Pentose phosphate pathway-glycogen metabolism

Lecture (6 hours) The pentose phosphate pathwayGlycogen metabolism Instructor: Dr Nguyen Thao Trang School of Biotechnology Semester I 2015-2016 Outlines of the PPP •  Functions of the pentose phosphate pathway •  Oxidative reactions: forming ribose 5-phosphate from glucose 6-phosphate •  Non-oxidative reactions: forming glycolysis intermediates •  Transition of carbon skeleton •  Control of the PPP Functions of pentose phosphate pathway •  To produce NADPH, the 2nd cellular currency NADPH is used for reductive biosynthesis of fatty acids and cholesterol •  To produce ribose 5-phosphate used for nucleic acid synthesis •  Despite their close chemical structures, NADPH & NADH are not metabolically interchangeable: -  NADH uses free energy of metabolite oxidation for ATP synthesis -  NADPH uses free energy of metabolite oxidation for reductive biosynthesis Functions of pentose phosphate pathway Biochemistry, Tymoczko, Berge, Strayer The pentose phosphate pathway (PPP) Oxidative phase Nonoxidative phase Fundamentals of biochemistry-Life at the molecular level, Voet, Voet, Pratt The pentose phosphate pathway (PPP) 1.  Oxidative phase: oxidation reaction Glucose 6-phosphate 6-Phosphoglucono-δ lactone Ribulose 5-phosphate 6-Phosphogluconate Biochemistry, Tymoczko, Berge, Strayer Overall: Formation of reducing power NADPH & ribulose 6-phosphate from the 1st phase (oxidative phase) of the PPP The pentose phosphate pathway (PPP) Non-oxidative phase: isomerization reactions Ribulose 5-phosphate Ribose 5- phosphate (R5P) Ribulose 5- phosphate (R5P) R5P + X5P Xylulose 5-phosphate (X5P) Glyceraldehyde + Sedoheptulose 3-phosphate 7-phosphate Fructose + 6-phosphate Glyceraldehyde 3-phosphate + Erythose 4-phosphate Fructose 6-phosphate Biochemistry, Tymoczko, Berge, Strayer Overall: generation of G3P & F6P from R5P & X5P in the 2st phase (non-oxidative phase) + Xylulose 5-phosphate Outlines of the PPP •  Functions of the pentose phosphate pathway •  Oxidative reactions: forming ribose 5-phosphate from glucose 6-phosphate •  Non-oxidative reactions: forming glycolysis intermediates •  Transition of carbon skeleton •  Control of the PPP Oxidative phase of the PPP Fundamentals of biochemistry-Life at the molecular level, Voet, Voet, Pratt •  Overall reaction: –  Conversion of glucose 6-phosphate (6C) into ribulose 6phosphate (5C) –  Generation of NADPH molecules Oxidative phase of the PPP •  Reaction1: –  G6P is considered the starting point of the pentose phosphate pathway –  This metabolite may arise through the action of hexokinase on glucose in glycolysis Biochemistry, Tymoczko, Berge, Strayer –  This metabolite may arise through glycogen breakdown 10 Outlines of glycogen metabolism •  Glycogen degradation •  Regulation of glycogen degradation •  Glycogen synthesis •  Regulation of glycogen synthesis •  Glycogen degradation and synthesis are reciprocally regulated 58 Glycogen synthesis Glycogen synthesis and degradation occur by separate pathways: Fundamentals of biochemistry-Life at the molecular level, Voet, Voet, Pratt 59 Step 1: Uridine diphosphate glucose (UDP-glucose) •  UDP-glucose is an activated form of glucose which acts as a glucose donor •  UDP-glucose is synthesized from glucose 1phosphate and UTP The reaction is catalyzed by UDP glucose pyrophosphorylase: •  Pyrophosphate PPi is rapidly hydrolysed to form orthophosphate This irreversible reaction drives the synthesis of UDP-glucose: Biochemistry, Tymoczko, Berge, Strayer Fundamentals of biochemistry-Life at the molecular level, Voet, Voet, Pratt 60 Step 2: Transfer of a glucose unit from UDP-glucose to a growing chain •  A new glycosyl group from UDP-glucose is added to nonreducing ends of glycogen: Glycogen synthase Biochemistry, Tymoczko, Berge, Strayer •  Glycogen synthase can only add glucosyl groups to a polysaccharide already containing > residues → glycogen synthesis requires a primer Glycogenin (primer) UDPglucose attached to OH group of Tyr 194 Glycogenin   Autoglycosylation Glycogenin   Glycogen synthase Glycogenin   Joseph Lomako, Wieslawa M Lomako, William J Whelan, BBA, 1673, 2004, 45–55 Biochemistry, Tymoczko, Berge, Strayer 61 Step 3: A branching enzymes forms α- 1,6 linkages •  A branching enzyme, amylo-(1,4→1,6)transglycosylase is catalyzed the formation of α-1,6 linkages by breaking the α-1,4 linkages conditions: A group of residues, typically will be transferred to form a new branch 1.  