Flavins: Riboflavin, FMN and FAD (Vitamin B2)

It became apparent during the 1920s that the
polyneuritis preventive factor, which was later designated
‘water-soluble B’ and subsequently ‘vitamin B’,
could prevent pellagra in humans as well as beriberi.
This discovery led to the replacement of the term
‘vitamin B’ by the terms ‘vitamin B1’ (the heat-labile,
anti-neuritic factor) and ‘vitamin B2’ (the heat-stable,
pellagra-preventive factor). Vitamin B2, present in
yeast extracts, was needed to prevent human pellagra
and an apparently similar canine disease called ‘black
tongue’. It was also required by rats to prevent a pellagra-
like dermatitis and to promote growth. At that
time vitamin B2 was assumed to be a single substance,
but it was later found that there are several vitamins
present in this heat-stable fraction of yeast.

The presence of water-soluble, fl uorescent, yellow
pigments in natural materials had been known for
some time. These pigments, known generically as
fl avins, were found in milk, liver, kidney, muscle, yeast
and plant materials. They were given specifi c names
according to their sources, e.g. lactofl avin (milk)
and hepatofl avin (liver). In 1933, Kuhn, György and
Wagner-Jauregg isolated from egg white a fl uorescent,
yellow, crystalline compound (‘ovofl avin’) which was
a growth-promoting factor for rats. The isolation of
other growth-promoting fl avins followed. By 1934,
Kuhn’s group had determined the structures of these
various fl avins and found them to be chemically identical.
Because each molecule contained a ribose-like
(ribitol) side chain, the term ‘ribofl avin’ was adopted.
Thus ribofl avin was the component responsible for
the rat growth-promoting activity in the aforementioned
vitamin B2 complex. The pellagra-preventive
factor and the rat anti-dermatitis factor subsequently
became known as niacin and vitamin B6, respectively.
Meanwhile, by 1932, Warburg and Christian had
isolated from yeast an enzyme, which dissociated into
a protein apoenzyme and a yellow prosthetic group
that was chemically similar to a fl avin. The yellow
pigment isolated from this ‘old yellow enzyme’ was a
vitamin-inactive, photo-derivative of a fl avin (lumifl
avin). Determination of this compound’s structure
proved useful to Kuhn for elucidating the structure
of ribofl avin. The synthesis of ribofl avin was accomplished
independently by Kuhn’s group and Karrer’s
group in 1935. By 1938, Warburg and Christian had
isolated and characterized fl avin adenine dinucleotide
(FAD) and shown it to be a coenzyme of D-amino
acid oxidase. The structure of the simpler coenzyme,
ribofl avin 5´-phosphate (FMN), was secured in the
previous year by Theorell.

The principal vitamin B2-active fl avins found in nature
are ribofl avin, ribofl avin-5´-phosphate (fl avin
mononucleotide, FMN) and ribofl avin-5´-adenosyldiphosphate
(fl avin adenine dinucleotide, FAD).
The structures of these compounds are depicted in
Fig. 12.1. The parent ribofl avin molecule comprises
a substituted isoalloxazine moiety with a ribitol side
chain.
The ‘mononucleotide’ and ‘dinucleotide’ designations
for FMN and FAD, respectively, are actually
incorrect but are nevertheless still accepted. FMN is
not a nucleotide, as the sugar group is not ribose, and
the isoalloxazine ring is neither a purine nor a pyrimidine.
FAD is composed of a nucleotide (adenosine
monophosphate, AMP) and the so-called fl avin pseudonucleotide.
In biological tissues, FAD and, to a lesser extent,
FMN occur almost entirely as prosthetic groups for
a large variety of fl avin enzymes (fl avoproteins). In
most fl avoproteins the fl avins are bound tightly but
noncovalently to the apoenzyme. In mammalian tissues
less than 10% of the FAD is covalently attached
to specifi c amino acid residues of four important
apoenzymes. These are found within succinate and
sarcosine dehydrogenases, monoamine oxidase and
gulonolactone oxidase in which FAD is peptidelinked
to an N-histidyl or S-cysteinyl residue via the
8-methyl group.