GPBR112 :: Lecture 15 :: ENZYMES

One of the unique characteristics of a living cell is its ability to permit complex reactions to proceed rapidly at the temperature of the surrounding environment.

  • The principal agents which participate in the remarkable transformations in the cell belong to a group of proteins named enzymes. In the absence of enzymes in the cell, these reactions would proceed too slowly.
  • Enzymes are proteins specialised to catalyse biological reactions with the following characteristics.

Characteristics of enzymes

  • Enzymes being proteins exhibit all properties of proteins.
  • They have their specific isoelectric points at which they are least soluble.
  • Like proteins, they can be denatured by changes in pH and temperature.
  • The enzyme-catalysed reactions occur below 100oC, at atmospheric pressure and nearby neutral pH.
  • Enzymes undergo physical changes during the reaction but revert to their original form at the end of the reaction.
  • Enzymes exhibit enormous catalytic power.The rates of enzymatically catalysed reactions are 106 - 1012 times greater than those of the corresponding uncatalysed reactions and several times greater than those of the corresponding chemically catalysed reactions.
  • For example the carbonic anhydrase enzyme catalyses the conversion of carbondioxide to carbonic acid.

                                            CO2 + H2O . H2CO3

  • In this reaction, each enzyme molecule can hydrate 105 molecules of CO2  per second.
  • Enzyme activity is regulated in a variety of ways, ranging from controls over the amount of enzyme protein synthesised by the cell or modulation of activity through reversible interaction with metabolic inhibitors and activators or through  isoenzymes.

Specificity of the enzymes

  • One of the characteristic feature which distinguishes enzymes from catalysts is their specificity.
  • Enzymes are specific in the reaction catalysed and in their choice of substrates.
  • It usually catalyses a single chemical reaction or a set of closely related reactions

Three kinds of specificities are observed.
i. Absolute specificity

  • When enzymes catalyse only one particular reaction they are said to exhibit absolute specificity.
  • e.g. Urease acts only on urea.

ii. Group specificity

  • Enzymes acting on a group of substances that possess a particular type of linkage common to that group of substances are said to exhibit group specificity.
  • Amylase hydrolyses the group of substances like starch, dextrin and glycogen, which have the same type of glycosidic linkages (α1,4).

iii. Optical specificity

  • Almost all enzymes show a high degree of optical specificity.
  • There are certain enzymes which catalyse the hydrolysis of same group of substances possessing same optical activity
  • Eg. D-amino acid oxidase acts on D-amino acid and L-amino acid oxidase acts on L-amino acid.
  • Maltase catalyses the hydrolysis of α-but not β- glycosides.

Classification of enzymes

  • In olden days enzymes have been named by adding the suffix -ase to the name of

            the substrate (the molecule on which the enzyme acts).

  • Ex. Urease (Substrate urea) Arginase (Substrate arginine)
  • Recent studies on the mechanism of enzyme catalysed reactions have led to a more rational classification of enzymes.
  • The International Union of Biochemistry (IUB) established a commission on enzyme nomenclature to adopt a systematic classification and nomenclature of all the existing and yet to be discovered enzymes.
  • This system is based on the substrate and reaction specificity.
  • Although, this International Union of Biochemistry system is complex, it is precise, descriptive and informative.
  • IUB system classifies enzymes into six major classes (should be written in specific order only)

1. Oxidoreductases
2. Transferases
3. Hydrolases
4. Lyases
5. Isomerases
6. Ligases

  • Again each class is divided into subclasses according to the type of reaction catalysed.
  • Each enzyme is assigned a recommended name usually a short for everyday use, a systematic name which identify the reaction it catalyses and a classification             number which is used where accurate and unambiguous identification of an              enzyme is required.

I. Oxidoreductases

  • Enzymes catalysing oxido-reductions between two substrates, S and S'.
S reduced + S' oxidised →      S oxidised + S' reduced

CH3-CH2-OH   +    NAD+        -------- →      CH3-CHO +    NADH +   H+
(reduced)        (oxidised)                     (oxidised)       (reduced)
Enzyme: Recommended name Alcohol dehydrogenase
Systematic name    Alcohol:NAD+ oxido-reductase
Enzyme Commission number           E.C.
First digit 1 indicates oxido-reductase (Major class)
Second digit 1 indicates enzymes acting on CH-OH group of donors (Sub-class)
Third digit 1 indicates NAD+ as the electron acceptor (Sub-sub class)
Fourth digit 1 indicates the specific enzyme
II  Transferases

  • Enzymes catalysing the transfer of a functional group (G) other than hydrogen between substrates.

                                      S - G + S'   --------->      S' - G + S
Example: Phosphorylation of glucose by hexokinase

Glucose  +  ATP  ------->   Glucose – 6- Phosphate  +  ADP

Enzyme : Recommended name: Hexokinase
Systematic name: ATP:D-hexose, 6- phosphotransferase
Enzyme commission No:
2 →     Transferase group (major class)
7 →     Transfer of phosphate group (sub-class)
1 →     Alcohol group as acceptor of phosphate group (Sub-sub-class)
1 →     Hexokinase
III  Hydrolases

  • Enzymes catalysing hydrolysis of ester, peptide or glycosidic bonds.
  • Example

                       Acetyl choline + H2O -----------> Acetic acid + Choline

Enzyme: Acetyl choline esterase
Systematic  name : Choline:acetyl hydrolase
E.C :
IV   Lyases

  • Enzymes catalysing the removal of groups from substrates by mechanism other than hydrolysis leaving a double bond in one of the products.
  • Example: convertion of malic acid to fumaric acid by fumarase

COOH – CH(OH) – CH2-COOH   --------> COOH – CH = CH –COOH + H2O
Malic acid                                                                 Fumaric acid

Enzyme : Fumarase (Fumarate hydratase)
Systematic name:  L. Malate hydrolyase
V.  Isomerases

  • Enzymes catalysing  interconversion of  optical, geometrical or positional isomers


All-trans retinal ----------->    11 cis-retinal

Enzyme Retinene isomerase
Systematic name : All-trans retinene:11-cis isomerase
VI.  Ligases

  • Enzymes catalysing the joining together of two compounds with the hydrolysis of a high energy compound.
  • Example      

Glutamic acid + NH3 ----------> Glutamine

Enzyme: Glutamine synthetase
L.Glutamate: Ammonia ligase

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