are electron and hydrogen carriers. The former is crucial in electron transport
associated with oxidative phosphorylation and ATP formation, the reduced
form of NADPH is essential to the biosynthesis of, for example sugars, lipids,
amino acids and nucleotides.
Vitamin B 5 or pantothenic acid is an essential part of coenzyme A (Figure 10.13),
which is the major carrier of metabolically active acyl (fatty acid residues)
in metabolism. Thus it is essential to many of the reactions involved in the
oxidation of lipids and in the synthesis of lipids including steroid hormones,
some neurotransmitters and hemoglobin.
The vitamers pyridoxine, pyridoxal and pyridoxamine all have vitamin B 6
activity (Figure 10.14), forming pyridoxal 5-phosphate, which is a cofactor for a
number of enzymes. These include glutamate decarboxylase that catalyzes the
formation of F aminobutyric acid, a neurotransmitter of the central nervous
system, and enzymes that catalyze transamination and decarboxylation
reactions of amino acids. Vitamin B 6 is therefore essential for the synthesis
of nonessential amino acids and in the catabolism of amino acids. Pyridoxal
5-phosphate is also a cofactor for glycogen phosphorylase of liver and muscle
tissues and helps regulate the actions of steroid hormones (Chapter 7) by
participating in the dissociation of hormone–receptor complexes from DNA.
DIET AND NUTRITIONCZhhVg6]bZY!BVjgZZc9Vlhdc!8]g^hHb^i]:YLddY '),
OOH HHO OROH HHO OHH HHH NN
NNNH 2H 2 NCH 2H HCH 2NCOOPOOO-OOP+Reactive siteO
PO-O-NAD+,R = HNADP+,R=Figure 10.12 The structures of NAD+ and NADP+.
OH HOHOOHH HHHHNN NNNHNH 2CH 2CH 2H 3 C CH 3CH 2CH 2CH 2CH 2OOOPO
POOO- OP
- O
- O
O-CCCNH
O
CSHPantothenate portionFigure 10.13 The structure of coenzyme A.CH 2 OHN CH 3HOH 2 CCHOCH 2 NH 2OHPyridoxine,R =Pyridoxal,R =Pyridoxamine,R =R+HFigure 10.14 The structures of
vitamin B 6.