CYTOPLASM 



135 



this the proteins clearly show acid or basic properties, this is brought 

 about by the side chains which in their turn carry free CO OH- or 

 NHg-groups. This happens when some members of the polypeptide 

 chains consist of dicarbo-amino acids or diamino acids (Fig. 88). 



Acid side chains 

 NH 



^CH-CH^-COOH 

 CO 



/ 



NH 

 \ 



/' 



CO 



Aspartic ocid 



CH-CH^-CHp-COOH 



Glutamic acid 



Lipophilic side chains 



,CH-CH,-CH 



/ ^CH- 



CO 



L Pucine 



/ 



NH 



ycH-cH,^^ 



CO 

 ^ Phenyl alanine 



Sulphur containing 



side chain 

 / 



NH 



yCH-CH.-SH 



CO 



Cysteine 



Basic side chains 



NH 

 \ 



^CH-CH2-CH^-CH2-NH2 

 CO 



Ornithine 



^LH-CH.-CH.-CHj-NH-C^ 



c'o ^^^ 



^ Arginine (Valine -^ Guanidme) 



Hydrophilic side chains 



NH 



.iH-CH^-OH 



CO 



Serine 



\ 



/ 



NH 



\h-chp-(^oh 



CO 



^ Tyrosine 



Possible chain end 



CH,^CH2 

 NH \ 

 \ ^CH! 



^•P Proline 



Fig. 88. Side chains R of the polypeptide chains. 



The common amino acids (valine, leucine, phenyl-alanine, etc.) 

 cannot exercise special influence on the reactivity of the proteins, but 

 they confer upon these proteins a pronounced lipidic character, since 

 the ends of the side chains consist of methyl or phenyl groups (Fig. 

 88). In many cases, however, the terminal groups carry an alcoholic 

 hydroxyl group (serine, tyrosine), on account of which a certain 

 hydrophily is maintained. 



A particularly important side chain is cysteine with its very reactive 

 sulphydryl group. As will be shown later, this group very easily forms 

 bridges between neighbouring polypeptide chains. In contrast to such 

 constituents of protein chains, capable of bonding and thus favouring 

 further polymerization, cyclic amino acids such as proHne can ter- 

 minate the main valency chains and thus limit the apparently endless 



