Biosynthesis of Riboflavin (Flavocoenzymes)
Flavocoenzymes are essential cofactors for
catalysis of a wide variety of redox reactions. Moreover, they are
involved in numerous other physiological processes involving light sensing,
bioluminescence, circadian time-keeping and DNA repair.
The compound is manufactured in relatively
large quantity (about 4000 metric tons per year) for use as a vitamin in
human and animal nutrition and as a colorant, and biotechnological aspects
were an important driving form for studies on its biosynthesis which
extend over a period of more than five decades. In fact, the manufacture
of the vitamin by fermentation has by now essentially replaced chemical
biosynthesis of one riboflavin molecule requires one molecule of GTP (1)
and two molecules of ribulose 5-phosphate (6). The imidazole ring of GTP
is hydrolytically opened by the GTP cyclohydrolase II (A), yielding a
4,5-diaminopyrimidine that is converted to
5-amino-6-ribitylamino-2,4(1H,3H)-pyrimidinedione (4)) by a sequence of
deamination (B), side chain reduction (C), and dephosphorylation (D).
Condensation of 5-amino-6-ribitylamino-2,4(1H,3H)-pyrimidinedione (5) with
3,4-dihydroxy-2-butanone 4-phosphate (7) obtained from ribulose
5-phosphate affords 6,7-dimethyl-8-ribityllumazine (8). This reaction is
catalysed by the 6,7-dimethyl-8-ribityllumazine synthase (F). Dismutation
of the lumazine derivative (riboflavin synthase; G) yields riboflavin (9)
and 5-amino-6-ribitylamino-2,4(1H,3H)-pyrimidinedione (5), which is
recycled in the biosynthetic pathway.
Structure and function of lumazine protein
A number of fluorescent proteins (e.g.
lumazine protein, yellow fluorescent protein, blue fluorescent protein)
have been found to serve as optical transponders in luminescent
photobacteria. They are brought to an excited state by Förster transfer
from bacterial luciferase. Their overall contribution to bioluminescence
is an increase in quantum yield and a shift of the emission spectrum.
Modulation of the binding site by site directed mutagenesis in conjunction with the spectroscopic techniques (multinuclear NMR, EPR, ENDOR, fluorescence spectroscopy and X-ray structure analysis) is expected to provide structural as well as dynamic information on protein-ligand interaction and its role for luminescence amplification by enhanced fluorescence quantum yield.