Enzyme Biology Quiz

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Questions and Answers

What is the general reaction catalyzed by ligases?

$A + B + ATP \longrightarrow A-B + ADP + Pi$ (correct)

$A-B + ADP + Pi \longrightarrow A + B + ATP$

$A-B + ATP \longrightarrow A + B + ADP + Pi$

$A + B + ADP + Pi \longrightarrow A-B + ATP$

Identify the incorrect statement about the Enzyme Commission (EC) system.

The third digit represents the enzyme's specific substrate. (correct)

The first digit of the EC code represents the enzyme's general reaction type.

The fourth digit corresponds to the specific enzyme being categorized.

The EC system utilizes a four-digit code for enzyme categorization.

Which of the following is NOT a characteristic of a coenzyme?

Dialysable

Direct participation in catalysis (correct)

Organic nature

Low molecular weight

Which of the following enzymes is an example of a simple-protein enzyme?

Papain (B) Signup and view all the answers

Which enzyme class would most likely catalyze the reaction $C_6H_{12}O_6 \longrightarrow C_6H_{12}O_6$?

Isomerases (D) Signup and view all the answers

Which of these is an example of a B-complex vitamin coenzyme?

NAD+ (C) Signup and view all the answers

What is the name of the complete functional enzyme, consisting of an apoenzyme and a coenzyme?

Holoenzyme (C) Signup and view all the answers

Which of the following is NOT an example of a reaction catalyzed by an oxidoreductase?

The hydrolysis of a peptide bond (C) Signup and view all the answers

Identify the type of bond that ligases typically catalyze the formation of.

All of the above (D) Signup and view all the answers

Which enzyme class would most likely catalyze the transfer of a phosphate group from ATP to glucose?

Transferases (D) Signup and view all the answers

What is the role of ATP in the reaction catalyzed by ligases?

ATP provides energy for the reaction. (C) Signup and view all the answers

Which enzyme class is characterized by the addition of water across a bond as a mechanism for breaking the substrate?

Hydrolases (A) Signup and view all the answers

Which of the following is NOT a characteristic of lyases?

They require the addition of water molecules. (B) Signup and view all the answers

What is the general reaction equation for an isomerase?

$A \longrightarrow A'$ (D) Signup and view all the answers

Which enzyme class would be involved in the breakdown of proteins into individual amino acids?

Hydrolases (D) Signup and view all the answers

Which enzyme class is NOT defined by the type of reaction it catalyzes?

Ligases (C) Signup and view all the answers

Which of the following is NOT a true statement about cofactors?

Cofactors are always metal ions. (A) Signup and view all the answers

Which of the following enzymes requires a metal ion that is tightly bound to the enzyme and is not dissociated?

Carbonic anhydrase (C) Signup and view all the answers

What is the primary role of S-Adenosylmethionine (SAM) in biochemical reactions?

Donates a methyl group (D) Signup and view all the answers

Which of the following is NOT a characteristic of an enzyme's active site?

The active site is rigid and unchanging. (A) Signup and view all the answers

What is the function of the coenzyme UDP in the synthesis of glycogen?

It carries monosaccharides like glucose and galactose. (C) Signup and view all the answers

What is the significance of the enzyme-substrate complex (ES)?

The ES complex is the intermediate state that facilitates catalysis and product formation. (D) Signup and view all the answers

Which of the following amino acids is most commonly found at the active site of enzymes?

Serine (B) Signup and view all the answers

What is the significance of the active site being flexible?

Flexibility allows for conformational changes to promote binding and catalysis. (B) Signup and view all the answers

Which of the following is NOT a limitation of the lock and key model?

It cannot explain the specificity of enzyme-substrate interactions. (B) Signup and view all the answers

What is the primary difference between the lock and key model and the induced fit theory?

The lock and key model assumes the active site is rigid, while the induced fit theory allows for conformational changes. (A) Signup and view all the answers

Which of the following statements about the induced fit model is TRUE?

The binding of the substrate induces a conformational change in the enzyme's active site. (D) Signup and view all the answers

What is a key advantage of the induced fit theory over the lock and key model?

It can accommodate the presence of allosteric modulators and competitive inhibitors. (D) Signup and view all the answers

What is the name given to enzymes that act within the cells in which they are produced?

Intracellular enzymes (C), Endoenzymes (D) Signup and view all the answers

Which of the following scientists is credited with giving the name “ENZYME” to these catalysts?

Friedrich Wilhelm Kuhne (C) Signup and view all the answers

Enzymes that catalyze the breakdown of carbohydrates are classified as:

Carbohydrases (D) Signup and view all the answers

Which enzyme would likely be involved in the conversion of L-malate to fumarate?

Fumarase (A) Signup and view all the answers

The enzyme named 'Sucrase' acts upon which substrate?

Sucrose (C) Signup and view all the answers

Which type of enzyme is responsible for catalyzing the removal of hydrogen atoms from a molecule?

Dehydrogenases (B) Signup and view all the answers

What is the primary function of a catalyst in a chemical reaction?

To decrease the activation energy of the reaction (B) Signup and view all the answers

Study Notes

Enzymes

Enzymes are biocatalysts synthesized by living cells.
They are proteins, colloidal, and thermolabile.
Enzymes are specific in their actions.
A catalyst increases the velocity or rate of a chemical reaction without being changed itself.
Friedrich Wilhelm Kuhne (1878) coined the term "enzyme" (En = in; Zyme = yeast).
James B. Sumner (1926) isolated and crystallized urease from jack beans, identifying it as a protein.

