17. The papaya fruit contains an enzyme called papain, which is used in cooking to tenderize meat. Papain works by cleaving the peptide bonds that are found in proteins. Shown below are the arrow pushing mechanisms for the first two steps of the multi-step process by which the enzyme cleaves a peptide bond. Draw the transition state for each step of the reaction that's shown below. In your transition state, show bonds that are breaking and forming using dashed lines. Be sure to show partial charges whenever a formal charge is developing or dissipating.
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Transition state 1: $$\mathrm{[Cys{-}S{-}H \cdots C_{peptide} {=} O_{peptide} \cdots HN_{peptide}]}$$ Show more…
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3. Inhibition of papain: The protease papain, which can be isolated from papaya fruit, is a monomer with an active site per protein molecule. The purified papain enzyme activity was measured at various substrate concentrations in the absence (-I) and presence (+I) of inhibitor and plotted as initial velocity on the following graph. Considering that the enzyme concentration was held constant in these experiments at 1.2 x 10 -7 M, answer the following questions about the enzyme. a. Calculate Km for the substrate in the absence and presence of inhibitor. b. Calculate Vmax for the substrate in the absence and presence of the inhibitor. c. Identify the type of inhibition. d. Calculate Kcat for papain with this substrate in the absence of inhibitor.
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Regulation of Enzymes The enzyme phosphofructokinase (PFK) catalyzes the phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate, a key regulatory step in the glycolytic pathway. Phosphofructokinase is an allosteric enzyme regulated by ATP (negative effector) and AMP (positive effector), thus indicating the cell's energetic needs. (a) If you carry out experiments to understand the kinetics of PFK in your lab, what type of substrate-activity curve (similar to the Michaelis-Menten kinetics plots but for allosteric enzymes) would you expect to see, knowing that PFK is an allosteric enzyme? (b) Sketch the expected substrate-activity curve (by hand is alright!!) and in the same plot draw/sketch the activity curves describing the effects of the allosteric regulators of the enzyme phosphofructokinase (PFK) ATP and AMP. Explain the observed effects of each allosteric regulator on the shape of its corresponding curve.
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3. Draw the transition state for the following SN2 reaction: 4. The following reaction conditions can give SN1 or E1 products, as shown. Draw out both complete mechanisms (notice that they start with the first step!), including all electrons, formal charges and curved arrows showing electron movement.
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