Glycogen synthase

Glycogen synthase (GS) converts glucose to glycogen. It takes short polymers of glucose and converts them into long polymers one by one into a polymeric chain for storage as glycogen.

Chemical equation

$UDPG + Glycogen_n \rightarrow UDP + Glycogen_{n+1}$

Rate equation

MWC model (Monod-Wyman-Changeux model) is used to model the reaction mechanism of this enzyme.. Glc6P activates GS by binding to an allosteric site of this enzyme.

$\frac{K_{cat,r}[GS]n\frac{[UDPG]}{K_{UDPG}} \left( 1 + \frac{[UDPG]}{K_{UDPG}} + \frac{[ATP]}{K'_{r,ATP}} \right)^{n-1}}{\left( 1 + \frac{[UDPG]}{K_{UDPG}} + \frac{[ATP]}{K'_{t,ATP}} \right)^n L_{0} \left( \frac{1 + \frac{[Glc6P]}{K_{t,Glc6P}} + \frac{[ATP]}{K_{t,ATP}} }{1 + \frac{[Glc6P]}{K_{r,Glc6P}} + \frac{[ATP]}{K_{r,ATP}} } \right)^n + \left( 1 + \frac{[UDPG]}{K_{UDPG}} + \frac{[ATP]}{K'_{r,ATP}} \right)^n }$

Parameter values

Parameter	Value	Units	Organism	Remarks
$n$	4	Dimensionless	Rabbit muscle
$K_{cat,r}$	1004	$min^{-1}$	Rabbit muscle
Total[GS]	$3 \times 10^{-3}$	mM
L	0.25	Dimensionless
$K_{UDPG}$	0.72	mM	Rat muscle
$Kr_{Glc6P}$	$6.76 \times 10^{-002}$	mM	Rat muscle
$Kt_{Glc6P}$	0.29	mM	Rat muscle
$K_{r,ATP}$	4.94	mM	Rat muscle
$K_{t,ATP}$	2.33	mM	Rat muscle
$K'_{r,ATP}$	15.92	mM	Rat muscle
$K'_{t,ATP}$	3.93	mM	Rat muscle

Parameters with uncertainty

• The parameter values with their uncertainty are mentioned in Table 3.5: Optimized kinetic parameters obtained using a scaled weighting strategy for data obtained by varying UDPG and a half-scaled strategy for data obtained by varying G6P .
• n is the number of allosteric active site which is constant.
• Total[GS] also considers to be constant as it represents the total concentration of enzymes.
• The value of L is calculated based on the activity ratio of the enzyme (The activity ratio is defined as the ratio of activity in the absence of glucose-6-P to that in its presence). The activity ratio for L is mentioned as to be in between $0.8 - 0.9$ in Wang (1993) et. al.. The Std. Dev. for this measurement is 0.025 which is 25% of the mean value. The same percent of error is considered for the value of L.
• Maximum error for all Km values listed for GS was 38%. For $K_{cat,r}$ this percent of error are considered.

Parameter	Value	Units	Organism	Remarks
$n$	4	Dimensionless	Rabbit muscle
$K_{cat,r}$	$1004 \pm 381.52$	$min^{-1}$	Rabbit muscle
Total[GS]	$3 \times 10^{-3}$	mM
L	$0.25 \pm 0.0625$	Dimensionless
$K_{UDPG}$	$0.72 \pm 0.16$	mM	Rat muscle
$Kr_{Glc6P}$	$0.0675 \pm 0.015$	mM	Rat muscle
$Kt_{Glc6P}$	$0.29\pm 0.093$	mM	Rat muscle
$K_{r,ATP}$	$4.94 \pm 1.59$	mM	Rat muscle
$K_{t,ATP}$	$2.33 \pm 0.89$	mM	Rat muscle
$K'_{r,ATP}$	$15.92 \pm 6.08$	mM	Rat muscle
$K'_{t,ATP}$	$3.93 \pm 1.13$	mM	Rat muscle