Enzymes are biological catalysts. A catalyst is any substance which makes a chemical reaction go faster, without itself being changed. A catalyst can be used over and over again in a chemical reaction: it does not get used up. Enzymes are very much the same except that they can be easily denatured by some means. All enzymes are made of protein; that is why they are sensitive to heat, pH and heavy metal ions. Unlike ordinary catalysts, they are specific to one chemical reaction. An ordinary catalyst may be used for several different chemical reactions, but an enzyme only works for one specific reaction. Enzymes must have the correct shape to do their job. Enzymes change their shape if the temperature or pH changes, so they have to have the right conditions.
Conventional chemical processes are generally severe and fibre damage may occur. However, enzymes are characterized by their ability to operate under mild conditions. As a result processes may take place without additional harm to the fibre. Enzymes are also readily biodegradable and therefore potentially harmless and environmentally friendly. This chapter discusses various properties of enzymes and applications of cellulases in textile processing.
Properties of the enzymes
a) Enzymes accelerate reaction:
Enzymes accelerate a particular chemical reaction by lowering the activation energy for the reaction. They achieve this by forming an intermediate enzyme substrate complex, which alters the energy of the substrate such that it can be more readily converted in to the product. The enzyme itself is released unaltered at the end of the reaction, thus acting as a catalyst. It can be schematically represented by the following equation:
Substrate + Enzyme → Substrate-enzyme complex → Substrate + Enzyme
Enzymes have an excellent catalytic power. They accelerate reactions, which are often undetectable in the absence of enzyme, by enormous amounts, sometimes several million fold. An outstanding example of this catalytic power is demonstrated by the enzyme triose phosphate isomerase. This enzyme accelerates the isomerisation of glyceraldehyde 3 phosphate thousand times compared with the rate in the presence of acetate ions.
Another example of the catalytic power of enzymes is the activity of peroxidase, an enzyme employed in the textile industry. One molecule of this enzyme can convert five million molecules of hydrogen peroxide to water and oxygen in one minute.
Enzymes do not alter the equilibrium position of the reactions they catalyze. The energy profile for a typical reaction shows that in order to proceed from reactants to products an energy barrier (∆G*) must be overcome. From the transition state theory of rate, the relationship between the rate constant (k) and the free energy of activation (∆G*) can be expressed as
K= kT/h.e-∆G*/RT = kT/h. e-∆H*/RT . e∆S*/R
k= Boltzman constant
h= Plancks constant
T= absolute temperature
R= Gas constant
∆H* & ∆S* are enthalpy and entropy of activation respectively.