Enzyme Nomenclature and Classification

Substrate acted upon: Enzymes are named by adding "-ase" to the substrate they act on.
- Carbohydrates — carbohydrases
- Proteins — proteinases
- Lipids — lipases
- Nucleic acids — nucleases
Type of reaction catalyzed: Enzymes are named based on the reaction they catalyze
- Hydrolases - catalyzing hydrolysis
- Isomerases - isomerization
- Oxidases - catalyzing oxidation
- Dehydrogenases - catalyzing dehydrogenation
- Transaminases - catalyzing transamination
- Phosphorylases - catalyzing phosphorylation
Substrate and reaction: Some enzymes are named to indicate both the substrate and the reaction type.
- Succinic dehydrogenase - catalyzes dehydrogenation of succinic acid
- L-glutamic dehydrogenase - catalyzes dehydrogenation of L-glutamic acid
Substance synthesized: A few enzymes are named based on the substance they synthesize.
- Rhodonase - forms rhodonate from hydrocyanic acid and sodium thiosulphate
- Fumarase - forms fumarate from L-malate

Endoenzymes and Exoenzymes

Endoenzymes: Enzymes that act inside the cells where they are produced. Examples include plant enzymes and metabolic enzymes.
Exoenzymes: Enzymes released by living cells that catalyze reactions outside the cell—in their environment. Examples include bacterial enzymes, fungal enzymes, and digestive tract enzymes.

International Union of Biochemistry (IUB) Nomenclature and Classification

The chemical reaction catalyzed is the key to enzyme classification.
In 1961, IUB classified enzymes into 6 major classes.
Each major class is further divided into subclasses (4 to 13).

Enzyme Classes (IUB)

1. Oxidoreductases: Enzymes that catalyze oxidation-reduction reactions between two substrates.
- Substrates often contain CH—OH, C=O, CH-CH, CH-NH2 and CH=NH groups.
- Examples: alcohol dehydrogenase, acyl-CoA dehydrogenase, cytochrome oxidase.
2. Transferases: Enzymes moving a group (other than hydrogen) between two substrates.
- These enzymes catalyze the transfer of one-carbon groups, aldehydic or ketonic residues, and acyl, glycosyl, alkyl, phosphorus, or sulfur-containing groups.
- Examples: acyltransferases, glycosyltransferases, hexokinase.
3. Hydrolases: Enzymes that promote hydrolysis of substrates by adding water across the bond to be split.
- Substrates include esters, glycosyl, ether, peptide, acid-anhydride, C-C, halide, and P-N bonds.
- Examples: glucose-6-phosphatase, pepsin, trypsin, esterases, glycoside hydrolases.
4. Lyases (Desmolases): Enzymes that remove groups from substrates to create double bonds.
- They act on C-C, C-O, C-N, C-S, and C-halide bonds.
- Examples: aldolase, fumarase, histidase.
5. Isomerases: Enzymes converting one isomer to another through intramolecular rearrangements.
- Examples: alanine racemase, retinene isomerase, glucosephosphate isomerase.
6. Ligases (Synthetases): Enzymes attaching two compounds by utilizing the energy from a pyrophosphate bond in ATP or similar compounds.
- They catalyze reactions forming C-O, C-S, C-N, and C-C bonds.
- Examples: acetyl-CoA synthetase, glutamine synthetase.

Chemical Nature of Enzymes

Simple enzymes: Contain only proteins. Examples include urease, amylase, and papain.
Complex enzymes: Include proteins (apoenzyme) and non-protein components (prosthetic group). The combination forms the holoenzyme.
Coenzymes: Organic, low molecular weight, and dialysable components associated with enzyme function. Often regarded as a second substrates or cofactors. Examples include B-complex vitamins.

Cofactors

Cofactors: Inorganic, low molecular weight, and dialysable components associated with enzyme function. Most are metal ions.
- Often associated with the protein (metalloenzymes).
- Examples: ATPase (Mg2+ and Ca2+), Enolase (Mg2+), Alcohol dehydrogenase, carbonic anhydrase, Alkaline phosphatase, carboxypeptidase, and aldolase (contain zinc).

Active Site

The active site is a small region where substrate binds and catalyses.
It exists due to the tertiary structure of the protein and consists of amino acids far apart in the linear sequence.
Active sites are pockets or crevices and are flexible to promote substrate binding.
Active site has binding site and catalytic site, and many enzymes use coenzymes or cofactors here
Common amino acids in active sites include serine, aspartate, histidine, cysteine, lysine, arginine, glutamate, and tyrosine.
Substrate binds to the enzyme to form an enzyme-substrate complex (ES), catalysis happens, and the product is released.

Mode of Enzyme Action

Lock-and-key model: Enzyme active site is rigid and pre-shaped, allowing only a specific substrate to bind. This model is outdated.
Induced-fit model: Enzyme active site changes shape after substrate binding to form stronger substrate binding, which facilitates catalysis. Supported by evidence like X-ray diffraction and understanding allosteric modulators and competitive inhibition.

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Description

Test your knowledge on enzyme classifications, characteristics, and reactions in this comprehensive quiz. Explore topics like ligases, coenzymes, and the Enzyme Commission system. Perfect for students of biochemistry and molecular biology!