Anabolism - Carbohydrates Lecture

OUTLINE: Introduction and review Transport Glycogenolysis Glycolysis Other sugars Pasteur: Anaerobic vs Aerobic Fermentations Pyruvate Krebs’ Cycle Oxidative Phosphorylation ANABOLISM: Carbohydrates Carbon Assimilation – Calvin Cycle Exam-1 material Exam-2 material Electron transport Chemiosmotic theory: Phosphorylation Fat Catabolism Fatty acid Catabolism Exam-3 material Mobilization from tissues (mostly adipose) Activation of fatty acids Transport; carnitine Oxidation: b-oxidation, 4 steps: Protein Catabolism Amino-Acid Degradation Dealing with the nitrogen; Urea Cycle Dealing with the carbon; Seven Families Nucleic Acid & Nucleotide Degradation PHOTOSYNTHESIS: Overview of Photosynthesis Key experiments: Light Reactions energy in a photon pigments HOW Light absorbing complexes-”red-drop experiment” Reaction center Photosystems (PS) PSII – oxygen from water splitting PSI – NADPH Proton Motive Force – ATP Overview of light reactions Stage One – Rubisco Carboxylase Know mechanism Oxygenase Glycolate cycle Stage Two – making sugar Stage Three - remaking Ru 1,5P2 Overview and regulation Calvin cycle connections to biosynthesis C4 versus C3 plants Kornberg cycle - glyoxylate Carbohydrate Biosynthesis in Animals precursors Cori cycle Gluconeogenesis reversible steps irreversible steps – four energetics 2-steps to PEP mitochondria Pyr carboxylase-biotin PEPCK FBPase G6Pase Glycogen Synthesis UDP-Glc Glycogen synthase branching Pentose-Phosphate Pathway Regulation of Carbohydrate Metabolism Anaplerotic reactions Pentose Phosphate Pathway (oxidative) (Non-oxidative) Pentose Phosphate Pathway, sometimes called the pentosephosphate shunt, is linked to Glycolysis Because it provides for essential components of these biomolecules, we will consider it part of anabolism. 1 Pentose Phosphate Pathway • Included in Anabolism because it provides the NADPH needed for so many biosynthetic reduction reactions, as well as the synthesis of pentoses. • But, can also be used for a shunt in the catabolism of Glc 6-P. • The main products are NADPH and ribose 5-phosphate (Rib 5-P). • Oxidative Phase oxidizes Glc 6-P to make NADPH • Non-oxidative Phase re-cycles pentoses to hexoses if not needed for biosynthesis. Uses of Pentose Phosphate Pathway (PPP): ØNADPH is an electron donor. – reductive biosynthesis of fatty acids and steroids – reductive biosynthesis of amino acids and nucleotide bases – repair of oxidative damage ØRibose 5-phosphate is a biosynthetic precursor of nucleotides. – used in DNA and RNA synthesis – or synthesis of some coenzymes Pentose Phosphate Pathway Oxidative Phase Generates NADPH and a Pentose Allosteric inhibition by product NADPH Mechanism lactone 2 Pentose Phosphate Pathway 5 Mechanism Mechanism Pentose Phosphate Pathway (aldose) Ru 5-P (ketose) (ketose) Phosphogluco isomerase 3C′s 2C′s (aldose) Phosphogluco isomerase (ketose) (ketose) (aldose) (aldose) 2C′s (ketose) (aldose) Non-oxidative Phase Interconverts Rib 5-P to form Glc 6-P • No ATP is used in the non-oxidative phase. • Complete conversion of ribose-5-phosphate to glucose-6phosphate requires 6 Rib 5-P (30 carbons) to make 5 Glc 6-P (30 carbons). 3 Pentose Phosphate Pathway (ketose) (aldose) (aldose) (ketose) Transaldolase Mechanism CH2OH : H+ | Protonated Schiff base (e– sink) E-NH2 + Ketose ⇌ E–N=C TPP | HOCH | HC–O-H | R :B Enamine carbanion This carbanion is protonated in aldolase In transaldolase, it makes a nucleophilic attack on the aldehyde of the secondsubstrate aldose. ℗ TPP Pentose Phosphate Pathway NADPH Regulates Partitioning into Glycolysis versus Pentose Phosphate Pathway Dihydroxyacetonephosphate • Used in tissues needing both Rib 5-P & NADPH phosphate phosphate • Used in tissues requiring more NADPH than Rib 5-P – such as the liver and adipose tissue – ATP is