Question

1. A common notation for the depiction of binding events in a multi-substrate enzyme is to show the enzyme as a horizontal line with arrows showing the binding or release of reactants and products. For example, the reaction catalyzed by Zn? -dependent aldolase is depicted in the following diagram: fructose 1,6-bisphosphate (FBP) glyceraldehyde 3-phosphate (GAP) dihydroxyacetone phosphate (DHAP) E E-FBP ZE -DHAP GAPE.DHAPE The reaction catalyzed by Schiff-base forming aldolase, on the other hand, is depicted: FBP DHAP E-FBP E-FBP - E-DHAP-GAP E -DHAP a. Diagram the reaction catalyzed by hexokinase if glucose must bind before ATP and ADP must dissociate before glucose 6-phosphate. b. Diagram the reaction catalyzed by glyceraldehyde-3-phosphate dehydrogenase.

Answer 1

Glucose 61-6-0 ATP ADP E+GI EIGI + ATP E+ G1-60 - ADP E 6. Gl-6-0

G3-P NAD, si NADH Bla 6.6 3-1 E.G-3-P. NAD.Pi - → E. BPG. NADH E.BG

Justification for a and b.

Horizontal line presentation is based on the sequence of binding of substrates with enzymes and vice versa (i.e. Release of products from enzyme).

In the first case, the sequence of events is well defined in the question. Firstly, enzyme will bind to glucose and then binding of ATP will take place. Subsequently, conversion will take place resulting in formation of glucose 6 phosphate. The release sequence is also defined in the question with ADP release happening before dissociation of enzyme and glucose 6 phosphate.

In the second case, sequence of events is not provided. Thus, the binding of substrate with enzyme is predicted based on affinity and energy kinetics. First, binding of enzyme with substrate will take place which further allows the binding of NAD and inorganic phosphate. Subsequently, conversion of substrate into NADH and BPG will occur. The dissociation of NADH will take place followed by BPG resulting in release of enzyme.