Lecture 5: Cell Metabolism Biology 219 Dr. Adam Ross
Cellular Respiration Set of reactions that take place during the conversion of nutrients into ATP Intricate regulatory relationship between several families of reactions Goal for this class is to understand relationships and regulatory mechanisms, not to memorize intermediate substrates, names of enzymes, or steps of reactions.
Two general classes of reaction Aerobic Requires oxygen Endurance exercise Anaerobic Does not require oxygen Strength exercise
Courtesy of Richard Wheeler via visual.ly
Glycolysis to ETC Blood vessel Electrons transferred by NADH Electrons transferred by NADH Cytoplasm Carrier protein Glucose Plasma membrane Electrons transferred by NADH and FADH 2 Glycolysis glucose pyruvate Transition Reaction Citric Acid Cycle Electron Transport Chain Oxygen Mitochondrion Extracellular fluid +2 ATP +2 ATP +32 ATP 36 ATP
Glycolysis Occurs in the cytosol Breakdown of glucose (6-C) into two pyruvate (3-C) molecules Multi stage pathway: Energy investment stage- adding ATP to isomerize glucose Cleavage stage- split 6-C into two 3-C molecules Energy harness stage- utilize free energy with 2NADH and 4ATP (2 net ATP) NADH is a carrier of high energy electrons which are used by the electron transport chain Pyruvate is the branch point between aerobic and anerobic metabolism Where the molecule is sent is determined by the oxygen status of the cells in need of energy
Glycolysis Start with Glucose Add 2 ATP End with 2 Pyruvate 4 ATP (2 net) 2 NADH
Glycolysis (in cytoplasm) Glycolysis Cytoplasm During the first steps, two molecules of ATP are consumed in preparing glucose for splitting. Glucose Energyinvestment phase 2 ADP 2 ATP During the remaining steps, four molecules of ATP are produced. 4 ADP The two molecules of pyruvate then diffuse from the cytoplasm into the inner compartment of the mitochondrion, where they pass through a few preparatory steps (the transition reaction) before entering the citric acid cycle. 2 Pyruvate 4 ATP 2 NAD + 2 NADH Energyyielding phase Two molecules of nicotine adenine dinucleotide (NADH), a carrier of high-energy electrons, also are produced. Figure 3.23
Pyruvate If sent to the aerobic pathway (cells have enough oxygen) Goes to transition step and TCA cycle in mitochondria TCA cycle can be called Kreb s cycle on Citric Acid cycle (in other classes) In Bio 219 it is called the Tri-Carboxlic Acid cycle (TCA cycle) If sent to the anaerobic pathway (cells are oxygen poor) Pyruvate is converted to lactate in the cytosol Via lactic acid pathway
Pyruvate
Transition reaction Before entering TCA cycle, pyruvate (3-C) must be converted pyruvate (3C) + CoA + NAD+ acetyl CoA (2C) + CO 2 + NADH (x 2 per glucose molecule) yield: 1 NADH (x 2) 1 for each Pyruvate product: 1 CO 2 (x 2) acetyl CoA is a key intermediate which transfers 2C (acyl) units to the Citric Acid Cycle
Transition reaction Start with: 2 pyruvate (3 carbon molecules) 2 Coenzyme A End with: 2 CO 2 2 NADH 2 Acetyl CoA (2 carbon molecule)
Transition Reaction Transition Reaction (in mitochondrion) Pyruvate (from glycolysis) A molecule of NADH is formed when NAD + gains two electrons and one proton. NAD + One carbon (in the form of CO 2 ) is removed from pyruvate. CO 2 NADH (electron passes to electron transport chain) Acetyl CoA CoA Coenzyme A The two-carbon molecule, called an acetyl group, binds to coenzyme A (CoA), forming acetyl CoA, which enters the citric acid cycle. Citric Acid Cycle
Tricarboxylic Acid Cycle Multi-step series of Redox reactions Occurs in the matrix of the mitochondria 2-C from acetyl CoA is broken down to C02 1 st step: Ac-CoA + oxaloacetate (4-C) > citrate (6-C) + CoA Subsequent steps: citrate is converted back to OAA and e- are captured 2-C atoms in citrate are fully oxidized to form CO2 Electrons are transferred to NADH and FADH2 Yield: 3 NADH + 1 FADH2 + 1 ATP (x2) Product: 2 CO2 (x2)
Tricarboxylic Acid Cycle The citric acid cycle also yields several molecules of FADH 2 and NADH, carriers of high-energy electrons that enter the electron transport chain. NADH Oxaloacetate Acetyl CoA Acetyl CoA, the two-carbon compound formed during the transition reaction, enters the citric acid cycle. CoA CoA Citrate NAD + Malate TCA Cycle NAD + CO 2 leaves cycle FADH 2 FAD ATP ADP + Pi NADH -Ketoglutarate Succinate NAD + CO 2 leaves cycle NADH The citric acid cycle yields One ATP from each acetyl CoA that enters the cycle, for a net gain of two ATP.
