70 UNITY AND DIVERSITY IN BIOCHEMISTRY 



(e) Nicotinamide mononucleotide — 



CONH, 



O 



C- C - CH,- O- P- OH 



OH 



H H H H 



This has been isolated from yeast. 



2. Dimicleotides 



{a) Diphosphopyridine-nucleotide (DPN) and triphosphopyridine-nucleo- 

 tide (TPN) — Diphosphopyridine nucleotide (DPN, coenzyme I, code- 

 hydrogenase I, cozymase), an important coenzyme of general utility, 

 contains the heterocyclic bases adenine and nicotinamide and two mole- 

 cules of D-ribose. The two component nucleotides are joined by a pyro- 

 phosphate bridge. A related compound, also widely distributed in the 

 biosphere, only differs from the above substance by having an additional 

 phosphate residue esterified at C-2' of the ribose molecule attached to 

 adenine. This substance is known as triphosphopyridine nucleotide 

 (TPN, coenzyme II, Co II or codehydrogenase II). 



,f^ 



^ 



\ 



CONH, 



HC 



H,NC = N 



I I 

 /N — C CH 



+ N 



I 

 HC 



I 

 HCOH 



I 

 HCOH 



i 

 HC 



I 

 HC- 



•C-N 



O 



CH, OH OH 



I I I 



o — p — O— P — 

 II II 

 o o 



DPN 



HCOH 



I 

 HCOH 



HC — 

 I 



CH, 

 I 

 — O 



O 



CONH, 



H,NC - N 



>N^ 



HC 



I 

 HCOH 



I O 

 HCOH 



HC 



I 



CH, 

 1 

 O— 



OH 

 I 

 -P — 



II 

 O 



O- 



TPN 



The pyridine nucleotides and their barium salts are very soluble in water. 

 The oxidized forms are written DPN+ or TPN+. DPN+ has two phosphate 

 residues bearing two primary acidic groups, whilst TPN+, has three 

 primary acid groups and, in addition, a secondary group. 



Reversible reduction of the pyridine ring transforms DPN+ into the 

 reduced dinucleotide DPNH and, similarly, for TPN+. 