The transferred block of residues must include the nonreducing terminal 2.  The transferred block must come from the chain containing at least 11 residues A branch is formed at least residues away from the existing one Biochemistry, Tymoczko, Berge, Strayer 3.  A new branch must be formed at least residues away from other branch points Importance of branching: - Increase the solubility of glycogen - Increases the rate of glycogen synthesis and degradation 62 Outlines of glycogen metabolism •  Glycogen degradation •  Regulation of glycogen degradation •  Glycogen synthesis •  Regulation of glycogen synthesis •  Glycogen degradation and synthesis are reciprocally regulated 63 Regulation of glycogen metabolism Glycogen degradation Glycogen synthesis 1.  Glycogen phosphorylase is the key regulatory enzyme in glucose degradation 2.  Glycogen synthase is the key regulatory enzyme in glucose synthesis 64 Regulation of glycogen synthesis •  Glycogen synthesis is controlled by glycogen synthase •  Activity of glycogen synthase is also regulated by covalent modification (phosphorylation) •  Glycogen synthase is phosphorylated at multiple sites by protein kinase A and glycogen synthase kinase (GSK): a form (active) Glycogen   synthase   b form (inactive) phosphorylation Protein kinase A GSK Recall glycogen phosphorylase in glycogen degradation: Phosphorylation Glycogen   synthase   Phosphorylation inactivates glycogen synthase (synthesis) but activates glycogen phosphorylase (degradation) ! Phosphorylase kinase 65 Outlines of glycogen metabolism •  Glycogen degradation •  Regulation of glycogen degradation •  Glycogen synthesis •  Regulation of glycogen synthesis •  Glycogen degradation and synthesis are reciprocally regulated 66 Glycogen degradation & synthesis are reciprocally regulated Biochemistry, Tymoczko, Berge, Strayer 67 Protein phosphatase reverses the enzymatic effects on glycogen metabolism (active) (inactive) (inactive) (active) (inactive) (active) Biochemistry, Tymoczko, Berge, Strayer 68 Regulation of protein phosphatase in muscle By mechanisms: In most tissues: small proteins are phosphorylated then bound to the catalytic site of PP1→ deactivate PP1 → glycogen synthesis is shut off In muscle: regulatory site of PP1 is phosphorylated then dissociated from glycogen→ deactivate PP1 → glycogen synthesis is shut off GM: regulatory site of PP1 PP1: catalytic site of PP1 Biochemistry, Tymoczko, Berge, Strayer 69 Insulin triggers glycogen synthesis by inactivating glycogen synthase kinase (GSK) a form (active) Glycogen   synthase   b form (inactive) phosphorylation Protein kinase A GSK Glycogen   synthase   •  When blood-glucose levels are high, insulin stimulates glycogen synthesis by inactivating GSK, the enzyme that maintains glycogen synthase in its phosphorylated, inactive state 1st step: Binding of insulin activates the tyrosine kinase receptor →phosphorylates the insulin receptor substrate (IRS) → activates protein kinases → phosphorylate & inactivate the GSK→ cannot keep glycogen synthase in its inactive form 2nd step: Protein phosphatase (PP1) dephosphorylates glycogen synthase→ converting it into its active form → promotes glycogen synthesis Biochemistry, Tymoczko, Berge, Strayer 70 Glycogen metabolism in liver is regulated by blood-glucose level Besides insulin, high glucose level also triggers glycogen synthesis in the liver: The amount of liver phosphorylase a decreases rapidly when glucose is added Phosphorylase a is the glucose sensor in liver cells The amount of glycogen synthase a increases after a lag period Biochemistry, Tymoczko, Berge, Strayer 71 Glucose regulation of glycogen metabolism in liver Inactivates glycogen degradation Biochemistry, Tymoczko, Berge, Strayer Activates glycogen synthesis 72 ... Ribulose 5- phosphate (R5P) R5P + X5P Xylulose 5 -phosphate (X5P) Glyceraldehyde + Sedoheptulose 3 -phosphate 7 -phosphate Fructose + 6 -phosphate Glyceraldehyde 3 -phosphate + Erythose 4 -phosphate. .. ribulose 6 -phosphate from the 1st phase (oxidative phase) of the PPP The pentose phosphate pathway (PPP) Non-oxidative phase: isomerization reactions Ribulose 5 -phosphate Ribose 5- phosphate (R5P)... 6 -phosphate •  Glucose 6 -phosphate can be used as a substrate either for glycolysis or for the pentose phosphate pathway depending on the cell’s need for biosynthesis (PPP) or for energy from metabolism