Allostery

2017-10-27


You measure MM curves for a variety of enzymes (solid curves). You add molecule $A$ to each solution and remeasure its MM curve (dashed line). For each curve: What changed ($K_{M}$ or $V_{max}$)? Is this a competitive or noncompetitive inhibitor?
$K_{M}$, competitive
$V_{max}$, noncompetitive
$V_{max}$, activator

Competitive inhibitor

  • Inhibitor competes with substrate for same site
  • This raises $K_{M}$: lower apparent affinity for substrate because it has to compete
  • No effect on $V_{max}$. If you add enough substrate, you swamp out competitor
  • Inhibitor is chemically similar to the substrate
  • Example: Ethanol competes with methanol for alcohol dehydrogenase, lowering the rate of formaldehyde production.

Noncompetitive inhibitor

  • Binds distant from the active site, altering active site to turn off activity
  • This leads to a drop in $V_{max}$. (lower $[E]_{T}$: Less of the enzyme is in the active form.)
  • No effect on $K_{M}$. What active enzyme is around has exact same affinity for substrate
  • The inhibitor need not have any chemical similarity to the substrate

Example noncompetitive inhibitor

Conceptual goals

  • Understand that binding at one site in a protein can alter activity at another site
  • This "allostery" arises because the "allosteric effector" interacts with one conformation, but not the other.
  • Understand how this applies to hemoglobin (BPG).

Skill goals

  • Determine how binding at on site affects activity at the other
  • Predict the effects of adding allosteric effectors to a system of equilibria.

Hemoglobin transports $O_{2}$ from the lungs to tissues


stephaniefuturedoc

Hemoglobin is a tetramer four proteins


Credit: Janet Iwasa (Utah)
Credit: Janet Iwasa (Utah)
Credit: Janet Iwasa (Utah)

This is through linked equilibria

$E_{active} + I \rightleftarrows E_{inactive} + I \rightleftarrows E_{inactive} \cdot I$

$[E]_{active} = [E]_{T}\theta_{active}$

$\theta_{active} = \frac{[E_{active}]}{[E_{active}] + [E_{inactive}] + [E_{inactive}\cdot I]}$

What are the ingredients you would need for allostery?

Ingredients:

  • Two different binding sites recognizing different things
  • Equilibrium between two (or more) protein shapes
  • Different "activities" (functions, properties, etc.) of each shape
  • Binding to one shape, but not the others

Summary

Allostery is when binding at one site in a protein alters activity at another site

  • Two different binding sites recognizing different things
  • Equilibrium between two (or more) protein shapes
  • Different "activities" (functions, properties, etc.) of each shape
  • Binding to one shape, but not the others

Noncompetitive inhibitors

BPG allosterically regulates $O_{2}$ binding in hemoglobin