• Most are proteins
• Primary purpose is to increase the rate of a chemical reaction
• Most reactions occur a million times faster (or more) with an enzyme
• Many can be isolated from cells and studied in a test tube
• They are highly specific for certain substrates (reactants)

Summary: Important Statements About Enzymes

• Most are named by adding the suffix “ase” to the root name of the substrate molecule it is acting upon (Hence: lipase, peptidase, sucrase)
• They occur in small amounts in the cell
• They are not consumed or degraded in a chemical reaction
• They often function within metabolic pathways and are regulated to meet the needs of the cell
• Even if one enzyme is non-functional, the results can be disastrous to the cell and the organism

Summary: Important Statements About Enzymes, cont’d

• Most are named by adding the suffix “ase” to the root name of the substrate molecule it is acting upon (Hence: lipase, peptidase, sucrase)
• They occur in small amounts in the cell
• They are not consumed or degraded in a chemical reaction
• They often function within metabolic pathways and are regulated to meet the needs of the cell
• Even if one enzyme is non-functional, the results can be disastrous to the cell and the organism
• They don’t affect the thermodynamics of a reaction, they only affect the rate of the reaction.

Summary: Important Statements About Enzymes, cont’d

Figure 3-8 Essential Cell Biology (© Garland Science 2010)

First Law of Thermodynamics

Second Law of Thermodynamics

Second Law of Thermodynamics

Spontaneous reaction

Requires energy input – not spontaneous

Figure 3-5 Essential Cell Biology (© Garland Science 2010)

Anabolic reactions are those that synthesize compounds. Energy is required for these reactions.

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Reactions that break down molecules are called catabolic reactions. Energy is released when molecules are broken down.

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ATP produced by catabolic reactions provides the energy for anabolic reactions. Anabolic and catabolic reactions are therefore coupled (they require each other) through the use of ATP.

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A sugar molecule has higher free energy (and more order) than CO2 and H2O

• The energy in one glucose molecule is used to produce 36 ATP.

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• ATP has approximately the right amount of energy for most cellular reactions.

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• ATP is produced and used continuously.

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• The entire amount of ATP in an organism is recycled once per minute.

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• Most cells maintain only a few seconds supply of ATP.

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Reactions with enzymes are up to 10 billion times faster than those without enzymes.

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Enzymes typically react with between 1 and 10,000 molecules per second.

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Fast enzymes catalyze up to 500,000 molecules per second.

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Substrate concentration, enzyme concentration, Temperature, and pH affect the rate of enzyme reactions.

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• At lower concentrations, the active sites on most of the enzyme molecules are not filled because there is not much substrate.
• Higher concentrations cause more collisions between the molecules. With more molecules and collisions, enzymes are more likely to encounter molecules of reactant.
• The maximum velocity of a reaction is reached when the active sites are almost continuously filled. Increased substrate concentration after this point will not increase the rate. Reaction rate therefore increases as substrate concentration is increased but it levels off.

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• Higher temperature generally causes more collisions among the molecules and therefore increases the rate of a reaction. More collisions increase the likelihood that substrate will collide with the active site of the enzyme, thus increasing the rate of an enzyme-catalyzed reaction.
• Above a certain temperature, activity begins to decline because the enzyme begins to denature.
• The rate of chemical reactions therefore increases with temperature but then decreases.

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• Each enzyme has an optimal pH.
• A change in pH can alter the ionization of the R groups of the amino acids. When the charges on the amino acids change, hydrogen bonding within the protein molecule change and the molecule changes shape. The new shape may not be effective.
• The diagram below shows that pepsin functions best in an acid environment. This makes sense because pepsin is an enzyme that is normally found in the stomach where the pH is low due to the presence of hydrochloric acid. Trypsin is found in the duodenum, and therefore, its optimum pH is in the neutral range to match the pH of the duodenum.

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Noncompetitive Inhibition

• Another form of inhibition involves an inhibitor that binds to an allosteric site of an enzyme. An allosteric site is a different location than the active site.
• The binding of an inhibitor to the allosteric site alters the shape of the enzyme, resulting in a distorted active site that does not function properly.
• The binding of an inhibitor to an allosteric site is usually temporary.

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Competitive Inhibition

In competitive inhibition, a similar-shaped molecule competes with the substrate for active sites.

Many enzymatic pathways are regulated by feedback inhibition. As an enzyme's product accumulates, it turns off the enzyme just as heat causes a thermostat to turn off the production of heat.

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The end product of the pathway binds to an allosteric site on the first enzyme in the pathway and shuts down the entire sequence.

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How would you regulate this pathway?

How would you regulate this pathway?