Foundations in Microbiology

Chapter8

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Fifth Edition

Talaro

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Microbial Metabolism: The Chemical Crossroads of Life

Chapter 8

Metabolism

The sum total of all chemical reactions & physical workings

occurring in a cell

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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)

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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

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Enzyme-substrate interactions

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• Exoenzymes – transported extracellularly, where they break down large food molecules or harmful chemicals; cellulase, amylase, penicillinase

• Endoenzymes – retained intracellularly & function there

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• 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

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• 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

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Transfer reactions by enzymes

1. Oxidation-reduction reactions – transfer of electrons

2. Aminotransferases – convert one type of amino acid to another by transferring an amino group

3. Phosphotransferases – transfer phosphate groups, involved in energy transfer

4. Methyltransferases – move methyl groups from one molecule to another

5. Decarboxylases – remove carbon dioxide from organic acids

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Metabolic pathways

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Control of enzyme activity

1. Competitive inhibition – substance that resembles normal substrate competes with substrate for active site

2. Feedback inhibition – concentration of product at the end of a pathway blocks the action of a key enzyme

3. Feedback repression – inhibits at the genetic level by controlling synthesis of key enzymes

4. Enzyme induction – enzymes are made only when suitable substrates are present

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Competitive inhibition

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Energy –capacity to do work or cause change

• Endergonic reactions – consume energy• Exergonic reactions – release energy

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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

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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

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NAD reduction

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Electron carriers

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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

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ATP

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Phosphorylation of glucose by ATP

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Formation of ATP

1. substrate-level phosphorylation2. oxidative phosphorylation3. photophosphorylation

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substrate-level phosphorylation

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Catabolism of glucose

1. Glycolysis2. Tricarboxylic acid cycle, Kreb’s cycle3. Respiratory chain, electron transport

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Metabolic strategies

Pathwaysinvolved

Final e- acceptor ATP yield

Aerobic respiration

Glycolysis, TCA, ET

O2 38

Anaerobic respiration

Glycolysis, TCA, ET

NO3-, So4

-2, CO3

-3

variable

Fermentation Glycolysis Organic molecules

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Overview of aerobic respiration

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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

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Glycolysis

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TCA cycle

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Electron transport system

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Chemiosmosis

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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

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Fermentation

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Products of fermentation

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• 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

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