Hello everyone, I would like to share another notes on enzymes and its types.
In this article I will discuss about:
- Mechanism of action
- Factors affecting Enzyme activity
- Competitive Inhibitor
- Non Competitive Inhibitor
- Un Competitive Inhibitor
- Enzyme kinetics
- Definition of Km or Michaelis constant.
What is Enzyme?:
enzymes are biological catalyst,mainly made up of protein(some are RNA type) that increase the speed of biochemical reactions taking place in the cell but remain unchanged at the end of the reaction.
- Enzymes are biocatalysts. They increase the speed of biochemical reactions but they remain unchanged at the end of the reaction.
- Enzymes are mainly protein in nature. Some are RNA types e.g. Ribozyme.
- They boost up the rate of biochemical reactions by decreasing the activation energy required to start the reaction.
- Enzymes are specific. Each enzyme has a specific shape, so only some specific substrates can fit into the active site of a particular enzyme.
- Activity of enzymes is regulated by some factors such as temperature, PH etc.
- Enzymes show inhibitory effects i.e. they are inhibited by inhibitors.
Mechanism of Enzyme Action:
There are two mostly accepted theory of Enzyme Action. These are:
- Lock and Key Theory
- Induced Fit Model
Lock and Key Theory:
The basic mechanism of enzymes can be explained via lock and key theory. According to this theory enzymes are specific to only specific substrate/substrates bind to the active site of the enzyme just like a specific lock is opened through a particular key. The active site is the particular region of the enzyme where substrates bind and therefore some molecular changes are occurred in the substrates leading to the reactions which ultimately results into products.
The active site of the enzymes is geometrically specific in shape which is further complementary to the shape of some specific substrates.
Induced Fit Model:
Induced fit model of enzyme activity is basically a theoretical model because of the absence of strong experimental results.
According to this theory, substrates play a pivotal role in determining the shape of the enzyme and there by supporting the partial flexibility of the active site of the enzyme leading to allow the better fit between the active site & the substrate. In short, it clarifies why certain mixes can tie to the protein yet don’t respond in light of the fact that the chemical has been misshaped to an extreme. Different particles might be too little to even think about inducing the correct arrangement and along these lines can’t respond. Just the best possible substrate is fit for inciting the best possible arrangement of the dynamic site.
According to international Union of Biochemistry (IUB) the present system of classification of enzyme is based on their reaction specificity. Six classes with several sub classes have been recognised.
This class includes enzymes which are concerned with biological oxidation and reduction and therefore with respiration and fermentation process. The important sub classes are:
I) Dehydrogenase: This catalyse reactions in which the electron transfer is accompanied by the transfer of hydrogen atoms. Example: alcohol dehydrogenase and glutamic dehydrogenase of animal liver.
II) Oxidases: in which molecular oxygen is one of the reactants. Example: cytochrome oxidase.
III) Peroxidases: it uses H2O2 as the oxidant.
IV) Oxygenases: It introduces molecular oxygen in place of a double bond in the substrate.
Enzymes of this class transfer a particular group from one substrate to another. The group maybe alkyl, glycyl, acyl, amino,sulphur or phosphorus.Transferase belong to different subclass according to the groups transferred by them. The sub classes are:
I) Acyltransferases: example choline Acyltransferase II) Glycosyltransferases: example phosphorylase III) Aminotransferases: example glutamic amino transferase IV) Kinases: example hexokinase.
They cleave their substrate by hydrolyzing a covalent bond between a carbon and some other atom. Hydrolases have different sub class according to the bond hydrolysed by them. There are 3 subclasses:
I) Proteases: These are enzymes that attack the peptide bonds of protein and peptides. It is customary to distinguish between the proteinases (endopeptidase) and the peptidases ( endopeptidase).
a) Peptidases: acts on peptide bonds adjacent to a free amino or carboxyl group. Among the principle types are the following_
i)Carboxypeptidases: It requires a free alpha carboxyl group in the substrate and split the peptide bond adjacent to this group,liberating a free amino acid.
ii)Aminopeptidases: It acts on the peptide bond adjacent to the free amino group of the simple peptide.
iii)Dipeptidase: It specifically acts on certain dipeptides. An example is glycylglycine dipeptidase.
Example: Pancreatic lipase belongs to the subclass of estarases as it hydrolysis an ester bond in a tri acylglycerol molecule.
This enzyme catalyse a reaction in which two parts of a molecule are separated with the formation of a double bond in one of them. The reaction catalyzed by them commonly reversible.
