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Plant Carbohydrate Biosynthesis 1. Glyoxylate cycle 2. Biosynthesis of starch and sucrose 3. Synthesis of cell wall polysaccharides 4. Integration of carbohydrate metabolism in the plant cell Glyoxylate cycle shares some enzymes with citric acid cycle p.624 Plants use glyoxylate cycle to convert lipids to carbohydrates Citric acid cycle and glyoxylate cycle are regulated reciprocally Starch biosynthesis is growing from reducing end Sucrose biosynthesis • Sucrose is synthesized in cytosol by sucrose 6-phosphate synthase and sucrose 6-phosphate phosphatase. Sucrose biosynthesis • Sucrose biosynthesis is beginning with dihydroxyacetone phosphate exported from the chloroplast. • Dihydroxyacetone phosphate is then converted to glyceraldehyde 3phosphate by triose phosphate isomerase. Sucrose biosynthesis • After condensation of glyceraldehyde 3phosphate and dihydroxyacetone phosphate by aldolase, fructose 1,6bisphosphate is dephosphorylated by FBPase-1 to produce fructose 6-phosphate. Sucrose 6-phosphate synthase catalyze the formation of sucrose 6phosphate Sucrose 6-phosphate phosphatase catalyze the formation of sucrose by dephosphorylation Regulation of sucrose biosynthesis FBPase-1/PP-PFK-1 Sucrose 6-phosphate synthase FBPase-1/PP-PFK-1 • FBPase-1 and PP-PFK-1 are regulated indirectly by the products of photosynthesis and oxidative phosphorylation. Sucrose 6phosphate synthase is also regulated • Sucrose 6phosphate synthase is regulate by phosphorylation/de phosphorylation. Starch biosynthesis is regulated by ADP-glucose pyrophosphorylase Plant cell wall biosynthesis • Plant cell wall is made of cellulose microfibrils, which is consisted of about 36 chains of cellulose, a polymer of b(14)glucose. Cellulose biosynthesis • Cellulose is synthesized by terminal complexes or rosettes, consisting of cellulose synthase and associated enzymes. Terminal complex (rosette) p.777 Cellulose synthase • Cellulose synthase has not been isolated in its active form, but from the hydropathy plots deduced from its amino acid sequence it was predicted to have eight transmembrane segments, connected by short loops on the outside, and several longer loops exposed to the cytosol. Initiation of new cellulose chain synthesis • Glucose is transferred from UDP-glucose to a membrane lipid (probably sitosterol) on the inner face of the plasma membrane. p.776 New cellulose chain synthesis (1) • Intracellular cellulose synthase adds several more glucose residues to the first one, in (b14) linkage, forming a short oligosacchairde chain attached to the sitosterol (sitosterol dextrin). New cellulose chain synthesis (2) • Next, the whole sitosterol dextrin flips across to the outer face of the plasma membrane, where most of the polysaccharide chain is removed by endo-1,4-bglucanase. New cellulose chain synthesis (3) • The dextrin primer (removed from sitosterol by endo-1,4-bglucanase) is now (covalently) attached to another form of cellulose synthase. New cellulose chain synthesis (4) • The UDP-glucose used for cellulose synthesis is generated from sucrose produced from photosynthesis, by the reaction catalyzed by sucrose synthase (this enzyme is wrongly named). New cellulose chain synthesis (5) • The glucose associated with UDP is a-linked. • Its configuration will be converted by glycosyltransferases so the product (cellulose) is b-linked. Biosynthesis of peptidoglycan • Peptidoglycan is the major component of bacterial cell wall. Peptidoglycan synthesis (1) 1. N-acetylglucosamine (GlcNAc) condenses with UTP to form UDP-GlcNAc. 2. UDP-GlcNAc reacts with PEP to form UDP-Mur2Ac. 3. Five amino acids are then added. Peptidoglycan synthesis (2) 4. The Mur2Acpentapaptide moiety is then transferred from UDP to dolichol. 5. Another GlcNAc is added to this molecule. 6. Five glycines are added to the lys residue of the pentapeptide. Peptidoglycan synthesis (3) 7. The whole disaccharide decapeptide is added to the nonreducing end of an existing peptidoglycan molecule. Peptidoglycan synthesis (4) 8. Transpeptidase catalyze a transpeptidation reaction to crosslink adjacent polysaccharide chains. Penicillin inhibit transpeptidase • Penicillins and related antibiotics contain the b-lactam ring. • Different substitution at position 6 determines their differential pharmacological properties. Acid stable Acid labile Penicillin actions • Penicillin acts as suicide inhibitors for transpeptidase. b-lactamase inactivates penicillin • A b-lactamase froms a temporary covalent adduct with the carboxyl group of the opened blactam ring, which is immediately hydrolyzed, regenerating active enzyme. glycolysis PPP photosynthesis