Module 8: Enzyme regulation

Allosteric enzymes

•  complex, highly regulated enzymes monitoring flow of biochemicals in metabolic pathways

  • Allosteric enzymes catalyze/control key steps in metabolic pathways

• Allosteric enzymes usually have multiple polypeptide chains that each have active sites

• Allosteric enzymes are called so because they're regulated by allosteric effectors

  • Small molecules that bind allosteric sites on the enzyme to regulate its activity

Allosteric enzymes T-state

T state: catalytically inactive and has a low affinity for substrates

• Allosteric inhibitors stabilize the T state (less active)

Allosteric enzyme R state

catalytically active and has a high affinity for substrates 

• Allosteric activators stabilize the R state (more active)

Allosteric effects

• allostery makes enzymes much more sensitive to substrate concentration

• no changes in secondary structure elements

ATCase

• In T state, steric hindrances prevents Asp binding domain from rotating into favorable position for substrate binding.

• CTP binding drives structural changes that remove barrier to rotation

  • CTP stabilizes the T state; CTP biding to the R state causes trimers to move closer together which reorients catalytic side chains to reduce catalysis

• The two substrate binding domains reorient in each catalytic subunit to give R state with more accessible substrate binding sites and proper orientation for catalysis.

  • ATP binds in R state

Proteins are modified by phosphate groups at which residues?

Serine, Threonine and Tyrosine

Glycogen phosphorylase

• glycogen phosphorylase a: phosphorylated at Ser14

  • phosphorylation of Ser14 opens up the active site and promotes the active conformation

  • phosphorylation promotes R-state

  • AMP is an allosteric activator that binds preferentially to the R-state and drives the same conformational changes

• glycogen phosphorylase b: unphosphorylated

PFK regulation 

• major enzyme in glycolysis in muscle fiber

• catalyzes reaction having a large delta G

• important regulatory role

• tetrameric protein with R & T states

PFK Substrates

F6P, ATP

Each subunit has a substrate binding site that accepts the enzymes F6P and ATP

PFK products

FBP, ADP

PFK inhibitors

ATP binds the T state

PFK activators

AMP/ADP and F2,6P bind at the regulatory site