CHAPTER VII 



PEPTIDES AND PROTEINS 



Modern concepts of protein structure are founded mainly 

 on the results obtained by subjecting proteins of animal ori- 

 gin to procedures involving partial and complete hydrolysis, 

 amino-acid analysis, ultracentrifugation, electrophoresis and 

 X-ray diffraction analysis. From the limited data available 

 there is, however, no reason to believe that the structure and 

 general physical properties of the proteins of micro-organ- 

 isms are in any way different from those of animals and 

 plants. A few representative microbial proteins have been 

 purified and in some cases crystallized, e.g. the enzymes 

 alcohol dehydrogenase, catalase, amylase and an extracellular 

 proteinase from Sac. cerevisiae, M. lysodeiktiais, B. suhtilis 

 and Strep, haemolyticus respectively and the toxins of CI. 

 botulinum and CI. tetani. Analyses of acid hydrolysates of 

 such proteins and of whole cells by classical precipitation 

 procedures, microbiological assay [41], the bacterial amino- 

 acid decarboxylases (p. 27), and chromatography on ion 

 exchange resins or paper have provided adequate evidence 

 that the proteins of micro-organisms are composed of the 

 L-stereoisomers of the same a-amino-acids as are those of 

 more complex multicellular organisms. 



Whilst there is no conclusive proof that D-amino-acids are 

 constituents of proteins and it is probable that any found in 

 acid or alkaline hydrolysates have arisen by racemization, 

 they are not metabolically inert, and indeed several of them 

 can be utilized by many micro-organisms [36]. The first step 

 in their metabolism may involve deamination by a D-amino- 

 acid oxidase (p. 11): alternatively, direct conversion to the 

 corresponding L-isomer may be accomplished by a racemase, 

 and at the present time two are known, specific for alanine 

 [46] and glutamic acid [31] respectively. Both possess a pros- 

 thetic group of py ridoxal phosphate and the alanine racemase 



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