HW Lecture #17: Intro to Metabolism: Overview and Review of Cells

Pg. 380; Study Exercise #2:

Explain the metabolic significance of reactions that function near equilibrium and reactions that function far from equilibrium:

Reactions that function near equilibrium (D G ~ 0) are driven in the forward or reverse direction primarily by concentration of substrate and product. These reactions are reversible.

"Enzymes that catalyze near-equilibrium reactions tend to act quickly to restore equilibrium concentrations, and the net rates of such reactions are effectively regulated by the relative concentrations of substrates and products." Pg. 358

Reactions that function far from equilibrium D G<<<0 are irreversible.

Far from equilibrium reactions:

Dictate direction of metabolic pathways (Reversal of the pathway must proceed via an alternate route).
Serve as committed steps when located at the beginning of the pathway.
Are sites of regulatory control (eg. Phosphofructokinase in glycolysis).

Enzymes that catalyze far from equilibrium reactions do not have enough activity to allow the reaction to come to equilibrium. Typically activity is inhibited by other molecules that bind to the enzyme and inhibit enzyme activity (allosteric effectors). It is like having one foot on the gas (lots of substrate), and the other foot on the brake (allosteric inhibitor). Once the allosteric inhibitor dissociates or an allosteric activitor binds, the enzyme has sufficient activity to catalyze the reactions. Pg. 358