Biosynthesis of carbohydrate polymers

• Starch in plants, glycogen in vertebrates

• These polymerization reactions utilize sugar nucleotides as activated substrates

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

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

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

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

• Branches can subsequently be modified by glycogen synthase

• Branches increase solubility of glycogen

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

Glycogenin stays bound to the

single reducing end of glycogen as glycogen synthase

takes over

Glycogen synthase and glycogen phosphorylase are reciprocally

regulated• Details in text

Starch synthesis• Analogous mechanism to glycogen

synthase, but starch synthase uses ADP-glucose

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

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

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

Rubisco catalyses the addition of CO2 to RuBP and cleavage to 3-phosphoglycerate

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

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

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

Transketolase reactions of the Calvin cycle

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)

An antiporter exchanges Pi with triose phosphates

Regulation of the Calvin cycle (Rubisco)

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

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

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