -> Fumarate Pyruvate -> lactate 6-Phosphogluconate -> Ribulose 5- phosphate Bicarbonate + pyruvate -> oxaloacetate Structure of only the cofactor Functional or catalytic role124. Consider the figure below, which is an alternate way to depict the energy changes occurring during a reaction from Substrate (S) to Product (P), when uncatalyzed (curve A) and when catalyzed by an enzyme (curve B). Note that curve B is not the same way we modeled an enzyme-catalyzed reaction in class; this model is a different, perhaps slightly more realistic, way of conceptualizing the energy changes over the course of a reaction than what we did in class. S and ES represent the transition states for reaction of the free substrate (S) or the enzyme-substrate complex (ES). T T activation energy for uncatalyzed reaction EST S edhosob nohtum od bluo woH abiow yedio 2 wwolaixa-y odi no vaigin oroda dapng odt ni o nfog) r gibadhoa P B ES EP progress of reaction activation energy for catalyzed reaction a) Explain briefly why, in curve B, the energy state of the enzyme-substrate complex is less than the energy state of the substrate alone. b) Suppose the enzyme in the diagram was mutated…
- 2. The enzyme exists in two interconvertible forms that differ markedly in their activities: Protomer (inactive) ⇒ filamentous polymer (active) Citrate and isocitrate bind preferentially to the filamentous form, and palmitoyl-CoA binds preferentially to the protomer. Select the true statements that explain how these properties are consistent with the regulatory role of acetyl-CoA carboxylase in the biosynthesis of fatty acids. High citrate and isocitrate levels indicate that there is plenty of ATP, reduced pyridine nucleotides, and acetyl-CoA for fatty acid synthesis. Palmitoyl-CoA is a feedback inhibitor, driving the equilibrium in the direction of the inactive (protomer) form. Palmitoyl-CoA triggers phosphorylation to inactivate the enzyme. Citrate and isocitrate are the precursors for acetyl-CoA synthesis in the mitochondria, thus providing the starting material for fatty acid biosynthesis. The protomer and filamentous polymer are the two subunits of acetyl-CoA carboxylase that hold…38. The shown reaction is one of the four repeating steps during fatty acid biosynthesis. Which of the following statements is correct? 유 CH3-C-CH₂-C-S-ACP A B OH 유 CH3-C-CH₂-C-5-ACP → A. The small molecule in box A is NADPH + H* B. It is the second reduction reaction during fatty acid biosynthesis C. Both A and B D. Neither A nor BChart is Given for you: Below is a chart of values for actual enzymes. Enzyme Km (M) kcat (1/s)Chymotrypsin 1.5 × 10^−2 0.14Pepsin 3.0 × 10^−4 0.5Tyrosyl-tRNA synthetase 9.0 × 10^−4 7.6Ribonuclease 7.9 × 10^−3 7.9 × 10^2Carbonic anhydrase 2.6 × 10^−2 4.0 × 10^5Fumarase 5.0 × 10^−6 8.0 × 10^2 Assume the enzyme concentration is equal across all samples (and is equal to 1). (Answer a and b only)a. Which enzyme will have the highest V0 at very high substrate concentrations? (1 M). Why? b. Which will have the highest V0 at very low substrate concentrations (5.0 × 10^−12). Why?
