So we have come to what in my mind is the most important topic of the course , yes , enzymes , these little protein buddies , as I like to call them , speed up chemical reactions within our body and allow us to live.

Enzymes are biological catalysts , hence they provide an alternative pathway to a reaction with a lower activation energy , activation is the minimum amount of energy required to initiate a chemical reaction.  It should be known that enzymes do no change the free energies of reactants or products and does not change the equilibrium either. An energy profile diagram for an enzyme catalyzed reaction can be seen below.


The rate at which enzymes work can be  expressed via the turnover number or Kcat , this is the number of molecules of substrate converted into product per enzyme per second. Naming enzymes allows us to identify the classes of enzymes by a single number , there are six classes of enzymes , 1)Oxidoreductases  , 2) Transferases, 3) Hydrolases, 4) Lyases  ,5) Isomerases, 6) Ligases.

Cofactors , a cofactor is a non protein component that makes the enzyme work . Prosthetic groups are coenzymes which are permanently associated , while Co substrates are transiently associated.

Apoenzyme + Cofator —– > Holoenzyme

Apoenzyme – the inactive protein part

Cofactor- non protein part

Holoenzyme – active protein

Active sites are the sites which the substrates bind to on the enzyme , the active site is usually a pocket of cleft. Interactions between the active site and subsrate occur via the same forces which stabilize the protein structure , ie. Hydrogen bonding , Van Der Waals interactions , hydrophobic interactions and electrostatic interactions.

There are a couple of hypothesis regarding the substrate , enzyme interaction. One such hypothesis is the Lock and Key hypothesis , which states that the active site of an enzyme is completely complementary in shape to the substrate it is catalysing. The Kushland’s Induced Fit hypothesis states that the shape of an enzyme active site is not exactly complementary to the shape if the substrate but rather it changes slightly in shape to accomadate the substrate , hence inducing its fit.

The following are graphs showing how different conditions , heat , temp etc , affect the rate of an enzyme catalyzed reaction.




As we have seen from the graphs above , enzymes are temperature sensitive and will denature after a certain has been reached. Enzymes have a  higher pH tolerance because of their internal hydrophobic interactions , hence their needs to be a substantial change in pH to destabilize the structure and denature the enzyme.

This brings us now to inhibitors , an inhibitor is any substance which diminishes the velocity of an enzyme catalysed reaction. There are different types of inhibitors that affect enzymes and they all have different ways in which they affect the rate of reaction.

Competitive inhibition – the inhibitor binds to the same site as the substrate would normally occupy. The line-weaver Burk plot and Michaelis Menten curve  for this type of inhibition  is seen below.


Non competitive inhibition- The inhibitor binds to different sites on the enzyme , and the inhibitor can bind to either a free enzyme of the enzyme-substrate complex. The Line-Weaver Burk Plot and Michaelis Menten curve for this type of inhibition is seen below.


Uncompetitive inhibition – The inhibitor binds to the enzyme-substrate complex , at a separate site from the substrate active site and not with the free enzyme.The The Line-Weaver Burk Plot for this type of inhibition is seen below.


Mixed inhibition- This occurs when the inhibitor binds at a separate site from the substrate active site , to either the free enzyme or the enzyme-substrate complex. The Line-Weaver Burk Plot for this type of inhibition is seen below.


We come now to the last point of this topic , which is Allosteric enzymes , they have more than one active site which cooperatively binds substrate molecules. The binding of one molecule causes a conformational change in the enzyme causing the other active site to gain an affinity to substrate and catalyse them.


Pictures  taken from ;

The BIOCHEMJM Youtube channel.

Revision World- Biology



Posted on April 7, 2013, in Uncategorized and tagged , . Bookmark the permalink. Leave a comment.

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