METABOLISME SEL

Microbial Metabolism: The Chemical Crossroads of Life Chapter 8

Metabolism The sum total of all chemical reactions & physical workings occurring in a cell

2 types of metabolism Anabolism - biosynthesis building complex molecules from simple ones requires energy (ATP) Catabolism - degradation breaking down complex molecules into simple ones generates energy (ATP)

Enzyme structure Simple enzymes – consist of protein alone Conjugated enzymes or holoenzymes – contain protein and nonprotein molecules apoenzyme –protein portion cofactors – nonprotein portion metallic cofactors – iron, copper, magnesium coenzymes -organic molecules - vitamins

Enzyme-substrate interactions

Exoenzymes – transported extracellularly, where they break down large food molecules or harmful chemicals; cellulase, amylase, penicillinase Endoenzymes – retained intracellularly & function there

Constitutive enzymes – always present, always produced in equal amounts or at equal rates, regardless of amount of substrate; enzymes involved in glucose metabolism Induced enzymes – not constantly present, produced only when substrate is present, prevents cell from wasting resources

Synthesis or condensation reactions – anabolic reactions to form covalent bonds between smaller substrate molecules, require ATP, release one molecule of water for each bond Hydrolysis reactions– catabolic reactions that break down substrates into small molecules, requires the input of water

Transfer reactions by enzymes Oxidation-reduction reactions – transfer of electrons Aminotransferases – convert one type of amino acid to another by transferring an amino group Phosphotransferases – transfer phosphate groups, involved in energy transfer Methyltransferases – move methyl groups from one molecule to another Decarboxylases – remove carbon dioxide from organic acids

Metabolic pathways

Control of enzyme activity Competitive inhibition – substance that resembles normal substrate competes with substrate for active site Feedback inhibition – concentration of product at the end of a pathway blocks the action of a key enzyme Feedback repression – inhibits at the genetic level by controlling synthesis of key enzymes Enzyme induction – enzymes are made only when suitable substrates are present

Competitive inhibition

Energy –capacity to do work or cause change Endergonic reactions – consume energy Exergonic reactions – release energy

Redox reactions always occur in pairs There is an electron donor and electron acceptor which constitute a redox pair The process salvages electrons & their energy. released energy can be captured to phosphorylate ADP or another compound

Electron carriers resemble shuttles that are loaded and unloaded with electrons and hydrogen most carriers are coenzymes, NAD, FAD, NADP, coenzyme A & compounds of the respiratory chain

NAD reduction

Electron carriers

ATP 3 part molecule consisting of adenine – a nitrogenous base ribose – a 5-carbon sugar 3 phosphate groups Removal of the terminal phosphate releases energy

ATP

Phosphorylation of glucose by ATP

Formation of ATP substrate-level phosphorylation oxidative phosphorylation photophosphorylation

substrate-level phosphorylation

Catabolism of glucose Glycolysis Tricarboxylic acid cycle, Kreb’s cycle Respiratory chain, electron transport

Metabolic strategies Pathways involved Final e- acceptor ATP yield Aerobic respiration Glycolysis, TCA, ET O2 38 Anaerobic respiration NO3-, So4-2, CO3-3 variable Fermentation Glycolysis Organic molecules 2

Overview of aerobic respiration

Overview of aerobic respiration Glycolysis – glucose (6C) is oxidized and split into 2 molecules of pyruvic acid (3C) TCA – processes pyruvic acid and generates 3 CO2 molecules Electron transport chain – accepts electrons NADH & FADH, generates energy through sequential redox reactions called oxidative phosphorylation

Glycolysis

TCA cycle

Electron transport system

Chemiosmosis

Fermentation Incomplete oxidation of glucose or other carbohydrates in the absence of oxygen Uses organic compounds as terminal electron acceptors Yields a small amount of ATP Production of ethyl alcohol by yeasts acting on glucose Formation of acid, gas & other products by the action of various bacteria on pyruvic acid

Fermentation

Products of fermentation

Many pathways of metabolism are bi-directional or amphibolic Metabolites can serve as building blocks or sources of energy Pyruvic acid can be converted into amino acids through amination Amino acids can be converted into energy sources through deamination Glyceraldehyde-3-phosphate can be converted into precursors for amino acids, carbohydrates and fats