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Enzymes11/5/07

Cell Biology 1 Metabolism

Preamble

First Law of Thermodynamics- energy can neither be created or destroyed, but it

may be transmuted

Second Law of Thermodynamics- entropy rules (chaos is the natural disorder of theuniverse)

A. Cellular Metabolism- refers to all of the enzyme-mediated (chemical)

reactions within a cell

Anabolic metabolism to buildCatabolic metabolism to break down

B. Activation Energy- the speed of a reaction depends on the amount of

activation energy required to break existing bonds

In either kind of reaction, additional energy must be

supplied to start the reaction. This energy is the activation

energy.

Notes

Catabolic Reactions Associated withDigestion (for Cell Respiration)

source: Madder, 9e

Endergonic- Refers to a chemicalreaction that consumes energy.

Exergonic - Describes a chemicalreaction that releases energy in the formof heat, light, etc.


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Cell Processes and Applications 2 Metabolism

Source:

http://faculty.clintoncc.suny.edu/faculty/Michael.Gregory/files/Bio%20100/Bio%20100%20Lectures/Enzymes/energy.htm

C. Catalyst

- a catalystlowers the activation energy needed (bystressing chemical bonds)

- a substance that speeds up a chemical reaction,- but is itself not a part of the final product- and would not cause a reaction that would not

normally occur given sufficient energy

- enzymes are the cells catalysts

Source:http://faculty.clintoncc.suny.edu/faculty/Michael.Gregory/files/Bio%20100/Bio%20100%20Lectures/Enzymes/energy.htm

D. Enzyme

- cells contain many different enzymes- each of which catalyses a different reaction

- enzymes cannot speed up reactions thatwould not occur on their own

- a given enzyme interacts with only one set ofreactants(customarily called substrates), oroccasionally with a few closely related ones

Exothermicvs ExergonicEndothermicvs Endogonic

Neither exergonic and exothermic aresynonymous nor endergonic andendothermic, however, if you find it easierto think of them as synonymous, then gofor it.

The -thermicterms tend to be limited todescribing heat energy (!H) while the -gonicterms are broader, referring to freeenergy (!G). That is, two possible thingscan drive a reaction spontaneouslyforward: A release of energyas reactants

go to products or an increase in entropyasreactants go to products.

!G takes into account not only changes ininternal energy but also changes in entropy

(!

S) that may accompany a reaction. Formost ordinary simple chemical reactions,

the entropy factor is not great, so chemistsusually talk about !H. For many biologicalreactions,the entropy factor is significant,so biochemists usually talk about !G. !G= !H - (T x !S ). Despite the signs, whichhave to do with the way these terms aredefined, !G is the sum of the 2 effects.

The -thermicterms more or less only dealwith the release of energywhile the -gonicterms refers to both the release of energyand an increase in entropy.


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Enzymes


- enzymes are mostly globular proteins- with one or more invaginations on their surface,

called the active site

- where catalysis occurs

Source: http://en.wikipedia.org/wiki/Enzyme

- in order for catalysis to occur the substrate must fitperfectly into this depression (Lock & Key theorydiagram)

- proteins are not rigid, so the enzyme may give alittle allowing an induced fit (diagram)

source: http://www.accessexcellence.org/AB/GG/

E. Factors Affecting Enzyme Activity

1. Temperature

- human enzymes work best between 35 and 40C

An allosteric enzyme has an active sitewhere substrate molecules bind and

another site that binds with intermediate orend-product molecules.


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- rate of chemical reaction is reduced by half forevery 10C drop

- below this temp. protein is not flexible to allowinduced fit

- and becomes deactivated(!denatured,because deactivation, unlike denaturation isreversible if the enzyme is warmed)

- above this temp. the H-bonds are too weak- upper limit of enzyme activity before being

denaturedis 40C

- damaged caused by mild heating may insome cases be reversible, but

- continued warming would continue todenature more and more of the enzyme until

no active enzyme remains, e.g., amylase

would be completely denatured at 80C

- Denaturing results in the! loss of the active site

2. pH- optimum pH between 6 to 8 (exceptions, e.g., pepsin

pH2)

- denatures protein by disrupting bond charges- disrupts H-bonds betweenRgroups! loss of the active site

3. Inhibitors

- Competitive Inhibitors blocks active site


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- A competitive inhibitor binds reversibly to theenzyme, preventing the binding of substrate. On theother hand, binding of substrate prevents binding of

the inhibitor, thus substrate and inhibitor compete

for the enzyme.Source: http://en.wikipedia.org/wiki/Enzyme

- Non-competitive Inhibitor- e.g., heavy metal poisoning, e.g, Lead or

Mercury

Source: http://en.wikipedia.org/wiki/Enzyme

- Binds to the enzyme and alters the shape of the activesite, so the substrate no longer fits

4. Cofactors

- often enzymes use additional chemical componentsto aid catalysis

- called cofactors,e.g.,

- some enzymes have metal ionslocked into theiractive sites

- ions that draw electrons from substrate molecules- e.g., carboxypeptidase has a zinc ion that draws

electrons from the bonds joining a.a.

- why the need for trace elements for good health,- nonprotein organic molecules used as cofactors are

called coenzymes

- vitaminsare coenzymes


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- an inactive enzyme (protein) is an apoenzyme- apoenzyme + coenzyme = holoenzyme (a fully

functional enzyme)

- in many enzyme-catalyzed oxidation-reductionreactions, energy bearing electrons are harvested- the electrons pass from active site to coenzyme

(electron acceptor)

- which carries them to another enzymecatalysing another reaction, and then returnsfor another load

- coenzymes shuttle energy in the form of an atomfrom one place in the cell to another (diagram)

- NB coenzymes do not provide E, they providethe atom, which in turn supplies the E to theenzyme mediated reaction

e.g., in Cell Respiration

- NAD(nicotinamide adenine dinucleotide)-aka Niacin-a coenzyme that functions as an electron acceptor in

many oxidation reactionsof the cell

-NADH2(NADre) is the reduced form, or energyrich form- a coenzyme that functions as an electron

donor, involved in reduction reactions


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Vocab Extras (Endocrine System):

Thyroxine [Thyroxin] (T4)

- Hormone secreted by the thyroid gland (neck)- Stimulates the body's oxidative metabolism;

regulates growth and development

Thyroid Stimulating Hormone (TSH)

- Secreted by the anterior pituitary- Stimulates the thyroid glandand therefore

thyroxine

- e.g., thyroxine action increases metabolic heatreducing cold stress

Source: http://www.biol.sc.edu/courses/bio102/f97-41.html

TSHRH= "Thyroid Stimulating Hormone Releasing Hormone"

TSH= "Thyroid Stimulating Hormone"TH= "Thyroxine"

4 cell metabolism

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