Enzyme Inhibition

reversible inhibitors

noncovalent binding of the inhibitor and can always be reversed, at least in principle, by removal of the inhibitor

In some cases, noncovalent binding may be so strong as to appear irreversible under physiological conditions.

irreversible inhibitors

molecule is covalently bound to the active sites of enzyme and inactivates it.

Almost all irreversible enzyme inhibitors are toxic substances, either natural or synthetic.

Such substances react with some functional group in the active site to leave it catalytically inactive or to block substrate binding.

CANNOT BE REMOVED

competitive inhibitor

competes with substrate for the enzyme active site. It increases K_M the apparent but does not change the observed V_max.

Vmax and Km of competitive

Vmax stays the same: Infinite Substrate (S) mathematically drowns out the inhibitor, eventually forcing 100% saturation.

Km goes UP: The inhibitor blocks free enzymes (E), so you have to add significantly more S to fight through the fakes and reach 50% speed.

uncompetitive inhibitor

binds tightly to the ES complex but shows low or zero affinity for the free enzyme.

A simple uncompetitive inhibitor binds at a second site on an enzyme surface (not the active site) and prevents conversion of the bound substrate to product

It reduces both the apparent V_max and apparent K_M These effects cannot be reversed by increasing [S].

Vmax and Km of uncompetitive

Vmax goes DOWN: The inhibitor permanently handcuffs ES complexes into ESI complexes. Even with infinite S, these paralyzed enzymes cannot produce product, lowering the absolute speed limit.

Km goes DOWN: The formation of ESI acts like a vacuum, constantly draining ES from the system. This pulls the E + S —> ES reaction forward to replace the missing ES, which fakes a higher substrate affinity.

Mixed Inhibition

This form of inhibition occurs when a molecule or an ion can bind to both the free enzyme and the ES complex

It reduces the apparent V_max at all [S] and increases the K_M apparent

Vmax and Km of Mixed Inhibition

Vmax goes DOWN: The inhibitor binds to ES to form paralyzed ESI complexes, ruining the absolute speed limit just like uncompetitive.

Km changes based on a tug-of-war: Binding to free E pushes Km up (competitive effect), while binding to ES pulls Km down (uncompetitive effect). Whichever target the inhibitor prefers determines the final Km.

Multisubstrate Reactions

Most biochemical reactions involve two or more substrates.

Multisubstrate reactions fall into several classes, depending on the order of substrate binding: random, ordered, or ping-pong

why is Multisubstrate Reactions important

When an enzyme binds two or more substrates and releases multiple products, the order of the steps becomes an important feature of the enzyme mechanism.

Random Substrate Binding

either substrate can be bound first, although in many cases one substrate will be favored for initial binding, and its binding may promote the binding of the other.

phosphorylation of glucose by ATP

Ordered Substrate Binding

one substrate must bind before a second substrate can bind significantly

This mechanism is often observed in oxidations of substrates by the cofactor NAD⁺ / NADH,

Ping-Pong Mechanism

One substrate is bound, one product is released, a second substrate comes in, and a second product is released.