3 CARBOHYDRATE BIOGENESIS
One of the most vital aspects of pharmacognosy which has gained paramunt importance and legitimate recognition in the recent past is the intensive and extensive studies involving the various biochemical pathways that has ultimately led to the production of ‘secondary constituents’ invariably employed as ‘drugs’. This type of specific and elaborated study is frequently termed as biogenesis or drug biosynthesis. It is quite pertinent to mention here that as it is absolutely necessary for a medicinal chemist to understand the intricacies of chemical synthesis of a potent drug substance, such as:
naproxen, chloramphenicol etc., exactly in the same vein a pharmacognosist must possess a thorough knowledge of the biogenesis of drugs of natural origin.
A Swiss chemist G. Trier, as far back in 1912, not only predicted but also postulated that amino acids and their corresponding derivatives invariably act as the precursors of relatively complex naturally occurring alkaloids mostly used as potent therapeutic agents. Interestingly, soonafter the second half of the twentieth century, there had been a tremendous progress in the era of isotopically labelled organic compounds that facilitated the affirmation as well as confirmation of the earlier
speculated theories. With the advent of most advanced knowledge in sciences it has been established that the carbohydrate biogenesis usually takes place due to the Photosynthesis from carbon dioxide (CO2) as the starting material occurring abundantly both in all plants and in certain purple bacteria as depicted below:
Calvin and coworkers established the various steps involved in the chemical reactions ultimately leading to the overall Eq. (a). They have also shown that D-ribose-1, 5-diphosphate is the primary acceptor of CO2. However, the exact mechanism of this particular step whereby CO2 gets assimilated has been studied at length with radio labelled 14CO2 and Chiorella (a fresh water algae).
Besides, the following Eq. (b) evidently illustrates the distribution of radiocarbon originating from 14CO2after completion of one full photosynthetic cycle:
From Eq. (b) it may be inferred that a triose phosphate having the identified radiocarbon distribution shall ultimately result after completing a single full cycle. It would most logically lead to hexose phosphate which should invariably contain relatively higher amounts of 14CO2 (i.e., radio labelled carbon), till such time after a series of recycling slots, give rise to an even distribution of radio active carbon spread over the entire hexose molecule.