Lactic acid pathway Pyruvate is sent here when cells are using anaerobic resporation 2 pyruvic acid + 2 NADH > 2 lactate and 2 NAD+ Generates 2 Lactate and NAD+ is regenerated for use in glycolysis
Electron transport chain Electron carrier proteins are in the inner membrane of the mitochondria 3 major complexes (I, III, IV) Cytochromes are iron containing electron carrier proteins in ETC NADH and FADH2 donate electrons to ETC Electrons move downhill (to lower energy stations) in ETC Oxygen is terminal electron acceptor
Potential energy Electron Transport Chain (inner membrane of mitochondrion) Electron Transport Chain The molecules of NADH and FADH 2 produced by earlier phases of cellular respiration pass their electrons to a series of protein molecules embedded in the inner membrane of the mitochondrion. High NAD + NADH 2e As the electrons are transferred from one protein to the next, energy is released and used to make ATP. FADH 2 FAD 2e 2e Membrane proteins 2e Eventually, the electrons are passed to oxygen, which combines with two hydrogens to form water. 2e H 2 O Low Energy released is used for synthesis of ATP 2 H + + 1 2 O 2
ATP Synthesis Chemiosmotic coupling mechanism ETC complexes act as H+ ion pumps Create H+ gradient, intermembrane space has high [H+] H+ moves down its concentration gradient through ATP-synthase This provides energy to phosphorylate ADP, creating ATP 3 H+ ions = 1 ATP
Complex Carbohydrates must first be broken down into glucose before entering glycolysis Fats and proteins enter the process at different steps
Summary
Glycogen metabolism Glycogen synthesis (glycogenesis) Glucose + ATP G-6-P glycogen Glycogen is stored mainly in liver and skeletal muscle cells Insulin stimulates glucose uptake into cells and glycogen synthesis Secreted when blood [glucose] is high Glycogen breakdown (glycogenolysis) Glycogen + Pi G-6-P glucose In liver glycogen glucose released to blood Glucagon stimulates glycogenolysis in liver when blood [glucose] is low In skeletal muscle: glycogen G-6-P metabolized by active muscle
Protein metabolism Protein catabolism (proteolysis) Hydrolysis of polypeptides into A.A. Deamination of amino acids Amino acids keto acids + NH3 Keto acids can enter TCA cycle NH3 converted to urea, and excreted via kidneys in urine Trans-amination: NH2 group can be transferred to another A.A.
Tissue utilization of proteins In every cell there is some baseline level of protein turnover Lost proteins are replaced by newly synthesized ones Protein metabolism is increased during starvation, injury, exercise and high protein diets Essential amino acids AA that the body cannot synthesize fast enough to survive Must eat to stay alive Non essential AA can be synthesized from essential AA There are a number of mechanisms by which AA are synthesized
Lipid metabolism Lipolysis- sign of an energy poor cell or organism Catabolism of lipids Hydrolysis of triglycerides Triglyeride + H2O = 3 fatty acids + glycerol Beta-oxidation Step wise oxidation of fatty acids Fats are broken down into 2-C units Then converted to acetyl CoA and enter the TCA cycle Occurs in mitochondria >100 ATP per fatty acid >2x more energy per gram than CH2O
Lipid synthesis Fas are synthesized from 2-C units of acetyl CoA Combined with glycerol to make triglycerides and phospholipids Most lipid synthesis occurs in smooth ER Insulin stimulates glucose uptake and triglyceride synthesis in adipose cells Lipid synthesis is a marker of an energy rich cell or organism
Tissue utilization of fatty acid Triglycerides stored mostly in adipose tissue Lipids are transported in the blood by lipoproteins (HDL and LDL) Liver, heart, and resting skeletal muscle use mainly fatty acids for energy Only broken down aerobically Excessive fat metabolism causes formation of ketone bodies Metabolic acidosis Type 1 diabetes mellitus
Gluconeogenesis Formation of glucose from non-carbohydrate sources Occurs mostly in the liver Important during fasting or glucose depletion (exercise, stress, low carb diet) Stimulated by cortisol and glucagon Pyruvate, lactate, glucogenic AA, and glycerol can be used
Interconversion of substrates Glucose acetyl CoA fatty acids (lipid synthesis) Amino acids keto acids glucose (gluconeogenesis) Glucose keto acids amino acids (transamination) *Note: Fatty acids can NOT be converted into glucose or amino acids
Regulation of blood glucose Insulin Made in the pancreas Released in response to an increase in blood [Glucose] Insulin binds to insulin receptor Causes cells to make more GLT transporters, and also causes the transporters to become more active Also causes fat to be stored rather than metabolized
Regulation of blood glucose Glucose is a very important molecule for survival Need it to make energy to survive Can be toxic if large concentrations are in blood Must have ways to prevent blood [glucose] from getting to high or low Insulin and glucagon There are others, but for the purpose to this class we will simplify the situation
Regulation of blood glucose Insulin is released from the β-cells of the pancreas In response to increased blood [Glucose] Causes cells in body to uptake glucose via GLT4 transporter Glycogenesis and fatty acid synthesis also occur
Regulation of blood glucose Glucagon produced by the α cells of the pancreas In response to low blood sugar Causes the inverse response of insulin Breakdown of glycogen into glucose in liver Stimulates gluconeogenesis from non CH2O precursors Eventually blood [Glucose] will rise, and the glucagon response will turn off
Regulation of appetite Hunger/ satiety is determined by hypothalamic activity in response to the nutritive state of the body Satiety center- in ventromedial nucleus (VMN) Artificial electrical stimulation elicits feelings of fullness Hyperphagia caused by a lesion on this center Anorexigenic molecules cause satiety Hunger center- in lateral hypothalamus (LHA) Artificial electrical stimulation elicits large appetite Ablation causes aphagia Orexigenic molecules stimulate hunger
Boron & Boulpaep 2005
Essential minerals Calcium Bones and intracellular signaling Copper Enzyme cofactor Magnesium Complexes with ATP Zinc Antioxidant, enzyme cofactor Phosphorous Bone, covalent modifications, ATP
Daily requirements None for carb or fat intake Protein 0.8g/kg body weight Higher in pregnant women, athletes, and postsurgical patients
Summary Use of various pathways is determined by nutritive state and presence or absence of different metabolic fuels Hormones help to regulate metabolic pathways Hormones are the response to a change in some homeostatic variable They help to induce the changes that will return the body to homeostasis