Enzymes

Enzymes

Enzymes are biological catalysts that speed up the biological reactions in the cells. They are protein in nature and help producing products rapidly by decreasing energy of activation without being changed.

Substrate

The substances to which enzymes are attached, making enzymes substrate complex ES and converting it to products quickly according to the need of cells.

Active Sites

Active sites are unique places on enzymes where the substrate is attached. As enzymes are proteins, so active sites are modified sequences of amino acids which serve best for linking with substrate.

Apo enzymes

The protein part of enzymes is called Apo enzymes.

Prosthetic Group

This part is covalently bonded with enzymes. It cannot be unattached from enzymes and helps increasing the efficacy of enzymes.

Prosthetic Groups are of Two Types

• Cofactor/Activators

The part of enzymes which are inorganic in nature and are required by the enzymes for accomplishing their activity. e.g Zn, Fe, Ca, Mg

Holoenzyme

An enzyme with its cofactor is called Holoenzyme.

• Coenzymes

They are organic parts  of enzymes and most of them are derived from vitamins e.g NAD, FAD, ATP.

They enhance the specificity of apoenzymes.

They help in transport of substrate.

Inhibitors

Any substance that decreases or stops the activity of enzyme catalyzed reation is called inhibitor. It inhibits the activity of enzyme. It can attach directly  to the enzymes or its activator.

Properties of Enzymes

• Enzymes are protein in nature; their non-protein part is called prosthetic group.
• They speed up the reaction without being utilized themselves and can be used again and again.
• Enzymes are very specific to their chemical reaction: that may be a single chemical reaction or a group of reactions.
• The enzyme does not change the equilibrium of of the reaction.
• Some enzymes chemically, immunologically or physically different but catalyze same reaction are called isozymes. e.g CPK (as CPK1 and CPK2) and LDH (as LDH1,LDH2,LDH3,LDH4,LDH5)
• Some RNA act like enzymes , catalyzing cleavage and synthesis of phosphodiester bonds, are called as ribozymes.
• Most enzyme show Michaelis Menton Equation.
• Graph between Initial Velocity and substrate has hyperbolic shape.
• Allosteric Enzymes give sigmoidal line in the graph.

Km :

Km is called as Michaelis Menten constant. Km is the property of enzymes and its substrate and is associated with the affinity for that substrate.

Mathematically, Km is equal to the substrate concentration where the reaction velocity is equal to 1/2Vmax

Small value of Km Indicates high affinity of enzyme for substrate.

Larger value of Km indicates low affinity of enzyme for substrate.

Commonly, enzymes experience two types of inhibitions during their activity.

Comeptitive Inhibition and Non Competitive Inhibition

Competitive Inhibition

The structure of inhibitor molecule is similar to that of the substrate,

The inhibitor get attached to the active site of enzyme.

It competes with the substrate molecule or for the enzyme.

It doesnot change the structure of enzymes.

The reaction can be reversed by increasing the substrate concentration. E.g Sulpha drugs given to bacteria with PABA and folic acid synthesis is inhibited.

Km is increased in competitive inhibition.

Vmax remains unchanged.

Inhibitor binds to the same site on the enzymes.

Non Competitive Inhibition

The structure of Inhibitor molecule is entirely different.

The inhibitor forms complex other than active site.

It doesnot compete with the substrate.

It alters the structure of the enzyme in such away that the substrate may get attached to the active site but products are not formed.

The reaction speed decreases gradually due to inhibitor linking till saturation is achieved.

e.g Cyanide combines with the prosthetic group of cytochrome oxidase and inhibits the electron transport chain.

Classification of Enzymes

Class	Subclass	Reaction catalyze	Example
Oxidoreductases	OxidasesAerobic DehydrogenasesAnaerobic Dehydrogenases Hyperperoxidases Oxygenases	Catalyze Oxidation-Reduction reactions	Dehydrogenases, Oxidases
Transferases	Transaminases Phospho transaminases (Kinases)Trans Methylases Transpeptidases Transcyclases	Catalyze Transfer of N, C or P containing group except H
Hydrolyses	Proteolytic (Exopeptidases and Endopeptidases)CarbohydrasesLipases, Cholesterases, Phospholipases Deaminases Deaminases Phosphatases Sulfatases	Catalyze cleavage of bonds by addition of water
Lyases		Catalyze C-C,C-S and certain C-N bonds	Fumarase, Carbonic Anhydrase, Decarboxylase
Isomerases		Catalyze racemization of optical or geometric isomers,	Methylmalonyl CoA multase
Ligases		Catalyze the joining of two molecules by forming C=O, C=S, C=N by using energy in the form of ATP	Pyruvate carboxylase, Succinic Thiokinase