Anabolism - Carbohydrate Synthesis In Animals

BI/CH 422/622 ANABOLISM I: Carbohydrates OUTLINE: Introduction and review Transport Glycogenolysis Glycolysis Other sugars Pasteur: Anaerobic vs Aerobic Fermentations Pyruvate Krebs’ Cycle Oxidative Phosphorylation 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 ANABOLISM I Carbohydrates Carbohydrate Synthesis in Animals 1 Gluconeogenesis Gluconeogenesis: Making “New” Glucose Precursors: From what compounds can glucose be made? • Animals can produce glucose from sugars or proteins. –sugars: pyruvate, lactate, or oxaloacetate –protein: from amino acids that can be converted to citric acid cycle intermediates (or glucogenic amino acids) • Animals cannot produce glucose from fatty acids. –product of fatty acid degradation is acetylCoA –There is no net conversion of acetyl-CoA to oxaloacetate in Kreb’s Cycle Plants, yeast, and many bacteria use the Kornberg Cycle to convert acetyl-CoA to oxaloacetate, thus producing glucose from fatty acids. Gluconeogenesis •Blood glucose is largely made in the liver, although other organs can reverse glycolysis, but not deliver free glucose into the blood •Synthesis of glucose from simpler compounds: called gluconeogenesis •Operates only if ATP and NADH are plentiful The Cori Cycle • Other tissues deliver carbon to liver from “waste” products (Ala and Cori Cycles). As you can see the two pathways operate in different tissues, but how is this controlled in a single cell? 2 Gluconeogenesis Glycolysis versus Gluconeogenesis 7 Glycolysis occurs mainly in the muscle and brain. PGI 6 • Opposing pathways that are both thermodynamically favorable: • Glycolysis: DG°’ = –35 kcal/mol • Gluconeogenesis: DG°’=–9 kcal/mol 5 Aldolase TIM 4 GAPDH 3 PGK 2 PGM 1 Enolase 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 – 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. Lets look at the energetics of making glucose….. Gluconeogenesis G3P PEP Pi 3PGA Favorable energetics for Calvin Cycle Changes in Free Energy During Glycolysis and the Citric Acid Cycle Pyruvate Pi Favorable energetics for Gluconeogenesis There are then 3 points that are novel for gluconeogenesis: 1. PyruvateàPEP (complicated) 2. Fru 1,6-P2 à Fru 6-P + Pi 3. Glc 6-Pà glucose + Pi 3 Gluconeogenesis The First Gluconeogenic Steps Travel Through Mitochondria 3’ 5’ • There are two ways from pyruvate to PEP. • Best place to assess whether there is ample ATP and NADH is the mitochondria. 4 6 TCA Cycle 5 citrate synthase 2 Acetyl-CoA 3 • The inner mitochondrial membrane is selectively permeable: Malate, PEP, and pyruvate, while oxaloacetate cannot escape. 2 • Oxaloacetate can be utilized in the citric acid cycle (Kreb’s cycle) if needed. pyruvate dehydrogenase Com plex 1 1 • Oxaloacetate can be converted to PEP or Malate in the mitochondria, then transported to cytosol for gluconeogenesis. • Will discuss the decision that pyruvate must make later….. There are two new enzymes here….. Gluconeogenesis Pyruvate to Phosphoenolpyruvate 4 Gluconeogenesis Pyruvate to Phosphoenolpyruvate ① ② DG°’ = –0.5 kcal/mol DG°’ = +0.7 kcal/mol • Requires two energy-consuming steps • The first step, pyruvate carboxylase (PC) converts pyruvate to oxaloacetate. – carboxylation using a biotin cofactor – This enzyme is only in the mitochondria; requires transport of pyruvate • The second step, phosphoenolpyruvate carboxykinase (PEPCK) converts oxaloacetate to PEP. – phosphorylation from GTP and decarboxylation – occurs in mitochondria or cytosol depending on the organism • During this 2-step conversion, the same carbon from CO2 is added and immediately removed from the structure. Lets look at the PC mechanism more closely….. Gluconeogenesis Biotin is a CO2 Carrier Pyruvate Carboxylase Mechanism 5 Gluconeogenesis Biological Tethers Allow Flexibility Gluconeogenesis Oxaloacetate to Phosphoenolpyruvate Phosphoenolpyruvate Carboxykinase (PEPCK) Phosphoenolpyruvate to Fru 6-P [PEPà2PGAà3PGAà1,3BPGAàGA3PàDHAPàFru 1,6P2] –3 –0.5 –1 0 –2 Fructose 1,6-bisphosphatase Glucose 6-phosphatase DG°’ = –3.3 kcal/mol Glucose 6-phosphate à glucose – Water hydrolyzes the His-Pi – DOES NOT generate ATP +5 –1.3 DG°’ = –4 kcal/mol • Catalyze reverse reaction of PFK in glycolysis • Fructose 1,6-bisphosphate à fructose 6-phosphate – – – – by fructose 1,6-bisphosphatase-1 coordinately/oppositely regulated with PFK cleaves phosphate with water DOES NOT generate ATP 6 Gluconeogenesis Why do we need glucose? RECALL: • Physiologically necessary: Brain, nervous system, and red blood cells generate ATP ONLY from glucose. • When we can’t get it from pyruvate, amino acids are utilized, which allows generation of glucose when glycogen stores are depleted: – – – during starvation during vigorous exercise can generate glucose from amino acids, but not fatty acids • Costs 4 ATP, 2 GTP, and 2 NADH. Net reaction: DG°’ = –9 kcal/mol 2 Pyruvate + 4 ATP + 2 GTP + 2 NADH + 2 H+ + 4 H2O à Glucose + 4 ADP + 2 GDP + 6 Pi + 2 NAD+ Glycogen Synthesis Storing a ready-reserve of carbohydrate 7 Glycogen Synthesis • Synthesis of glycogen requires two enzymes, whereas glycogen degradation, only used phosphorylase. Both pathways use a-phosphoglucomutase; the start and end junction is Glc 1-P. • Blood glucose must be: – – – – Phosphorylated: Glc à Glc-6-P Then converted: Glc 6-P à Glc 1-P Activated with UDP (UDP-Glc is the precursor) Added to glycogen Glycogen Synthesis Glycogen Is Synthesized by Glycogen Synthase Glycogenin Starts a New Glycogen Chain Takes 2 “ATPs” to put Glc into polymer (Glc1P & UDP-Glc), but get 33 ATP of potential energy. 8 Glycogen Synthesis General Structure of a Glycogen Particle fifth – 12th tier (last tier is unbranched) Calculate the number of Glc residues in a glycogen molecule: 29 – 212 x 12-14 = 6,000–57,000 Glc per glycogen Exponent= number of (MW = 106 – 107) tiers Synthesis of Branches in Glycogen Pentose Phosphate Pathway Providing reduced electrons and ribose 9 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. Pentose Phosphate Pathway Dr. Kornberg: 10