NUTRITION IN SPORT

and maintaining muscle glycogen stores for com-
petition and training.

Regulation of

muscle glycogen synthesis

Figure 7.1 illustrates the metabolic reactions
controlling glycogen synthesis. Upon crossing
the sarcolemma, glucose is rapidly converted to
glucose-6-phosphate (G6P) by the enzyme hex-
okinase. The G6P is then converted to glucose-1-
phosphate (G1P) by phosphoglucomutase. Next,
uridine triphosphate and G1P are combined to
form uridine diphosphate (UDP)-glucose, which
serves as a glycosyl carrier. The glucose attached
to the UDP-glucose is then transferred to the ter-
minal glucose residue at the non-reducing end of
a glucan chain to form an a(1Æ4) glycosidic
linkage. This reaction is catalysed by the enzyme
glycogen synthase. The initial formation of
the glucan chain, however, is controlled by the
protein glycogenin, which is a UDP-glucose-
requiring glucosyltransferase. The first step
involves the covalent attachment of glucose to a
single tyrosine residue on glycogenin. This reac-
tion is brought about autocatalytically by glyco-
genin itself. The next step is the extension of the
glucan chain which involves the sequential addi-
tion of up to seven further glucosyl residues. The
glucan primer is elongated by glycogen syn-
thase, but only when glycogenin and glycogen
synthase are complexed together. Finally, amylo

98 nutrition and exercise

(1,4Æ1,6) transglycosylase catalyses the transfer

of a terminal oligosaccharide fragment of six or

seven glucosyl residues from the end of the

glucan chain to the 6-hydroxyl group of a glucose

residue of the same or another chain. This occurs

in such a manner as to form an a(1Æ6) linkage

and thus create a compact molecular structure.

The synthesis of the glycogen molecule is termi-

nated by the dissociation of glycogen synthase

from glycogenin (Smythe & Cohen 1991; Alonso

et al. 1995).

Following its depletion by exercise, muscle

glycogen synthesis occurs in a biphasic manner

(Bergström & Hultman 1967b; Piehl 1974; Ivy

1977; Maehlum et al. 1977; Price et al. 1994). Ini-

tially, there is a rapid synthesis of muscle glyco-

gen that does not require the presence of insulin

(Ivy 1977; Maehlum et al. 1977; Price et al. 1994).

In normal humans, the rate of synthesis during

this insulin-independent phase has been found

to range between 12 and 30mmol · g–1 wet

weight · h–1and to last for about 45–60 min. The

second phase is insulin dependent (Ivy 1977;

Maehlumet al. 1977; Price et al. 1994) and in the

absence of carbohydrate supplementation occurs

at a rate that is approximately seven- to 10-fold

slower than that of the rapid phase (Price et al.

1994). If supplemented immediately after exer-

cise with carbohydrates, however, the rate of

synthesis during the slow phase can be increased

several-fold, and if supplementation persists, the

muscle glycogen level can be increased above

+

Hexokinase

Glucose

Glucose transporter

Glycogen synthase

+ glycogenin

DI

Glycogen

UDP-glucose

UDP-glucose

pyrophosphorylase

Glucose-1-phosphate

Phosphoglucomutase

Glucose-6-phosphate

Fig. 7.1The metabolic reactions

and the enzymes controlling the

reactions that are responsible for

the synthesis of muscle glycogen.

Enzymes are in italic. See text for

details.