ENZYMES 155 



molecules joined together by a-glucoside linkages. The hydrolysis of 

 starch only takes place in the presence of amylases, ^-amylase attacks the 

 long chains of amylose and hydrolyses it completely. With amylopectin 

 (or with glycogen), ^-amylase acts on the outer chains but its action is 

 stopped at the point of branching. When acting on amylopectin, j8- 

 amylase hydrolyses it to maltose to the extent of 65% and the residue is 

 attacked by a-amylase. 



Whilst a-amylase appears to be very widely distributed in living beings, 

 up to the present /3-amylase has not been detected in animals. 



A whole series of other polyases exists and they are more or less widely 

 distributed in the biosphere : cellulases, dextranases, lichenases, inulinases, 

 pectin-polygalacturonidases, mucopolysaccharases, lysozymes, hyaluroni- 

 dases, etc. 



(c) Esterases 



These enzymes hydrolyse esters of organic acids and alcohols. They are 

 widely distributed but, in general, their specificity is of a low order. 



It is possible to divide them into the true esterases (catalysing the hydro- 

 lysis of all esters of monocarboxylic acids and monohydric alcohols) and the 

 lipases (catalysing the hydrolysis of esters of fatty acids and glycerol). 

 However it is not always easy to make this distinction because of the low 

 specificity of these enzymes. 



In certain regions of the biosphere are localized esterases possessing a 

 more marked specificity : chlorophyllase, pectin-esterases, cholinesterases, etc. 



Besides the esterases for esters of organic acids, there are esterases for 

 the esters of inorganic acids. A widely distributed group is that of the 

 sulphatases or esterases for esters of sulphuric acid. 



(d) Phosphatases 



Phosphatases are present throughout the biosphere. They can be divided 

 into four groups: — 



1 . Phosphomonoesterases 



O 

 R-O— P— OH + H2O ^ ROH + H3PO4 



OH 



These are of low specificity as far as the alcohol radical R is concerned. 

 In this category are included alkaline phosphatase, acid phosphatase, 

 acetylphosphatase, hexosediphosphatase, etc. 



