5. What is the reaction catalyzed by triosephosphate isomerase: (give only the n

Question

5. What is the reaction catalyzed by triosephosphate isomerase: (give only the names of substrate and product) The 0" for this reaction as used in glycolysis is +7.9 klinol. Based on this information What can you conclude about the relative concentrations of substrate and products at equilibrium? (circle one) What can you conclude about the relative concentrations of substrate and products at standard conditions? What can you conclude about the relative values of Q and Keq in a cell that is using glycolysis? What can you conclude about the Go, ofthe reaction without the enzyme present? This diagram shows the first two steps of a coenzyme that (xylulose) is the substrate of the reaction transketolase reaction mechanism. The ring is from thiamine, a is part of the enzyme. The way lines shows amino acid side chains of the enzyme. The sugar 6. CH,OH CH2OH COOH 0 COOH coO HO_ Ho OH lys What catalytic strategy is being shown in the first step of the mechanism? What catalytic strategy is being shown in the second step of the mechanism? What is the role of the lysine? In the next step, part of the original substrate acts as a leaving group to produce one of the products of the reaction. What is that product?

Explanation / Answer

5. (a) Substrate = Ketose Dihydroxyacetone phosphate (DHAP)

         Product   = Aldose Glyceraldehyde-3-phosphate (GAP)

(b) At equilibrium, {S} > {P}

Explanation : At equilibrium, about 95% of the triose phosphate is dihydroxyacetone phosphate.


(c) At standard conditons,{S} < {P}

Explantion : The reaction is not at equlibrium,therefore Ketose Dihydroxyacetone phosphate (DHAP) is converted to glyceraldehyde 3-phosphate (GAP) by Triose Phosphate Isomerase (TPI) through the formation of an intermediate, "enediol".Depending upon the reaction conditions,concentration of substrate and products keep on varying.

(d) Q > Keq

Explanation - Since Q tells us in which direction,reaction is taking place and since with the help of enzymes it is going in the forward direction thus,its value is going to be greator than Keq.


(e) In the absence of enzyme,the reversible conversion of Aldose Glyceraldehyde-3-phosphate (GAP) to Ketose Dihydroxyacetone phosphate (DHAP) will happen at a very slow rate through an unstable enediol intermediate.The uncatalyzed reaction is so slow such that the aldose and the ketose are both stable compounds in the absence of a catalyst.

Therefore, G >7.9 KJ/mol -1 which indicates that the tendency of reaction to go in the forward reaction is decreasing.

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