NUCLEOTIDES AND COENZYMES 



tain a nitrogenous base (the cyano-complex of a cobalt por- 

 phyrin) linked to a nucleotide (the 2'- or 3'-ribotide of ben- 

 zimidazole). Likewise, its function is obscure, although the 

 possible oxido-reduction of the cobalt porphyrin lends support 

 to the hypothesis that it mediates the reduction of disulfide 

 groups. 



Glucose- 1,6-diphosphate functions catalytically (A'^ = 

 4 X 10"'^ M) in the conversion of glucose- 1 -phosphate to glucose- 

 6-phosphate, but here again we are dealing with a special case, 

 since the catalyst actually "primes the pump" and is itself used 

 up in the reaction, viz. 



Ci - P CiP Ci 



C6\ 



CiP 



CsP 



) 



+ 



GeP GeP 



In a like manner, 2,3-diphosphoglycerate participates in the 

 conversion of 2- to 3-phosphoglycerate. 



There are also several substances, such as the AMP-ATP 

 complex of Kiessling and Meyerhof (26) (see also Ohlmeyer 

 (34)), the prosthetic group associated with rabbit muscle phos- 

 phorylase (10), the second prosthetic group of the crystalline 

 yeast flavoprotein (4), or the factor replacing autoxidizable dyes 

 involved in the enzymatic oxidation of reduced pyridine nucleo- 

 tides (19), which would appear to be coenzymes because of their 

 structure, function, or association with highly purified enzymes. 



It is probably correct to say, however, that most of the 

 major coenzymes or growth factors have been discovered — 

 major in the sense of the amounts existing in tissues — but there 

 are still undoubtedly new frontiers to be explored in that area 

 which Lardy (29) has so aptly termed "the micro-catalytic 

 vitamins." 



499 



