Samenvatting "Biochemistry: A Short Course" (John L. Tymoczko) Section 11 (H24, H25 en H26)

Section 11 - Glycogen Metabolism and the
Pentose Phosphate Pathway

Chapter 24 - Glycogen Degradation
● Glycogen → large, branched polymer of glucose residues
○ Most glucose residues are linked by α-1,4-glycosidic bonds
○ Branches at about every 12th residue are created by α-1,6-glycosidic bonds
○ Not as energy-rich as fatty acids
○ The readily mobilized glucose from glycogen is a good source of energy for
sudden, strenuous activity → can also be used for anaerobic activity
○ Main sites of storage: Liver and skeletal muscle
○ In the liver, glycogen synthesis and degradation are regulated to maintain the
concentration of glucose in the blood required to meet the needs of the
organism as a whole


24.1 - Glycogen Breakdown Requires Several Enzymes
● Efficiënt breakdown of glycogen to provide glucose 6-phosphate for further
metabolism requires 4 enzyme activities
○ 1 to degrade glycogen
○ 2 to remodel glycogen so that it remains a substrate for degradation
○ 1 to convert the product of glycogen breakdown into a form suitable for further
metabolism
● Phosphorylase cleavers glycogen to release glucose 1-phosphate
○ Glycogen phosphorylase​ (key regulatory enzyme in glycogen breakdown)
■ Cleaves its substrate by the addition of orthophosphate (P​i​) to yield
glucose 1-phosphate
● Phosphorolysis:​ the cleavage of a bond by the addition of P​i
■ Glycogen(n) + P​i​ ⇌ glucose 1-phosphate + glycogen(n-1)
■ Phosphorylase catalyzes the sequential
removal of glycosyl residues from the
nonreducing ends of the glycogen
molecule
● P​i​ splits the glycosidic linkage
between C-1 of the terminal
residue and C-4 of the adjacent one




■ The phosphorolytic cleavage of glycogen is energetically
advantageous because the released sugar is already phosphorylated

, ● A debranching enzyme is also needed for the breakdown of
glycogen
○ α-1,6-glycosidic bonds are not susceptible to
cleavage by phosphorylase → stops cleaving α-1,4
linkages when it reached a residue 4 residues away
from a branch point
○ Transferase​ shifts a block of 3 glucosyl residues
from one outer branch to another → exposes a
single glucose residue joined by an α-1,6-glycosidic
linkage
○ α-1,6-glucosidase (debranching enzyme)
hydrolyzes the α-1,6-glycosidic bond → release of
a free glucose molecule
○ The free glucose molecule is phosphorylated by
hexokinase
● Phosphoglucomutase​ converts glucose 1-phosphate into glucose 6-phosphate
○ To catalyze this shift → enzyme exchanges a phosphoryl group with the
substrate
○ The active site of mutase contains a phosphorylated serine residue
■ The phosphoryl group is transferred from the serine residue to the C-6
hydroxyl group of glucose 1-phosphate to form glucose
1,6-biphosphate
■ The C-1 phosphoryl group of this intermediate → shuttled to the same
serine residue → formation of glucose 6-phosphate and the
regeneration of the phosphoenzyme




● The liver contains ​glucose 6-phosphatase​ → the muscle does not
○ A major function of the liver is to maintain a nearly constant concentration of
glucose in the blood
○ Glucose 6-phosphatase​ enables glucose to leave the liver instead of
phosphorylated glucose
■ Cleaves the phosphoryl group to form free glucose and P​i
■ Glucose 6-phosphatase + H​2​O → glucose + P​i