Reflections on enzyme :)

In my previous posts on enzymes I did a little a history and I did a little introduction, I do hope that you found it interesting and informative. 🙂 

In this blog post I would like to go a little more indept. 

Michealis – Menten. 

Leonor Michaelis Image  and

Maude Menten   Image

came up with a simple model that accounts for most of the features of enzyme – catalyzed reactions. 

In the model the enzyme will reversibly combine with the substrate and form an enzyme – substrate complex, this will yield a product and regenerate the free enzyme. This is represented by the following equation

 Image

In the Michaelis -Menten equation it describes how reaction velocity varies with substrate concentration.

 Image

The Km in the equation represents Michaelis constant.

There were a lot of Assumptions made during the deriving of the Michaelis- Menten rate equation. And they are…..

1. Relative concentrations of E and S: It says that the [S] is greater that the [E], which means the percentage of total substrate bound by the enzyme at any given time is small 

2. Steady- state assumption : This says that the ES complex in unchanged over time, the rate of formation of ES is equal to that of the breakdown of ES. This is similar to transition state, and if you cant remember what the transition state is you can just go and read my previous post on enzymes. 

3. Initial Velocity: The rate of the reaction is measured as soon as the enzyme and substrate are mixed. At this point the concentration of the product is small. 

Now we move on to some important conclusions about Michaelis – Menten Kinetics. 

1. Characteristics of Km 

    Km – (Michaelis constant) it is characteristic of an enzyme and its substrate. Reflecting the affinity of the enzyme for that substrate. It is numerically equal to the substrate concentration at which the reaction velocity is equal to 1/2 Vmax. It also does not vary with the concentration of enzymes. 

When you have a small Km it reflects a high affinity of the enzyme for the substrate.

When you have a large Km it reflects a low affinity of the enzyme for the substrate.  

2. Order of reactions. 

When the [S] is much less than Km the velocity if the reaction would be proportional to the [S]. This is said to be a first order reaction with respect to [S].

When the [S] is much greater than Km the velocity if the reaction would constant and equal to Vmax. The rate would now be independent of the [S] , and it is termed zero order with respect to [S].

 

Lineweaver – Burk Plot

This is another method that can be used to calculate Km and Vmax, it also determines the mechanisms of actions of enzymes inhibitors.

Why create a new method when there is already one ?

Well when you plot Vo against [S], it is not always possible to determine when Vmax is reached, this is due to the gradual upward slope of the hyperbolic curve at high [S]

But in a Lineweaver – Burk Plot a straight line is obtained if 1/Vo is plotted against 1/ [S]

The equation describing the Lineweaver – Burk plot is as follows

Image

The intercept on the x-axis is equal to -1/Km , and the intercept on the y-axis is equal to 1/Vmax.

 

Inhibition of enzyme activity!!!

There are substances that diminish the velocity of an enzyme-catalyzed reaction and these are known as inhibitors, there are two types : irreversible and reversible.

When we talk about irreversible inhinbitors the name says it all, basically they can’t form their original shape, because  the inhibitor would bind to the enzyme via covalent bonds, and we all know how strong these bonds are.

However In reversible inhibitors, they bind via noncovalent bonds, this results is the dissociation of the inhibitor, and recovery of enzyme activity.

There are four types of reversible inhibitors and they are

Competitive inhibitor

Non competitive inhibitor

Uncompetitive inhibitor

Mixed Inhibitors

 

Starting with competitive inhibition.

The inhibitor resembles the substrate so it competes with it for the active site, however it binds reversibly and since it is just an inhibitor no products will be formed, it slows the rate of the reaction.

Michaelis – Menten Vs Lineweaver for competitive inhibition

 

 Image

Non- competitive Inhibition

In this type of inhibition there is no competition for the active site, the inhibitor rather binds to either the free enzyme or the ES complex, this prevents the reaction from occurring. It is recognised by its effect on Vmax, it decreases Vmax. It however does not affect Km.

 Image

 Line weaver – Burk plot of non competitive inhibition

 Image

Uncompetitive inhibition

The inhibitor binds only to the enzyme-substrate complex at a separate site from the substrate active site, and not with the free enzyme, the inhibitor does not resemble the substrate. Both Vmax and Km are reduced to the same amount.

Image 

Mixed Inhibition

In this inhibition the inhibitor does not resemble the substrate as it does not bind to the substrate active site but rather to the free enzyme or enzyme substrate complex. Vmax would always be reduced while Km can either be increased or decreased.

 

 Image

Allosteric enzymes!

These are enzymes with more than one active sites, which cooperatively bind substrate molecules, when this substrate molecule binds to the enzyme it induces a conformational change in the enzyme which affects the affinity of the other active sites for substrates.

They are multisubunit proteins, with  an active site on each subunit.

They are regulated by effectors, that bind non covalently to any other part of the enzyme but the active site.

This effector alters the affinity of the enzyme for its substrate

There are two types of effectors – those that inhibit enzyme activity (negative effectors), and those that increase enzyme activity (positive effectors)

 

Homotropic effectors

When the substrate itself acts as an effector.

Heterotropic effectors

When the effector is different from the substrate.

 

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