Enzyme Regulation

Why does the body regulate enzymes?

Because cells need to control how fast metabolic pathways run

• Without regulation…

  • Too much product can be made

  • Energy/resources get wasted

  • Cell balance is disrupted

Two ways enzyme activity is regulated

1. Regulation of gene expression

—→ Controls how much enzyme is made

2. Regulation of enzyme activity

—→ Changes activity of enzymes already present

Competitive Inhibition

A molecule competes with the substrate for the active site

• Inhibitor looks similar to substrate

• Blocks substrate from binding

• Slows reaction

Why does increasing substrate concentration reverse competitive inhibition?

Because more substrate molecules outcompete the inhibitor for the active site

• More substrate = better chance the substrate binds instead

Noncompetitive Inhibition

Inhibitor binds somewhere other than active site

• This changes enzyme shape —→ active site works poorly

  • They bind to allosteric site = non-direct change to active site

Irreversible Inhibition

Inhibitor permanently disables enzyme by forming covalent bonds

• Enzyme can’t function again

• Covalent bonds with key amino acids = so strong the enzyme can’t break them

Allosteric Site

A separate spot on an enzyme

• A regulator molecule binds to it = changing the enzyme’s activity

  • Changes enzyme shape

  • Turns enzyme OFF or ON

Why do many allosteric enzymes have quaternary structure?

Because they often have:

• Regulatory subunits

• Catalytic subunits

What is Feedback Inhibition?

When the end product of a pathway STOPS the first enzyme from working

• Prevents overproduction of product

Why is feedback inhibition important?

• Prevents waste

• Saves energy

• Prevents overproduction of product

• Maintains balance

ATP Feedback

When a cell has plenty ATP…

• ATP binds to enzyme (in its own production pathway) —→ enzyme changes shape —→ pathway slows down —→ less ATP made —→ ATP levels drop —→ enzyme reactivates —→ STARTS AGAIN

  

Phosphofrucktokinase (PFK)

Regulates glycolysis

• Activated by Adenosine monophosphate (AMP)

• Inhibited by ATP

• Inhibited by Citrate

How are branched pathways regulated?

Different end products inhibit their own pathways

Positive Feedback

When a product or signal increases its own production —→ making the process go faster

Negative Feedback

When a product or signal slows down or stops its own production —→ keep things balanced