Lesson 18 Enzymes-I


 In this lesson, we will study about enzymes, their classification and characteristics.

18.2   Enzymes

Enzymes are biologic polymers which catalyze biochemical reactions. They are functional units of cell metabolism. Most of the enzymes are proteins. Measurement of activities of certain enzymes is an important tool in diagnosing diseases. Enzymes are important practical tools in medicine, chemical industry, food processing and agriculture.

18.3   Constituents of Enzymes                                                  

Almost all of the enzymes are proteins. Some enzymes (e.g. pancreatic ribonuclease) are made up of only polypeptides and contain no groups or atoms other than amino acid residues. Some enzymes require additional chemical component called cofactor. The cofactor can be of two types:-

  1. Inorganic ions like Fe2+,Zn2+,Mg2+ etc.
  2. Coenzyme: which is a complex organic group.

Figure 18.1 Figure showing constituents of enzyme molecules

There are some enzymes which require both coenzyme and metal ions for   activity. In some enzymes, coenzymes or metal ions are loosely bound to the protein. In other enzymes it is tightly and permanently bound and is called prosthetic group. A complete catalytically active enzyme together which its coenzyme or metal ion is called holoenzyme. The protein part of an enzyme is known as apoenzyme. Coenzyme and metal ions are stable to heat while the protein part is unstable and denatured on heating.

In an enzyme, the inorganic part is essential for activity of some enzymes, while the coenzyme functions as carries of specific functional groups.

Table18.1 Some enzymes containing Inorganic elements as cofactors

Inorganic Part



Alcohol dehydrogenase

Ni 2+




K+, Mg2+

Pyruvate kinase


Table18.2   Coenzymes which function as carries of functional group


Group of atoms transferred

Thiamine pyrophosphate


Coenzyme A

Acyl groups

Pyridoxal Phosphate

Amino groups

Co enzyme B12

H atoms and alkyl groups


18.4   Classification of Enzymes

Enzymes are classified into six major groups depending on the type of reactions they catalyze. The complete rules of nomenclature and classification can be found here.


Table18.3    International classification of enzymes



Type of reaction Catalyzed




Transfer of electrons

Alcohol dehydrogenase, peroxidases, catalases, etc



Transfer of groups.(e.g. methyl, NH2)

Phosphorylases, transaminases, hexokinase, etc.



Hydrolytic cleavage of C-C, C-N,C-O & some other bonds

α & β amylase, urease, etc



Addition of groups to double bond or cleavage of C-C, C-N, C-O & other bonds by elimination, resulting in formation of double bonds or rings.




Geometric or structural changes within one molecules

Racemases, cis-trans isomerases, etc



Joining together of two molecules coupled with hydrolysis of diphosphate bond in ATP or similar molecules

Pyruvate carboxylase

18.5   Characteristics of Enzymes

18.5.1  Enzymes enhance rate of chemical reaction by lowering their activation energy.

Explanation:- During a chemical reaction the reactant molecules are first activated(their energy is increased). The difference in energy between activated state and initial state is known as activation energy. Once the molecules are activated they are converted to products. Enzymes lower the activation energy by providing an alternative path to reach that state. Thus in given time period more number of molecules reach the activation state and reaction is catalyzed.

18.5.2 The substrate concentration has significant effect on rate of enzyme catalyzed reactions and a quantitative relationship exist between the two.

Figure 18.2  Figure showing effect of substrate concentration on initial rate of reaction         

Explanation:-   Consider a case where enzyme concentration is held constant. Also for simplicity consider that the enzyme acts on only on the substrate. If substrate concentration is low,   the initial rate of reaction will also be low but it will increase as the former increases. As the substrate concentration increases, the   increase in rate becomes smaller and smaller and approaches a constant value. At this rate the enzyme is saturated with the substrate. This phenomenon is known as the saturation effect exhibited by enzyme molecules.

If an enzyme acts on only one substrate, the relationship between initial rate of reaction and substrate concentration (known as Micheles-Menten equation) can be written aseq-1


            Vo = Initial rate at substrate concentration [S]

            Vmax =  Maximum rate               

            KM = Micheles-Menten constant

18.5.3 Enzymes have an optimum pH at which their activity is maximum.

For example, pepsin is an enzyme which is released in stomach and hydrolyses peptide bonds of proteins has its maximum activity at pH 1.5. The enzyme catalse has optimum pH 7.6.

18.5.4 The activity of enzyme can be determined quantitatively.

Explanation:- There are various methods of determining enzyme activity. By international agreements, 1.0 unit of enzyme  activity is defined as that amount causing transformation of 1.0 micromole of a substrate per minute at 25°C under optimum conditions.

18.5.5 Enzymes are specific in their action.

Explanation:- some enzymes act on a a single substance and will not attack other  closely related molecules. There are some enzymes which are broad in their action and will act on compounds having common structural feature.

18.5.6 Most enzymes can be inhibited by specific chemical reagents

References & Further Reading:

1.         Lehninger A. (1987), “Principles of Biochemistry”. B S Publishers & Distributers, pp. 207-221.

2.         http://www.chem.qmul.ac.uk/aubmb/enzymes/rules.html

3.         Satyanarayana U. & Chakrapani U. (2011), “Biochemistry”. Books and Allied (P) Ltd. pp. 85-96.

Last modified: Saturday, 5 October 2013, 4:28 AM