Enzymes and Their Classification

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Questions and Answers

What is the turnover number in enzyme-catalyzed reactions defined as?

The speed at which substrates bind to the enzyme

The number of catalyzed reactions taking place per minute

The number of enzymes present in the reaction

The number of substrate molecules converted to product per enzyme molecule per second (correct)

Which model describes the active site's role in enzyme activity?

Complex molecular machine facilitating substrate conversion (correct)

Simple binding site for maximum turnover

Passive receptor for substrates

A static entity that requires substrates to fit perfectly

What describes the conversion of zymogens into active enzymes?

It is a process that increases enzyme specificity.

It occurs through reversible binding of substrates.

It is influenced by external environmental factors.

It involves the cleavage of peptide bonds. (correct)

Which class of enzymes is primarily responsible for catalyzing oxidation-reduction reactions?

Oxidoreductases Signup and view all the answers

What happens to reaction velocity as substrate concentration increases, until a certain point?

It increases until a maximal velocity is reached. Signup and view all the answers

What term refers to the complete form of an enzyme including its nonprotein components?

Holoenzyme Signup and view all the answers

What is the optimum temperature range for most human enzymes?

35°C to 40°C Signup and view all the answers

Which enzyme class includes enzymes that catalyze the transfer of a phosphoryl group?

Transferases Signup and view all the answers

Which of the following statements best describes the specificity of enzymes?

Enzymes are highly specific and usually catalyze only one type of reaction Signup and view all the answers

What term describes the energy barrier that separates reactants from products?

Activation energy Signup and view all the answers

What is the relationship between enzyme-catalyzed reactions and uncatalyzed reactions regarding speed?

Enzyme-catalyzed reactions are faster, often by a factor of 10³ to 10⁸ Signup and view all the answers

Which enzyme is an example of an oxidoreductase?

Lactate dehydrogenase Signup and view all the answers

Which factor does NOT typically affect enzyme activity?

Time of day Signup and view all the answers

Which molecule acts as a cofactor when it is a metal ion for enzyme activity?

Zinc ion (Zn2+) Signup and view all the answers

What type of reaction do hydrolases catalyze?

Hydrolysis Signup and view all the answers

How does temperature affect reaction velocity until optimal conditions are met?

It increases until peak velocity is reached. Signup and view all the answers

Which of the following factors primarily affects the velocity of enzyme-catalyzed reactions?

The concentration of substrates and products Signup and view all the answers

Which type of enzymes is responsible for catalyzing structural rearrangements within a molecule?

Isomerases Signup and view all the answers

Which enzyme splits pyruvate into acetaldehyde and carbon dioxide?

Pyruvate decarboxylase Signup and view all the answers

What distinguishes an apoenzyme from a holoenzyme?

An apoenzyme lacks its nonprotein component and is inactive Signup and view all the answers

What aspect of enzyme specificity ensures that enzymes catalyze only specific reactions?

Active site shape and chemical environment Signup and view all the answers

What defines maximal velocity (Vmax) in an enzyme-catalyzed reaction?

The point where all enzyme active sites are fully occupied. Signup and view all the answers

What is the systematic name of the enzyme that catalyzes the conversion of L-alanine to 2-oxoglutarate?

L-alanine:2-oxyglutarate aminotransferase Signup and view all the answers

Which of the following is not a common name for oxidoreductases?

Hydrolases Signup and view all the answers

Study Notes

Enzymes (1)

Enzymes are protein catalysts that speed up chemical reactions without being consumed.

Enzyme Classification

Most enzymes are named by adding the suffix "-ase" to the name of their substrate or a descriptive term for the reaction they catalyze.
Urease has urea as a substrate.
Alcohol dehydrogenase catalyzes the removal of hydrogen from alcohols.
International Union of Biochemistry and Molecular Biology (IUBMB) categorizes enzymes according to the general class of organic chemical reaction they catalyze.
The IUBMB scheme assigns a unique EC number to each enzyme.

Six Classes of Enzymes

Oxidoreductases: Catalyze oxidation-reduction reactions. Commonly called dehydrogenases, oxidases, peroxidases, or oxygenases, reductases.
- Lactate dehydrogenase is an example (EC 1.1.1.27) converting L-Lactate to pyruvate.
Transferases: Catalyze group transfer reactions. Some require coenzymes.
- Kinases transfer a phosphoryl group from ATP.
- Alanine transaminase (EC 2.6.1.2) transfers an amino group from L-alanine to alpha-ketoglutarate.
Hydrolases: Catalyze hydrolysis reactions.
- Pyrophosphatase (EC 3.6.1.1) is an example.
Lyases: Catalyze the cleavage of C-C, C-S, and C-N bonds, resulting in the formation of double bonds.
- Pyruvate decarboxylase (EC 4.1.1.1) splits pyruvate into acetaldehyde and carbon dioxide.
Isomerases: Catalyze structural changes within a single molecule (isomerization reactions).
- Alanine racemase (EC 5.1.1.1) converts L-alanine to D-alanine.
Ligases: Catalyze the joining of two substrates, requiring the input of chemical potential energy (e.g., from ATP). Often referred to as synthetases.
- Glutamine synthetase (EC 6.3.1.2) joins glutamate and ammonia to produce glutamine.

Holoenzymes

Some enzymes require molecules (other than proteins) for activity, called coenzymes or cofactors.
The active enzyme with its nonprotein component is a holoenzyme.
Apoenzyme refers to the enzyme without its non-protein component, which is inactive.
Metal ion cofactors (e.g., Zn2+, Fe2+)
Small organic molecules (coenzymes).

Vitamins as Coenzymes

Several B vitamins act as coenzymes in enzymatic reactions.

Active Sites

Active sites are complex and not passive. They employ diverse chemical mechanisms to facilitate substrate conversion to product.

Catalytic Efficiency

Enzyme-catalyzed reactions proceed much faster than uncatalyzed reactions (10³ to 10⁸ times faster).
Turnover number (k) represents the number of substrate molecules converted to product per enzyme molecule per second. Typically 10² to 10⁴ s⁻¹.

Enzyme Specificity

Enzymes are highly specific, interacting with one or a few substrates and catalyzing only one type of reaction.

Enzyme Regulation

Enzyme activity can be regulated (increased or decreased) to meet cellular needs.

Proenzymes/Zymogens

Some enzymes are produced in an inactive form, called proenzymes or zymogens.
Zymogens are converted into active forms through the cleavage of peptide bonds.
Chimotrypsinogen is an example.

Multienzyme Systems

Several enzymes can work together in a pathway.
Substrates are passed along to different enzymes in the pathway.

Energy Changes in Reactions

Chemical reactions have an energy barrier (activation energy) that enzymes lower.
This allows reactions to occur faster.

Factors Affecting Enzyme Reaction Rates

Substrate Concentration: Increasing substrate concentration increases reaction rate until saturation is reached.
Temperature: Reaction velocity increases with temperature up to a point. Beyond that, the enzymes can denature and lose activity. Optimal temperature varies by enzyme.
pH: The pH affects the ionization of active site groups, which affects enzyme function. Optimal pH varies by enzyme.

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Description

This quiz covers the basics of enzymes, including their role as protein catalysts, naming conventions, and classification according to the IUBMB. Explore the six classes of enzymes and their specific functions in biochemical reactions.