Allosteric enzymes

Allostery

Enzymes don’t follow Michaelis-Menten kinetics

Sigmoidal curves are typical- cooperative, multiple states- individual states=hyperbolic, combined=sigmoidal

Feedback inhibition

Homotropic/homoalloster effectors

Substrates act as effectors, may bind to active or regulatory sites

Increase or decrease activity

Heterotropic/heteroallostery effectors

Non-substrate effectors, bind to regulatory sites

Increase or decrease activity

Main structural feature of most allosteric enzymes?

Quaternary structure

Effectors bind to regulatory subunit if it exists or regulatory site on catalytic subunits

Can’t use Km

Symmetry model of allostery

Switch all together, each oligomeric can exist in either the R or T state, ligands can bind to either state and shift equilibrium, overal symmetry is maintains (all subunits in same confirmation), D symmetry is common

Sequential model of symmetry

Each subunit can exist in either T or R state, rest is same besides overal symmetry is not maintained

Aspartate transcarbamolyase (ATCase)

6 catalytic subunits, 6 regulatory, D3 symmetry

feedback inhibition from CTP (its product)

Activated by ATP

Follows symmetry model

Steric bar until binding occurs and subunits can separate (R state)

Feedback inhibition

Concentration if end product often signals the amount of activity required, feedback inhibitors are often allosteric modulators

How do we study the R state

Something that looks like intermediate but won’t react, traps it in R state

competitive inhibitor

In ATCase use PALA

Allostery of ATCase

• CTP= heterotropic inhibitor

• ATP heterotropic activator

• Aspartate homotropic activator

Phosphofructokinase 1

Homotetramer, D2 symmetry, each unit contains an active and a regulatory site

Active and regulatory sites are far apart, quaternary structure brings them together

Allostery of phosphofructokinase 1

• ATP- homotropic inhibitor (rare case)

• Fructose-6-phosphate- substrate, homotropic activator

• AMP- Heterotropic activator, competes with ATP, overcomes inhibition from ATP

• Phosphoenolpyruvate- heterotropic inhibitor

What changes are seem in the T to the R state of PFK1

Glu 161-Arg 162

Substrate is negatively charged so needs the positive charge for affinity