6.4-6.5: enzymes, metabolism, and enzyme regulation

What are enzymes?

Proteins or RNA molecules that act as catalysts.

______, or reactants, are molecules that bind to an enzyme and are converted to products in chemical reactions.

substrates

Which is true about enzymes?

• They are altered by the chemical reaction and can only be used once.

• They lower the activation energy of a reaction by binding to the substrates.

• They are consumed in the chemical reaction and can only be used once.

• They raise the activation energy of a reaction by binding to the substrates.

They lower the activation energy of a reaction by binding to the substrates.

Select the true statements about cellular enzymes.

• Each enzyme in a cell catalyzes one or a few specific reactions.

• Each enzyme in a cell can catalyze many (tens to hundreds) different reactions.

• All cells in an organism contain the same, full set of enzymes.

• Different cells contain different sets of enzymes.

• Each enzyme in a cell catalyzes one or a few specific reactions.

• Different cells contain different sets of enzymes.

What describes the active site of an enzyme?

It is the part of the enzyme where the substrate fits.

_______are proteins or RNA molecules that act as catalysts to speed up reactions in living organisms.

enzymes

How many different kinds of enzymes are known?

Thousands

Enzyme-mediated chemical reactions take place when the substrate molecules bind to the ______ _______ of an enzyme.

active site

When a reactant binds to the active site of an enzyme, a(n) _____-_______ complex forms

enzyme substrate

order the steps in enzyme catalysis

1. substrate binds to enzyme

2. amino acids from enzyme interact with substrate

3. bonds in substrate(s) are broken and/or created; products form

4. products dissociate from the enzyme

Which of the following is the best description of the fit between an enzyme and its substrate?

• The enzyme is rigid and its substrate must fit very well, like a lock and key.

• Because the enzyme and its substrate fit very well, only a single substrate can be bound to an enzyme at any given time.

• The active site of the enzyme is permanently changed once it encounters the substrate for the first time.

• As the substrate binds to the enzyme, the shape of the active site changes to accommodate the reaction.

As the substrate binds to the enzyme, the shape of the active site changes to accommodate the reaction.

The bacterial enzyme pyruvate dehydrogenase contains multiple copies of each of three enzymes for a total of 60 protein subunits. This is an example of _____

a multienzyme complex

Select the ways in which multienzyme complexes improve catalytic efficiency.

• Because products do not leave the complex, there are no unwanted side reactions.

• All of the reactions inside the complex can be controlled as one unit.

• The concentration of substances inside the multienzyme complex can be raised several-fold compared to cytoplasm.

• The product of one reaction can be delivered to the next enzyme more easily.

• Because products do not leave the complex, there are no unwanted side reactions.

• All of the reactions inside the complex can be controlled as one unit.

• The product of one reaction can be delivered to the next enzyme more easily.

Select the true statements about ribozymes.

• Ribozymes serve as structural framework molecules with no catalytic function.

• Ribozymes can greatly accelerate the rate of a reaction.

• Some ribozymes catalyze reactions on themselves.

• Ribozymes have much lower substrate specificity than protein enzymes

• Ribozymes can greatly accelerate the rate of a reaction.

• Some ribozymes catalyze reactions on themselves.

Select the events that must occur before bonds in the substrate are broken during an enzyme catalyzed reaction.

• An enzyme-product complex must be formed.

• The substrate-enzyme complex must be broken.

• The substrate must bind the enzyme.

• Amino acids in the enzyme must interact with the substrate.

• A substrate-enzyme complex must form.

• The substrate must bind the enzyme.

• A substrate-enzyme complex must form.

• Amino acids in the enzyme must interact with the substrate.

Temperature affects chemical reactions in the following way: increasing the temperature of an uncatalyzed reaction ______ the rate of that reaction

increases

The binding of a substrate usually produces a slight change in the enzyme's shape resulting in a better enzyme-substrate fit; this is called ______ fit

induced

When several enzymes that catalyze different steps of a sequence of reactions are associated with one another in a noncovalently bonded assembly, they form a(n) _______ complex

multienzyme

A more efficient way to catalyze a sequence of related biochemical reactions is to combine several proteins into a ______ complex; in this way, the product of each reaction can be delivered to the next enzyme, without being released to diffuse away.

multienzyme

Which of the following statements, that describe catalysis of protein synthesis in ribosomes, are true?

