Lab Exam

glycogenesis

• synthesized from alpha-D-glucose
• in cytosol
• requires enerrgy supplied by ATP & uridine triphosphate (UTP)

Glucose-1-phosphate + UTP = UDP-glucose by UDP-glucose pyrophosphorylase + pyrophosphate (pp_i)

source of all glycosyl residues - glycosyl residues added to growing glycogen molecule

the hydrolyisis is exergonic - this ensures that UDP- glucose phosphorylase reaction proceeds in the direction of UDP- glucose production

• makes alpha(1→4) linkage in glycogen 
• can NOT initiate chain synthesis usisng free glucose 
• can only unse existing chains of glucose & requires a primer 

primer is a fragment of glycogen 

if there is no fragment GLYCOGENIN can serve as an acceptor of glucose 

glycosyl unit binds to the Thyrosine- 194 of the glycogenin 

→ reaction is catalyzed by glycogenin (Enzyme) via autoglucosylation 

Glycogenin can catalyze the transfer of at least 4 molecules of glucose from UDP - Glucose 

→producing a short alpha(1→4) linked glucosy chain  = primer

- transfer of glucose from UDP-glucose to nonreducing end of growing chain by glycogen synthase 

- new glycosidic bonds- between anomeric hydroxyl group of carbon 1 of activated glucose & carbon 4 of accepting glucosyl residue 

Glycogen has branches every 8 glycosyl residues - makes it highly branched, tree like structure 

more soluble than unbranched amylase 

branching increases number of non reducing ends to which new glucose can be added

branching increases the size of glycogen molecule

made by branching enzyme= amylo-alpha(1-4)→(1-6)- transglycosylase

• it removes a set of 6-8 gycosyl residues from the non- reducing end - braking alpha(1-4) bond 
• attaches this to a non terminal glycosyl residue by a alpha(1-6) linkage 
• after, both the new & old non reducing ends can be elongated again by glycogen synthase 

• degradation of glycogen 
• primary product is glucose-1-phosphate - obtained by braking alpha(1-4) glycosidic bonds 
• second product free glucose - from each alpha(1-6) linked glycosyl residue (branch point)

• glycogen phosphorylase cleaves alpha (1-4) glycosidic bonds between the glycosyl residues at non reducing ends - phosphorylysis- producing glucose-1-phosphat 
• phosphorylysis until 4 glycosyl residues remain on each chain at a brach point ≈ dextrin structure

⇒ phosphorylase can NOT degrade it further 

• phosphorylase requires pyridoxal phosphate as coenzyme - Vitamin B6

• Phosphorylase cleaves alpha(1-4) glycosidic bonds between the glycosyl residues at the non reducing ends getting Glucose-1-phosphate
• Coenzyme is pyridoxal phosphate (Vitamin B6) 

branches removed by 2 enzyme activities of the debranching enzyme (bifunctional protein)

1. oligo-alpha-(1-4) → alpha(1-4)glucantransferase

• removes outer 3 of 4 glycosyl residues 
• transfers them to the nonreducing end of another chain 
• one alpha(1-4) bond broken & one alpha(1-4) bond made 

1. Amylo-alpha(1-6) glucosidase

• removes glucose attached in a alpha(1-6) linkage hydrolytically 
• releases this way free nonphosphorylated glucose 
• glycosyl chain can be degraded again by glycogen phosphorylase (until 4 glucosyl units in next branch are reached again) 

Isomerization in the cytosol from Glucose-1-phosphate to glucose-6-phosphate

1. Glucose-1-phosphate isomerization to Glucose-6-phosphate in cytosol by phosphoglucomutase
2. G-6-P transported to ER by Glucose-6-phosphate translocase
3. G-6-P dephosphorylated in ER --> G-6-P to glucose by glucose 6 phosphatase 
4. Glucose transportet from ER to cytosol 
5. Hepatocytes release the glycogen-derived glucose into the blood 

→helps to maintain blood glucose level 

1. Glucose-1-phosphate isomerization to Glucose-6-phosphate in cytosol by phosphoglucomutase
2. in muscle there is NO Glucose-6-Phosphatase → no dephosphorylation to glucose 
3. Glucose-6-phosphate enters glycolysis to provide energy for muscle contraction