7. Cyclic nucleotide phosphodiesterase type 5 (PDE5) is a human enzyme that cata

Question

7. Cyclic nucleotide phosphodiesterase type 5 (PDE5) is a human enzyme that catalyzes the following hydrolysis reaction of cyclic nucleotides cyclic nucleotide phosphate +H2O5'-nucleotide phosphate A. Below are shown two kinetic plots of PDE5 with two different substrates: cyclic AMP and cyclic GMP. Calculate the KM and VmEx for cGMP and cAMP for PDES. Include units in your answer 3 10 20 -1.5-10-0.3 0.0 1.0 13 20 0 001 0.0 001 02 B. The enzyme reaction shown was performed in a 1 ml volume with a concentration of PDES of 0.01 mg/ml. The enzyme has a molecular weight of 50,000 daltons and has one active site. Calculate the turnover numbers for PDE5 with cGMP and with cAMP as substrates. Show your work. C. Based on the specificity constantsca/m) for each substrate, which substrate shows a greater catalytic efficiency with PDES?

Explanation / Answer

ans a- for cyclic GMP:

x-intercept = -1/Km

in plot, x-intercept= -0.5 therefore, -0.5= -1/Km i.e., Km= 1/0.5= 2uM-1

y- intercept= 1/Vmax

in plot, y-intercept= 5 thus, Vmax= 0.2 umol/min

for cyclic AMP, as calculated above;

Km= 100umol and Vmax= 0.1 umol/min

ans b-

turnover number with cyclicGMP= Vmax/Et= 0.2/0.01= 20/min

turnover number with cyclicAMP= Vmax/Et= 0.1/0.01= 10/min

ans c-

specificity constant for enzyme with cGMP= 20/2= 10

speicificity constant for enzyme with cAMP= 10/100= 0.1

thus, cGMP shows greater efficieny with PDE5.

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