CH 7 - Part 2

Allosteric Enzymes: Catalysts and Information Sensors

Key Properties of Allosteric Proteins

• Allosteric enzymes control the flux of biochemical reactions in metabolic pathways.

• Their regulatory properties enable the generation of complex metabolic pathways.

Regulation by Products of Metabolic Pathways

• The conversion of A to B is the committed step, meaning once A is converted to B, B is destined to become F.

• Allosteric enzymes catalyze the committed step of metabolic pathways, while Michaelis-Menten enzymes facilitate the remaining steps.

• Committed steps are irreversible under cellular conditions.

• Feedback inhibition regulates the amount of F synthesized.

• The pathway product F inhibits enzyme e1 by binding to a regulatory site distinct from the active site.

• Regulation of metabolic pathways can be complex, with allosteric enzymes potentially inhibited or stimulated by multiple regulatory molecules.

Deviation from Michaelis-Menten Kinetics

• The reaction velocity of allosteric enzymes exhibits a sigmoidal relationship with substrate concentration, unlike the hyperbolic relationship seen in Michaelis-Menten kinetics.

Dependence on Quaternary Structure

• All allosteric enzymes display quaternary structure, featuring multiple active sites and regulatory sites.

• The concerted model explains the kinetics of allosteric enzymes:

  • The enzyme exists in two quaternary structures: T (tense) and R (relaxed).

  • T and R are in equilibrium: T \rightleftharpoons R

  • The T state is less active but more stable.

  • The R state is enzymatically more active but less stable.

  • All active sites must be in the same state (either all T or all R).

• Substrate binding to one active site traps other active sites in the R state.

• This disruption of the T \rightleftharpoons R equilibrium by substrate binding favors further substrate binding which known as cooperativity.

Threshold Effect

• Allosteric enzymes are more sensitive to changes in substrate concentration near their K_M values compared to Michaelis-Menten enzymes.

• This heightened sensitivity is known as the threshold effect.

Sequential Model

• The sequential model proposes that subunits of allosteric enzymes undergo sequential changes in structure, in contrast to the concerted model where all subunits change simultaneously.

Modulation of the T \rightleftharpoons R Equilibrium

• Allosteric regulators disrupt the R \rightleftharpoons T equilibrium upon binding to the enzyme.

• Inhibitors stabilize the T state, while activators stabilize the R state.

• Homotropic effect: Disruption of the T \rightleftharpoons R equilibrium by substrates.

• Heterotropic effect: Disruption of the T \rightleftharpoons R equilibrium by regulators.

Clinical Insight: Loss of Allosteric Control

• Phosphoribosylpyrophosphate synthetase (PRS) is an allosteric enzyme in the purine nucleotide synthesis pathway.

• A mutation leading to the loss of regulatory control (while maintaining catalytic activity) causes overproduction of purine nucleotides, subsequently converted to urate.

• Overproduction of urate results in gout.

Quick Quiz 2

What would be the effect of a mutation in an allosteric enzyme that resulted in a T/R ratio of 0?

Problem-Solving Strategies 2

PROBLEM: Examine the metabolic pathway shown below. Which of the enzymes shown as “e” with a numeric subscript is likely to be the allosteric enzyme that controls the synthesis of G?