• Researchers now know that ribosomal RNA is actually a type of catalytic protein.

• Ribosomal proteins are the key catalysts in a ribosome, and ribosomal RNA provides the structural framework that orients the catalytic proteins.

• Ribosomal RNA functions as a catalyst.

• Ribosomes are essentially large ribozymes.

• Ribosomal RNA functions as a catalyst.

• Ribosomes are essentially large ribozymes.

Most enzymes only function in a narrow range of temperatures and have an optimum temperature at which they work best. Why?

Lower temperatures prevent the formation of the substrate-enzyme complex while higher temperatures can denature the enzyme.

Select the true statements about the effect of temperature on an uncatalyzed chemical reaction.

• Increased temperature raises the activation energy of the reaction

• Increased temperature decreases the rate of the reaction.

• Increased temperature increases random molecular movements which increases stress on bonds.

• Increased temperature increases the rate of the reaction

• Increased temperature increases random molecular movements which increases stress on bonds.

• Increased temperature increases the rate of the reaction.

Changes in pH affect an enzyme because the interaction of charged amino acid residues in the enzyme are affected by ______

hydrogen ion concentrations

The optimum pH of most enzymes is:

6 to 8

Select the ways in which multienzyme complexes improve catalytic efficiency.

• All of the reactions inside the complex can be controlled as one unit.

• The product of one reaction can be delivered to the next enzyme more easily.

• Because products do not leave the complex, there are no unwanted side reactions.

• The concentration of substances inside the multienzyme complex can be raised several-fold compared to cytoplasm.

• All of the reactions inside the complex can be controlled as one unit.

• The product of one reaction can be delivered to the next enzyme more easily.

• Because products do not leave the complex, there are no unwanted side reactions.

Increasing the temperature outside the optimal range for an enzyme can lead to ________ of the enzyme, altering the enzyme's structure and function.

denaturation

The stomach enzyme pepsin functions well at pH values that are very ______

low/acidic

Most enzymes in the human body work best at neutral pH. Select the reasons why changing the pH of the fluid in which a human enzyme is dissolved (for example to a pH of 3) will affect the enzyme's ability to function.

• The balance between positively and negatively charged amino acids in the enzyme is shifted.

• Enzymes cannot maintain their three-dimensional shape at extreme pHs, such as a pH of 3.

• The enzyme loses its side chains at an extreme pH, such as a pH of 3.

• The enzyme is dissolved into amino acids.

• The balance between positively and negatively charged amino acids in the enzyme is shifted.

• Enzymes cannot maintain their three-dimensional shape at extreme pHs, such as a pH of 3.

What happens if a chemical substance binds to an enzyme and alters its shape?

The enzyme's activity can be increased or decreased.

A substance that binds to an enzyme and decreases its activity is an _____

inhibitor

How can some enzymes function well at a very low pH?

They maintain their three dimensional shape even at a very low pH.

a ______ inhibitor binds to an area outside of the active site of an enzyme and impairs proper functioning of the enzyme.

noncompetitive

What are competitive inhibitors?

Competitive inhibitors bind to the active site of an enzyme and prevent substrates from binding.

What describes the activity of a particular enzyme?

It can be affected by substances which bind the enzyme and alter its shape.

Enzyme inhibitors _______

bind to an enzyme and decrease its activity

An allosteric enzyme is an enzyme that ______

can exist in either an active or an inactive conformation

______ inhibitors bind to the active site of an enzyme and prevent the substrate from binding.

competitive

A molecule that binds to a site on the enzyme other than the active site and inhibits the enzyme's activity is a(n) ______ inhibitor

allosteric

Select the true statements about allosteric sites on enzymes.