Example: L–aromatic amino acid decarboxylase of liver is a c – c lyase because it cleaves a c – c bond in 3 hydroxytryptophan.
Isomerase includes enzymes which catalyse interconversion of optical ,geometrical or positional Isomerase. They change their substrate to their isomer by intermolecular rearrangement and are divided into sub classes according to the type of isomerisation.
Example: Phosphohexose Isomerase of muscles is an aldose ketose Isomerase catalysing the interconversion of glucose (aldohexose) and fructose ( ketohexose).
These enzymes bond two substrate molecules together by forming a covalent bond.These reactions are endergonic,energy for driving them is provided by the simultaneous cleavage of some high energy bond.
Example: Acetyl CoA carboxylase of liver belongs to the subclass of C – C ligase as it joins the carbon of CO2 to the methyl-c of acetyl CoA to form molonyl CoA at the cost of a phosphate bond of ATP
Factors affecting Enzyme activity:
There are certain factors that affects enzyme activity. These factors are as below….
Each enzym shows the highest activity at a particular temperature,called its optimum temperature.
Further increase in temperature the activity will show a decline. At high temperature there is a loss of enzyme activity because denaturation of protein occurs at this temperature. At low temperature the Enzyme substrate interaction and consequently the catalytic action are minimised or suspended,but the stability of the Enzyme is enhanced.
the enzymes are influenced by pH changes since they are proteins and have an ionic character due to amino and carboxylic groups. If the pH is too low or too high the enzyme activity will loss due to the denaturation of protein.
There are certain compounds also known as antimetabolites which when added to the substrate combine with the enzyme reversibly irreversibly to block the production of end products such substances are known as inhibitor.
If the substrate concentration is increased from a very low level, the enzyme activity at first rises proportionately but with further increase in the substrate concentration, the rate of rise in the initial velocity declines progressively .Ultimately at a high substrate concentration the initial velocity reaches a maximum value Vmax which cant be exceed by any further rise in substrate concentration.
Cofactors can be metals or coenzyme and their primary function is to assist Enzyme activity.
They are able to assist in performing certain necessary reactions the Enzyme cannot perform alone.
Enzyme without their necessary cofactor are called apoenzyme ,it is the inactive form. Cofactor with an apoenzyme called holoenzyme which is the active form.
- inhibitor bears close structural similarity with substrate and competes with it later, attaching to the substrate binding site of enzyme.
- A high concentration of substrate may overcome competitive inhibition by forcing out Inhibitor molecule from EI complex.
- High substrate concentration produces the same maximum velocity as in inhibited state.The higher the inhibitor concentration the greater is the substrate concentration is required for reaching Vmax.
Competitive Inhibition Graph
Non competitive inhibition:
- Inhibitor bears no structural similarity with substrate and binds the enzyme at a site other than the substrate binding site.
- Inhibitor binds with free enzyme and enzyme substrate complex to form EI and ESI complex.
- By increasing substrate concentration Inhibitor can’t be dislodged from the EI or ESI complex.So original Vmax can’t be regained by enhancing substrate concentration. Thus Vmax is reduced.
Non competitive inhibition Graph:
Un competitive Inhibition :
- Inhibitor need not be a structural analogue of the substrate.
- It binds only to the enzyme substrate complex to form ESI complex and thereby probably be formed the structure and configuration of the active site to enhance the affinity of the enzyme for the substrate already bound to the enzyme. This inhibits the release of product from the enzyme.
- The original Vmax can’t be regained by enhancing the substrate concentration because the inhibitor can’t be dislodged from the ESI complex.
- As the inhibitor can bind only to the ES complex to affect the V0, the inhibition of the enzyme action and the fall of V0 are negligible at low substrate concentration.
Un competitive inhibition Graph:
E + S ⇌ ES & ES ⇌ E + P
velocity of the total reaction is proportional to the concentration of ES complex.
V ∝ [ES]
V = K [ES]…………..(I)
The maximum reaction rate (Vmax) occur at high substrate concentration so as to utilise all the available enzyme.
SO,concentration of [ES] = total enzyme [E total].
=> [ES] = [E total] × K
=> Vmax = K × [E total]……………..(ii)
Divide equation (i) by (ii)
Michelis Constant or Km:
It is the value of moler concentration of a substrate at which half the maximum velocity is attended.
- It is independent of enzyme concentration.
- It is the measure of affinity of the enzyme for a substrate and is inversely related to the substrate affinity.
- It’s unit is mole/liter.