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Biosynthesis of carbohydrate polymers
• Starch in plants, glycogen in vertebrates
• These polymerization reactions utilize sugar nucleotides as activated substrates
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Why sugar nucleotides?• Their formation is metabolically irreversible,
contributing to the irreversibility of pathways in which they are intermediates
• Nucleotide moiety provides potential interactions
• The substrate is activated because the nucleotidyl group is a good leaving group
• Tags the substrate, marking it for storage
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Glycogen synthesis
• Glucose 6-phosphate is isomerized to glucose 1-phosphate by phosphoglucomutase
• UDP-glucose pyrophosphorylase converts glucose 1-phosphate to UDP glucose using UTP and producing pyrophosphate
• Glycogen synthase attaches the UDP-glucose to the nonredcuing end of a branched glycogen molecule
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Making bonds in glycogen
• Glycogen synthase requires as a primer an (1-4) poly glucose chain or branch having at least eight glucose residues.
• Glycogen synthase cannot make the (1-6) bonds found at branch points; these are formed by glycosyl (4-6) transferase
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Branching glycogen
• Glycosyl (4-6) transferase catalyzes the transfer of a terminal fragment of six or seven glucose residues from the non-reducing end of a glycogen branch (having at least 11 residues) to the C6 hydroxyl group of a glucose residue at a more interior position of a glycogen molecule, generating a new branch
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• Branches can subsequently be modified by glycogen synthase
• Branches increase solubility of glycogen
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Where does the primer come from?
• Glycogenin builds primers for glycogen synthase• Tyrosine-194 of this protein is the the site of
covalent glucose attachment (via UDP-glucose)• This modified glycogenin binds to glycogen
synthase, and the glycogen-bound glucose molecule is extended up to seven residues using UDP-glucose
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Glycogenin stays bound to the
single reducing end of glycogen as glycogen synthase
takes over
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Glycogen synthase and glycogen phosphorylase are reciprocally
regulated• Details in text
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Starch synthesis• Analogous mechanism to glycogen
synthase, but starch synthase uses ADP-glucose
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UDP-sugars are used in synthesis of other biomolecules
• UDP-glucose for sucrose synthesis
• UDP-galactose for lactose synthesis
• UDP-glucose for vitamin C
• UDP-glucosamine for peptidoglycan
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A discussion of carbohydrate biosynthesis must encompass photosynthesis (chapter 20)
• Photosynthetic organisms assimilate or fix CO2 via the Calvin cycle
• This cycle has three stages:– Fixation – making 3-phosphoglycerate– Reduction – generating glyceraldehyde 3-
phosphate– Regeneration – making ribulose 1, 5
bisphosphate from triose phosphates
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Stage I is mediated by Rubisco• Rubisco is considered the most abundant
protein on Earth (located in chloroplast)
• Rubisco stands for ribulose 1,5-bisphosphate carboxylase/oxygenase
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Rubisco catalyses the addition of CO2 to RuBP and cleavage to 3-phosphoglycerate
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Stage II
• The first step is catalyzed by 3-phosphoglycerate kinase, which converts 3-phosphoglycerate to 1,3 bisphosphoglycerate using ATP
• This compound is reduced using NADPH by glyceraldehyde 3-phosphate dehydrogenase to glyceraldehyde 3-phosphate
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Stage II (cont)
• DHAP is formed by triose phosphate isomerase then a portion transported to the cytosol for either glycolytic metabolism or production of starch or sucrose as a storage and transport media
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Each CO2 fixed consumes a molecule of RuBP
• Therefore, RuBP must be regenerated.
• This is accomplished by a pathway including variable number carbon intermediates reminiscent of non-oxidative branch of PPP
• Enzymes included in this stage include transaldolase and transketolase
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Transketolase reactions of the Calvin cycle
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The result of the Calvin cycle
• The net result is the conversion of three molecules of CO2 and one molecule of phosphate into a molecule of triose phosphate. (One molecule of glyceraldehyde 3-phosphate is the net product of this carbon assimilation pathway)
• This result comes from (uses) 6 NADPH and 9 ATP – supplied by photosynthesis (light)
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An antiporter exchanges Pi with triose phosphates
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Regulation of the Calvin cycle (Rubisco)
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Four essential Calvin cycle enzymes are regulated by light
• Ribulose 5-phosphate kinase
• Fructose 1,6-bisphosphatase
• Sedoheptulose 1,7 bisphosphatase
• Glyceraldehyde 3-phosphate dehydrogenase
• Regulation mediated by disulfide bond formation and disruption
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Rubisco is an oxygenase
• Evolution has made Rubisco somewhat of an inefficient enzyme as it has a difficult time discriminating between O2 and CO2
• Using oxygen results in a metabolically useless molecule, phosphoglycolate
• Carbon is salvaged from phosphoglycolate by photorespiration
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Plants can minimize photorespiration
• Photorespiration is wasteful• Tropical plants employ a more complex
pathway for fixing CO2
• This pathway fixes CO2 on PEP using PEP carboxylase and subsequently donates the CO2 to Rubisco
• These are known as C4 plants, in contrast to C3 plants which only use the Calvin cycle