Chapter 20 Carbohydrate Biosynthesis in Plants and Bacteria Anabolic pathways use energy from ATP and NADH or NADPH to synthesize cellular components from simple precursor molecules o Generally reductive Anabolism and catabolism proceed simultaneously in a dynamic steady state Biosynthesis occurs primarily in the plastids Plants are autotrophs able to convert carbon from CO2 into organic compounds o Plastids are membrane bound organelles unique to photosynthetic organisms The movement of intermediates between cellular compartments is an important aspect of metabolism Plants are not motile so they must have sufficient metabolic flexibility to allow them to adapt to changing conditions Plants have thick cell walls made of carbohydrate polymers which must be assembled outside the plasma membrane and which constitute a significant proportion of the cell s carbohydrate 20 1 Photosynthetic Carbohydrate Synthesis 2 assimilation a process by which CO2 is converted to simple organic CO compounds o Also called CO2 fixation or carbon fixation o CO2 is assimilated via a cyclic pathway called the Calvin cycle photosynthetic carbon reduction cycle Key enzymes are regulated by o Reduction of disulfide bonds by electrons flowing from photosystem I o Changes in pH and Mg2 concentration that result from illumination o Conventional allosteric regulation by one or more metabolic intermediates o Covalent modification phosphorylation Plastids Are Organelles Unique to Plant Cells and Algae a family of self reproducing organelles bounded by a double membrane Plastids and containing a small genome that encodes some of their proteins o Reproduce by binary fission Chloroplasts the sites of CO2 assimilation o Contain the enzymes for CO2 assimilation in the stroma colorless plastids that lack chlorophyll and other pigments found in Amyloplasts chloroplasts o No internal membranes o Packed with starch granules in plant tissues rich in starch Chloroplasts can be converted to proplastids by the loss of their internal membranes and chlorophyll and proplastids are interconvertible with amyloplasts o Both amyloplasts and proplastids can develop into chloroplasts The inner membranes of all plastids are impermeable to polar and charged molecules Carbon Dioxide Assimilation Occurs in Three Stages Carbon fixation reaction condensed the first step in the assimilation of CO2 where CO2 is o Ribulose 1 5 bisphophate o Forms two molecules of 3 phosphoglycerate a five carbon acceptor In the second stage 3 phosphoglycerate is reduced to triose phosphates Overall three molecules of CO2 are fixed to three molecules of ribulose 1 5 bisphophate to form six molecules of glyceraldehyde 3 phosphate in equilibrium with dihydroxyacetone phosphate In the third stage five of the six molecules of triose phosphate are used to regenerate three molecules of ribulose 1 5 bisphosphate o The sixth triose can be used to make hexoses for fuel and building materials sucrose for transport to nonphotosynthetic tissues or starch for storage Reductive pentose phosphate cycle phosphate pathway but in the opposite direction converting hexose phosphates to pentose phosphates the same reactions as in the pentose o Uses the same enzymes as the oxidative pathway and more that make it irreversible o All 13 enzymes are in the chloroplast stroma Stage 1 Fixation of CO2 into 3 Phosphoglycerate o 3 phosphoglycerate is an early intermediate in photosynthesis o C 3 plants plants in which 3 phosphoglycerate is the first intermediate o Ribulose 1 5 bisphosphate carboxylase oxygenase catalyzes incorporation of CO2 into an organic form the enzyme that Aka rubisco Catalyzes the covalent attachment of CO2 to ribulose 1 5 bisphosphate and cleavage of the unstable six carbon intermediate to form two molecules of 3 phosphoglycerate o Two forms of rubisco Form I is found in vascular plants algae and cyanobacteria Form II is confined to certain photosynthetic bacteria Two subunits similar to the large subunits of the plant form Plant rubisco has a complex form I structure with eight identical large subunits each with a catalytic site and eight identical small subunits of uncertain function o Plants need a lot of rubisco because it acts slowly o The Mg2 ion on a carbamoylated Lys side chain in plant rubisco brings together and orients the reactants at the active site and polarizes the CO2 opening it to nucleophilic attack by the five carbon enediolate reaction intermediate formed on the enzyme o Rubisco is a target of regulation Inactive until carbamoylated on the amino group of Lys201 Ribulose 1 5 bisphosphate inhibits carbamoylation by binding tightly to the active site Rubisco activase overcomes this inhibition by promoting ATP dependent release of the ribulose 1 5 bisphosphate o Activated by light through a redox mechanism 2 carboxyarabinitol 1 phosphate synthesized in the dark is a potent inhibitor of carbamoylated rubisco Either broken down when light returns or is expelled by rubisco activase Stage 2 Conversion of 3 Phosphoglycerate to Glyceraldehyde 3 Phosphate o Essentially reverse of the corresponding steps in glycolysis with one exception The nucleotide cofactor for the reduction of 1 3 bisphosphoglycerate is NADPH rather than NADH o 2 phosphoglycerate kinase the transfer of a phosphoryl group from ATP to 3 phosphoglycerate yielding 1 3 bisphosphoglycerate an enzyme found in the stroma that catalyzes o NADPH donates electrons in a reduction catalyzed by the chloroplast specific isozyme of glyceraldehyde 3 phosphate dehydrogenase producing glyceraldehyde 3 phosphate and Pi o Triose phosphate isomerase interconverts glyceraldehyde 3 phosphate and dihydroxyacetone phosphate Stage 3 Regeneration of Ribulose 1 5 Bisphosphate from Triose Phosphates o Ribulose 1 5 bisphosphate is constantly regenerated in a series of reactions that constitute the cyclic pathway The product of the first assimilation reaction undergoes transformations that regenerate ribulose 1 5 bisphosphate Intermediates include three four five six and seven carbon sugars o The combination of glyceraldehyde 3 phosphate and dihydroxyacetone phosphate to make fructose 1 6 bisphosphate is catalyzed by aldolase The fructose 1 6 bisphosphate is cleaved to fructose 6 phosphate and Pi by fructose 1 6 bisphosphatase Strongly exergonic and essentially irreversible o The combination of fructose 6 phosphate with glyceraldehyde 3 phosphate to form xylulose 5 phosphate and eyrthrose 4 phosphate is catalyzed by