Cellular oxidations and the synthesis of 

 amino-acids and amides in plants 



by 



E. W. YEMM 



Botany Department, University of Bristol 



INTRODUCTION 



The biochemical mechanisms engaged in the synthesis of amino-acids and proteins in 

 the cell have recently been extensively studied. There is now much evidence that 

 amino-acids are directly involved in the biosynthesis of proteins, but two distinct 

 hypotheses have been put forward with regard to the way in which specific peptide 

 structures are built up. The first of these, the so-called 'template' hypothesis, was 

 advanced primarily to account for the reduplication of protein structures; it suggests 

 that amino-acids are orientated on specific surfaces in the cell and are there con- 

 densed en bloc in a single-step reaction. In a review of this mechanism, Dounce (1952) 

 considers that transphosphorylations, mediated by nucleic acids, may provide the 

 energy coupling necessary to promote the reaction. The second hypothesis, developed 

 mainly by Fruton (1952) and Waelsch (1952), suggests that a preliminary synthesis 

 of amino-acid amides or simple peptides takes place, followed by a conversion to 

 proteins by transamidation and transpeptidation reactions controlled by specific 

 transferring enzymes in the cell. As distinct from the 'template' hypothesis, this 

 transamidation mechanism implies an active formation of amides and simple pep- 

 tides and a close coupling between these syntheses and the exergonic reactions of 

 cell respiration. It is the main objective of this paper to consider some further evidence, 

 which has recently been obtained, bearing on this point. An attempt has been made 

 to trace some of the stages by which simple inorganic forms of nitrogen are assimi- 

 lated by plant cells. Under favourable conditions a rapid formation of amino-acids 

 and amides from ammonium salts or nitrates takes place, and affords an opportunity 

 of examining the relation between these syntheses and the breakdown of carbohy- 

 drates in cellular oxidations. 



CELLULAR RESPIRATION AND THE ASSIMILATION OF NITROGEN 



It is well established that the rate of respiration of plants and micro-organisms may 

 be greatly increased during the assimilation of nitrogen. Kellner (1874) first showed 

 that pea seedlings respired more rapidly when supplied with nitrates, and his obser- 

 vations have been confirmed and extended to other species by Hamner (1936), 

 Hoagland (1944), Woodford and Gregory (1948), Humphries (1951), and Syrett 

 ( I 953) > Our work in this direction has been carried out mainly with young seedlings 



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