Allosteric Enzymes

What are sequential reactions characterized by?

Sequential reactions are characterized by the formation of a ternany complex consisting of two substrates and the enzyme

What is a ternary complex?

Enzyme bound to two substrates

Are sequential reactions ordered or random?

Sequential reactions may be *ordered* (S1 always binds before S2) or random *(order of substrate binding doesn’t matter)*

What are double displacements (ping pong) characterized by? \*review\*

Double displacements (or ping pong) are characterized by the formation of a **substituted enzyme intermediate**

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(One of more products are released before all substrates bind the enzyme)

What are allosteric enzymes?

Allosteric enzymes catalyze the **committed step** of metabolic pathways

What is the committed step?

The committed step is the step in which the cell commits to forming a product (as the formed intermediate serves no purpose to the cell and thus, making it is wasting energy unless it is converted to the final product.

What is feedback inhibition?

It’s where an inhibitor binds the regulatory site on the enzyme, which inhibits activity

How do metabolic pathways communicate?

Allosteric enzymes recognize both inhibitory molecules and stimulatory molecules

What is feed forward activation?

Look at slide 10

Why do allosteric enzymes display *sigmoidal* kinetics?

Due to cooperativity in substrate binding to the allosteric enzyme

Which form of enzyme is catalytically active?

The relaxed form

Describe the tense form of an enzyme

The tense form is less active but more stable and more common

Note: R and T are in equilibrium

What is the allosteric constant?

Allosteric constant = L0= T/R (typically in the hundreds)

What does the symmetry rule dictate?

That all active sites of an enzyme must be in the same state

Which state does substrate bind more readily to the enzyme?

Substrate binds more readily to R than T

Describe the concerted model for allosteric enzymes

* When \[S\] is low, most of the enzyme is in the stable tense form
* As \[S\] increases, it binds an active site on R, trapping all other active sites in R state (as per the symmetry rule)
* As most active sites are in R state, it is easier for substrate to bind to R state enzymes (velocity increases)
* The binding of substrate to relaxed enzymes becomes even easier as more active sites are in the R form

What is cooperativity?

It is where the binding of substrate disrupts the T-R equilibrium in favour of R

What is the threshold effect?

It’s the phenomenon in which allosteric enzymes are more sensitive to \[S\] near KM than Michaelis-Menten enzymes with the same Vmax

How do regulators modulate allosteric enzymes?

Negative regulators shift equilibrium to the less active T form (by stabilizing that form)

Positive regulators shift equilibrium to the more active R form (by stabilizing that form)

What are heterotropic effects?

Regulators shift the sigmoidal curve left (activator) or right (inhibitor)

Which way do inhibitors shift the sigmoidal curve?

Right

Which way do activators shift the sigmoidal curve?

Left (because less \[S\] is needed to increase velocity)

What are homotropic effects

Homotropic effects are effects of *substrate* on allosteric enzyme (the sigmoidal curve is a homotropic effect)

What is the sequential model for allosteric enzymes?

The binding of substrate to one active site influences the binding of substrates to neighbouring active sites

Do allosteric enzymes behave according to concerted or sequential model?

Many allosteric enzymes may behave according to a combination of the concerted and sequential models