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Unit E: Enzymes

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An enzyme is a protein molecule that functions as an organic catalyst to speed a chemical reaction.

An enzyme brings together particular molecules and causes them to react.

The reactants in an enzymatic reaction are called the substrates for that enzyme.

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Metabolic Pathways and Enzymes

Cellular reactions are usually part of a metabolic pathway, a series of linked reactions, illustrated as follows:

E1 E2 E3 E4 E5 E6

A → B → C → D → E → F → GHere, the letters A-F are reactants or

substrates, B-G are the products in the various reactions, and E1-E6 are enzymes.

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Energy of Activation

The energy that must be added to cause molecules to react with one another is called the energy of activation (Ea).

The addition of an enzyme does not change the free energy of the reaction, rather an enzyme lowers the energy of activation.

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Energy of activation (Ea)

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Enzyme-Substrate Complexes

Every reaction in a cell requires a specific enzyme.

Enzymes are named for their substrates:Substrate EnzymeLipid LipaseUrea UreaseMaltose MaltaseRibonucleic acid Ribonuclease

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Only one small part of an enzyme, called the active site, complexes with the substrate(s).

The active site may undergo a slight change in shape, called induced fit, in order to accommodate the substrate(s).

The enzyme and substrate form an enzyme-substrate complex during the reaction.

The enzyme is not changed by the reaction, and it is free to act again.

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Enzymatic reaction

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Induced fit model

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Factors Affecting Enzymatic Speed

Enzymatic reactions proceed with great speed provided there is enough substrate to fill active sites most of the time.

Enzyme activity increases as substrate concentration increases because there are more collisions between substrate molecules and the enzyme.

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Temperature and pH

As the temperature rises, enzyme activity increases because more collisions occur between enzyme and substrate.

If the temperature is too high, enzyme activity levels out and then declines rapidly because the enzyme is denatured.

Each enzyme has an optimal pH at which the rate of reaction is highest.

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Rate of an enzymatic reaction as a function of temperature and pH

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A cell regulates which enzymes are present or active at any one time.

Genes must be turned on or off to regulate the quantity of enzyme present.

Another way to control enzyme activity is to activate or deactivate the enzyme.

Enzyme Production

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Enzyme Inhibition

Enzyme inhibition occurs when an active enzyme is prevented from combining with its substrate.

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Competitive Inhibition

Competitive inhibitors compete with the substrate to bind to the active site.

The greater the concentration of the competitive inhibitor, the less the enzyme is able to function because less active sites are present for the substrate to bind to it.

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Non-Competitive Inhibitors

Non-competitive inhibitors do not combine to the active site but combine to another part of the enzyme, changing the shape of the enzyme which prevents the substrate from binding to the active site.

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Heavy Metal Ions

Heavy metal ions change the shape of enzymes which decreases the chance of a substrate binding to the enzyme.

The shape changes because heavy metal ions have a greater affinity for electrons so the electrons are disturbed in the organic catalyst (enzyme).

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Negative Feedback

When the product of a metabolic pathway is in abundance, it may bind competitively with the enzyme’s active site, a simple form of negative feedback.

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Enzyme CofactorsPresence of enzyme cofactors may be

necessary for some enzymes to carry out their functions.

Cofactors add stability to the electron shifts in a reaction.

Inorganic metal ions, such as copper, zinc, or iron function as cofactors for certain enzymes.

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CoenzymesOrganic molecules, termed coenzymes,

must be present for other enzymes to function.

Some coenzymes are vitamins. A deficiency in certain vitamins results in

the lack of a particular coenzyme and therefore a lack of enzymatic functions. In humans, this eventually results in vitamin-deficiency symptoms (ex. Lack of vitamin C leads to Scurvy)

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Thyroxin

The thyroid gland produces thyroxin.

Thyroxin increases metabolic rate and regulates growth and development throughout the body.

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Chapter Summary

Metabolic pathways are a series of enzyme-catalyzed reactions.

Each reaction requires a specific enzyme.Substrate concentration, temperature, pH,

enzyme concentration, heavy metal ions, and inhibitors affect the rates of reactions.

Most metabolic pathways are regulated by feedback inhibition.