plentiful • Used in tissues requiring more Rib 5-P than NADPH – such as the lymphocytes and fast growing tissues/cancer cells 4 Pentose Phosphate Pathway Glc 6-P Dehydrogenase Deficiency* • Can be fatal in cases of high oxidative stress. Hemolytic anemia 500:1 – certain drugs, herbicides, and some foods (fava beans) • Resistance to malaria due to high oxidative stress in red blood cells • X-linked heterozygous advantage Essential for red-blood cells *Total lack of G6PDH is lethal Pentose Phosphate Pathway Glc 6-P Dehydrogenase (G6PD) Deficiency* Dr. Kornberg: Lecture 02.15.17 (35:00-41:36) 6.5 min 5 Pentose Phosphate Pathway Reactive Oxygen Species (ROS) Glutathione (GSH) Fe 2+-catalyzed Fenton reaction gECG Pentose Phosphate Pathway How do we get NADPH from PPP, which is made in the cytosol, into mitochondria? •Ubiquinone is naturally “leaky” and facilitates partial reduction of non-Complex III targets. Fe2+ (aconitase) –Single electron transfers result in free radicals. •One method by which the cell can correct free-radicals is the production of reduced glutathione (GSH), which fuels the glutathione shuttle Glc 6-P Dehydrogenase (G6PD) Deficiency Reactive Oxygen Species (ROS) Can Damage Biological Macromolecules Glutathione 6 Pentose Phosphate Pathway Converting Cytosolic Electron Carriers (NADPH from PPP & NADH from glycolysis) to the Mitochondria Nicotinamide nucleotide transhydrogenase NADPH antiporter NADP+ Malate-Aspartate Shuttle • This more complicated shuttle is mostly present in liver, heart, and kidney. • It moves NADH & NADPH equivalents from cytosol to NADH & NADPH equivalents in the mitochondria. antiporter or NADPH Regulation of Carbohydrate Metabolism Catabolism vs. Anabolism 7 Gluconeogenesis Glycolysis versus Gluconeogenesis 7 Glycolysis occurs mainly in the muscle and brain. PGI 6 Aldolase 5 • Opposing pathways that are both thermodynamically favorable: • Glycolysis: DG°’ = –35 kcal/mol • Gluconeogenesis: DG°’=–9 kcal/mol TIM 4 GAPDH 3 PGK 2 PGM 1 Enolase – operate in opposite direction • end product of one is the starting compound of the other • Seven Reversible reactions are used by both pathways. • Three ”glycolysis-specific” steps are reversed with Four “gluconeogenesis-specific” steps. Gluconeogenesis occurs mainly in the liver and kidney cortex. • Irreversible reactions of glycolysis must be bypassed in gluconeogenesis. – no ATP generated during gluconeogenesis; instead 6 ATPs and 2 NADH needed per Glc. – Some different enzymes results in the different pathways – differentially regulated to prevent a futile cycle Catabolism vs. Anabolism Regulation of Carbohydrate Metabolism Gene controlled Glycogenolysis versus Glycogen Synthesis Catabolism Glycogen Glycogen Phosphorylase Glycogen Synthase Anabolism Phosphorylase Glycogen Synthase Hexokinase Glucose 6phosphatase Phosphofructokinase-1 Fructose 1,6bisphosphatase -1 Pyruvate Kinase – PEPCK – Pyruvate carboxylase UDP-Glc Glycolysis versus Gluconeogenesis • Regulated enzymes often correspond to points in the pathways that have the same substrate and product, but a different enzyme. • Also where there are junctions. • Can you name those enzymes? Acetyl-CoA UDP-glucose pyrophosphorylase Glc 1-P phosphoglucomutase Pyruvate Dehydrogenase Complex 8 Regulation of Carbohydrate Metabolism Regulation of Pyruvate Dehydrogenase Complex • Allosteric regulation by energy charge and substrate/product –ADP & pyruvate activates –ATP/NADH & acetyl-CoA inhibit • Regulated by reversible phosphorylation of E1 –phosphorylation: inactive –dephosphorylation: active • PDH kinase and PDH phosphatase are part of mammalian PDH complex. –Kinase is activated by ATP. • high ATP à phosphorylated PDH à less acetyl-CoA made • low ATP à kinase is less active and phosphoprotein phosphatase removes phosphate from PDH à more acetyl-CoA made 9