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)

– See condensation or dehydration synthesis below

• Catabolism - degradation– breaking down complex molecules into simple ones

– generates energy (ATP)

– See hydrolysis below

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RNA enzymes - ribozymes

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Activation Energy

• For a reaction to occur, an energy barrier must be surmounted

• Enzymes make the energy barrier smaller

activation energywithout enzyme

activation energywith enzyme

energyreleased

by thereaction

products

starting substance

Figure 6.12aPage 105

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Figure 6.8 Activation Energy Initiates Reactions

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Figure 6.9 Over the Energy Barrier

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Transition State

• Unstable at highest energy level

(substrate + activation energy)

• Point when a reaction can easily run in either

direction, to product or back to a reactant

• Substrate is bound most tightly to an enzyme in

this state

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Would take so long that it practically does not happen

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Enzyme structure

• Simple enzymes – consist of protein alone

• Conjugated enzymes or holoenzymes – contain protein and nonprotein molecules– apoenzyme –protein portion (core)– cofactors – nonprotein portion

• metallic cofactors – iron, copper, magnesium

• coenzymes -organic molecules - vitamins

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Enzyme-substrate interactionsLock and key vs Induced Fit model

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Enzyme-substrate interactionsLock and key vs Induced Fit model

http://wps.prenhall.com/wps/media/objects/724/741576/Instructor_Resources/Chapter_24/Text_Images/FG24_03-31UNab.JPG

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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. Phosphorylatoin– transfer phosphate groups, involved in energy transfer

1. Photophosphorylation2. Substrate level phosphorylation3. Oxidative phosphorylation

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Metabolic pathways

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Control of enzyme activity

1. Control of enzyme activity directly2. Control of enzyme activity by

regulating enzyme synthesis

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Control of enzyme activity directly

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

2. Noncompetitive inhibiot – substance that binds to the cofactor and distorts the shape of the active site

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

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Competitive inhibition

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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

• Represible enzymes – Enzymes not produced when the end product of the pathway is present

Control of enzyme synthesis

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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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