224 BIOCHEMICAL SYSTEMATICS 



O 



O O 



benzoquinone naphthoquinone 



quinones are known to occur only in certain echinoderms and insects. 

 They are rare in algae but common in fungi. In the vascular plants, 

 with the exception of certain quinones which are believed to function 

 as important coenzymes, quinones are of restricted occurrence. 

 Although not widespread among plant families, in those families in 

 which quinones do occur, the compounds may be characteristic for 

 the family. 



The chemical properties of quinones include relative ease of 

 oxidation and reduction. In this connection 2-methyl,3-phytyl, 

 1-4-naphthoquinone (Vitamin K), found in high concentration in 

 chloroplasts, has been proposed as a coenzyme involved in electron 

 transport, particularly in the processes following the primary photo- 

 chemical event of photosynthesis. 



Recently, another group of coenzymes of a quinoid nature 

 referred to collectively as coenzyme Q (or ubiquinone) has been 

 described. This group is thought to participate in electron transport 

 between cytochromes b and c. The basic structure of coenzyme 

 Q involves a 2,3,5-tri-methyl benzoquinone substituted with isoprenoid 

 side chains at the 6-position. 



CH 



3 



CH3 \ 



CH2CH=C — CH2/nH 



At least five naturally occurring homologues of coenzyme Q 

 have been described, the differences involving the number of isoprene 

 units attached. Lester and Crane (1959) studied the distribution 

 of the coenzyme Q series in animals, plants, and microorganisms. 

 Coenzyme Q was found in all higher plants examined (six genera) and 

 among red, brown, and green algae. One bluegreen alga, Anacystis 

 nidulans, did not yield any coenzyme Q. The higher animals and 

 plants were found to contain, usually, coenzyme Qio, with Q9 appear- 

 ing in a few cases. (The subscript refers to the number of isoprene 

 units in the side chain.) Among microorganisms, there was considera- 

 bly greater variation in the types present; for example, in ascomycetes 

 coenzymes Qio, Q9, Qs, Q7, and Qs were present. A particular quinone 



