metabolism part 1.pdf

8/18/2019 Metabolism part 1.pdf

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Figure 8.1


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Enzymes

Catalyze the chemical reactions of life

• Enzymes: an example of catalysts, chemicals

that increase the rate of a chemical reactionwithout becoming part of the products or being

consumed in the reaction


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Activation energy is the energy required to bring all molecules

in a chemical reaction into the reactive state


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Enzymes overcome activation energy


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How Enzymes Lower Ea By increasing concentrations of substrates at active site

of enzyme By orienting substrates properly with respect to each

other in order to form the transition-state complex

Increasing thermal energy to increase molecular

velocity Induced fit model for enzyme-substrate interaction

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Enzyme Structure

Most- protein

Can be classified as simple or conjugated

– Simple enzymes- consist of protein alone – Conjugated enzymes- contain protein and non-

protein molecules


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Apoenzyme – protein component of an enzyme

Cofactor

– nonprotein component of anenzyme

prosthetic group – firmly attached

coenzyme – loosely attached, can

act as carriers/shuttles

Holoenzyme = apoenzyme +

cofactor7

Conjugated enzyme/Holoenzyme


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Cofactors: Supporting the Work of

Enzymes Metallic cofactors

– Include Fe, Cu, Mg, Mn, Zn, Co, Se – Metals activate enzymes, help bring the active site and

substrate close together, and participate directly inchemical reactions with the enzyme-substrate complex

Coenzymes – Organic compounds that work in conjunction with an

apoenzyme to perform a necessary alteration of asubstrate

– Removes a chemical group from one substrate moleculeand adds it to another substrate

– Vitamins: one of the most important components ofcoenzymes


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conjugated enzyme (holoenzyme)


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Location and Regularity of Enzyme

Action Either inside or outside of the cell

Exoenzymes break down molecules outside

of the cell

Endoenzymes break down molecules inside

of the cell


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Rate of Enzyme Production

Enzymes are not all produced in the cell in

equal amounts or at equal rates

– Constitutive enzymes: always present and inrelatively constant amounts

– Regulated enzymes: production is either

induced or repressed in response to a change inconcentration of the substrate


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Figure 8.6


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Synthesis and Hydrolysis Reactions


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Transfer Reactions by

Enzymes Oxidation-reduction reactions

– A compound loses electrons (oxidized) – A compound receives electrons (reduced)

– Common in the cell – Important components- oxidoreductases

Other enzymes that play a role in necessarymolecular conversions by directing the transfer offunctional groups:

– Aminotransferases – Phosphotransferases – Methyltranferases – Decarboxylases


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The Sensitivity of Enzymes to Their

Environment Enzyme activity is highly influenced by the

cell’s environment

Enzymes generally operate only under thenatural temperature, pH, and osmotic pressureof an organism’s habitat

When enzymes subjected to changes in normalconditions, they become chemically unstable(labile)

Denaturation: the weak bonds that maintainthe native shape of the apoenzyme are broken


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Energy in Cells

– Exergonic reaction: a reaction that releases

energy as it goes forward – Endergonic reaction: a reaction that is driven

forward with the addition of energy


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Figure 8.13


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How does a cell produce

ATP?


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Oxidative phosphorylation and

Photophosphorylation use theElectron Tranport Chain via proton

motive force to produce ATP


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Electron Carriers

located in plasma membranes of

chemoorganotrophs in bacteria and archaeal

cells located in internal mitochondrial

membranes in eukaryotic cells

examples of electron carriers include NAD, NADP, and others

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ec ron ranspor a n


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ec ron ranspor a n(ETC)

Electron carriers

organized into ETC – first electron carrier

having the most negative

E’o

– the potential energy

stored in first redox

couple is released and

used to form ATP – first carrier is reduced

and electrons moved to

the next carrier and so on25


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Electron Carriers NAD

– nicotinamide

adenine

dinucleotide NADP

– nicotinamide

adeninedinucleotide

phosphate


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Electron Carriers FAD

– flavin adenine

dinucleotide

FMN

– flavinmononucleotide

– riboflavin phosphate

coenzyme Q (CoQ) – a quinone

– also called

ubiquinone

riboflavin


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Electron Carriers Cytochromes

– use iron to transfer

electrons

iron is part of a heme

group Nonheme iron-sulfur

proteins

– e.g., ferrodoxin

– use iron to transport

electrons

iron is not part of a

heme group


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Figure 8.15

Th E bd M h f


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The Embden-Meyerhof

Pathway Occurs in cytoplasmic matrix of most

microorganisms, plants, and animals

The most common pathway for glucose

degradation to pyruvate in stage two ofaerobic respiration

Function in presence or absence of O2

Two phases – Six carbon phase

– Three carbon phase

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Summary of Glycolysis

glucose + 2ADP + 2Pi + 2NAD+

2 pyruvate + 2ATP + 2NADH + 2H+


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Figure 8.17


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The Tricarboxylic Acid Cycle

Also called citric acid cycle and Kreb’s cycle

Common in aerobic bacteria, free-living

protozoa, most algae, and fungi Major role is as a source of carbon

skeletons for use in biosynthesis

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The Respiratory Chain: Electron

Transport and Oxidative

Phosphorylation

The final “processing mill” for electrons

and hydrogen ions

The major generator of ATP

A chain of special redox carriers that

receives electrons from reduced carriers

(NADH and FADH2) and passes them in a

sequential and orderly fashion from one

redox molecule to the next


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Figure 8.18


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Glycolysis

2 ATPs

2 NADH

Kreb‘s Cycle

2 ATPs8 NADH

2 FADH2

ETC

10 NADH X 2.5 = 25 ATPs

2 FADH2 X 1.5 = 3 ATPs

Total net ATPs

Bacteria = 32

Eukaryotes = 30

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