• This is where a non-competitive inhibitor would bind.

• The binding of a substance to the allosteric site can switch an enzyme between its active and inactive configurations.

• This is where a competitive inhibitor would bind.

• This is where a substrate would bind.

• This is where a non-competitive inhibitor would bind.

• The binding of a substance to the allosteric site can switch an enzyme between its active and inactive configurations.

______ are small chemical components, usually inorganic ions, such as Zn²⁺, Cu²⁺, and Mn²⁺, that are often found in the active site of an enzyme, participating directly in catalysis.

Cofactors

______ enzymes are enzymes that can exist in either an active or inactive conformation.

allosteric

Which of the following is an accurate description of a coenzyme?

• Organic molecules that temporarily attach to an enzyme and promote a chemical reaction without being changed during the reaction

• Small inorganic ions that temporarily attach to the outside of an enzyme and promote a chemical reaction

• Small molecules permanently attached to the outside of an enzyme that aid in catalysis

Organic molecules that temporarily attach to an enzyme and promote a chemical reaction without being changed during the reaction

An allosteric ________ binds to a site in the enzyme other than the active site and by doing so it keeps the enzyme in its active configuration.

activator

the _____ site is a specific region of an enzyme to which non-competitive inhibitors bind in order to regulate enzyme activity.

allosteric

What organic molecules can accept electrons from the active site of one enzyme and then transfer them to another enzyme where the electrons are released to a substrate in a different reaction? (small organic molecules that temporarily attach to an enzyme and promote the chemical reaction catalyzed by the enzyme.)

Coenzymes

What term refers to small chemicals, usually inorganic ions, that temporarily attach to the surface of an enzyme and promote a chemical reaction?

Cofactors

How does a noncompetitive enzyme inhibitor work?

It binds to a site other than the active site and prevents the proper functioning of the enzyme.

In many oxidation-reduction reactions, electrons pass in pairs from the active site of the enzyme to a(n) _______ that serves as an electron acceptor, which can then release the electrons to the substrates of another reaction.

coenzyme

What describes metabolism?

Metabolism refers to the sum of all chemical reactions that occur within an organism.

What is the general role of anabolic reactions in the metabolism of a cell?

Anabolic reactions are involved in the synthesis of larger molecules from smaller precursor molecules.

Cells harvest energy during _____ reactions and expend energy during ____ reactions.

catabolic; anabolic

Which of the following are included in metabolism?

• Only the breakdown of organic molecules

• Only the synthesis of organic molecules

• Both the synthesis and breakdown of organic molecules

• Neither the synthesis nor the breakdown of organic molecules

Both the synthesis and breakdown of organic molecules

Each ______ in a biochemical pathway acts on the products of the previous reaction.

enzyme

What type of reactions are involved in the synthesis of larger molecules from smaller precursor molecules inside of living cells?

anabolic

Select reasons why metabolic pathways are regulated.

• By regulating metabolic pathways, cells waste less energy.

• By regulating metabolic pathways, cells conserve raw materials.

• By regulating metabolic pathways, cells can produce compounds in excess.

• Regulation allows cells to operate efficiently.

• By regulating metabolic pathways, cells waste less energy.

• By regulating metabolic pathways, cells conserve raw materials.

• Regulation allows cells to operate efficiently.

What correctly describes the differences between anabolic and catabolic reactions.

• Catabolic reactions release energy, while anabolic reaction need energy input.

• In catabolic reactions large molecules are broken down, while in anabolic reactions large molecules are made from smaller ones.

In _____ inhibition, the end product of a pathway affects the first enzyme in the series to shut down the pathway.

feedback

a ______ _______ is a sequence of reactions occurring in an individual cell that are organized so that the product of one reaction becomes the substrate of the next.

metabolic pathway

In order to function efficiently and to conserve energy and raw materials, a cell _____

maintains biochemical pathways active only when their products are needed

Which of the following acts as an inhibitor of the first enzyme reaction in a metabolic pathway during feedback inhibition?

• final substrate

• first substrate

• first product

• final product

the final product