AP Biology : Understanding Proteins

Study concepts, example questions & explanations for AP Biology

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Example Questions

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Example Question #1 : Macromolecules

Which of the following statements about enzymes is false?

Possible Answers:

Enzymes are consumed during reactions

Enzymes accelerate the rate of a chemical reaction by lowering the activation energy necessary for the reaction to proceed

Cofactors and coenzymes, such as metals and vitamins, can affect the ability of the enzyme

Competitive inhibitors can block enzymatic activity by binding in the active site

Correct answer:

Enzymes are consumed during reactions

Explanation:

Enzymes are not changed or consumed by the reactions they catalyze, but can be altered by environmental conditions. They work in three-dimensional active sites to bind specific substrates and lower the activation of certain reactions, subsequently increasing the reaction rate. Reaction rate can be further increased when enzymes react with cofactors or coenzymes, but decreased when enzymes are blocked from their specified active sites by competitive inhibitors.

Example Question #2 : Macromolecules

The monomers of all biological macromolecules are combined by which type of bond?

Possible Answers:

Hydrogen bond

London dispersion forces

Ionic bond

Covalent bond

Correct answer:

Covalent bond

Explanation:

Macromolecules, such as proteins, nucleic acids, and polysaccharides, are composed of monomers. Each polymer is made from at least two smaller monomers. Protein monomers are amino acids, nucleic acid monomers are nucleotides, and polysaccharide monomers are monosaccharides. In order to form polymers, the monomers must form covalent bonds with one another.

For proteins, these covalent bonds are peptide bonds, and for saccharides they are glycosidic linkages.

Example Question #1 : Macromolecules

What event would activate a G protein?

Possible Answers:

Phosphorylation of GTP to GDP

Phosphorylation of GDP to GTP

Hydrolysis of GTP to GDP

Replacement of GDP with GTP

Hydrolysis of GDP to GTP

Correct answer:

Replacement of GDP with GTP

Explanation:

G proteins are second messengers involved in cell signaling and propagation or effects within the cell. G protein receptors in the plasma membrane bind to extracellular ligands, causing them to recruit G proteins. Inactive G proteins carry ADP. Once they bind to G protein receptors in the membrane, this GDP molecule is removed, and a GTP molecule is substituted to activate the G protein.

The activated G protein then binds another protein and hydrolyzes GTP to GDP to activate this protein and stimulate cellular effects.

Example Question #2 : Macromolecules

Amino acids are most closely related to which macromolecule?

Possible Answers:

Disaccharide

Polypeptide

Triglyceride

Nucleic acid

Correct answer:

Polypeptide

Explanation:

An amino acid is the monomer unit of the polymer known as a polypeptide. Polypeptide chains form the primary structure of proteins.

A monosaccharide is the simplest unit of a carbohydrate; a monosaccharide dimer is a disaccharide. Triglycerides are a simple form of lipid and nucleic acids are primarily composed of nucleotide monomers.

Example Question #2 : Macromolecules

How is protein primary structure formed?

Possible Answers:

Peptide bonds between the amine group of one amino acid and the carboxylic acid group of another

Hydrogen bonding between the hydrogen of one amino acid and the carboxylic acid of another

Hydrogen bonds between the amine group of one amino acid and the side chain of another

Peptide bonds between side chains (R-groups)

Correct answer:

Peptide bonds between the amine group of one amino acid and the carboxylic acid group of another

Explanation:

Peptide bonds form between the amine group of one amino acid and the carboxylic acid of another via a covalent linkage. The formation of a polypeptide chain from amino acid residues constitutes the protein primary structure.

Secondary structure is formed by hydrogen bonding between the amino and carboxyl backbone units of the polypeptide. Tertiary structure is formed by disulfide covalent bonds, hydrophobic interactions, and R-group hydrogen bonding. Quaternary structure is the joining of multiple polypeptide subunits.

Example Question #3 : Macromolecules

Peptide neurohormones are primarily synthesized in what cellular structure?

Possible Answers:

Rough endoplasmic reticulum

Nucleolus

Cytoplasmic ribosomes

Smooth endoplasmic reticulum

Nucleus

Correct answer:

Rough endoplasmic reticulum

Explanation:

Peptide nuerohormones are synthesized in the rough endoplasmic reticulum of the cell body of the neuron. They are then packaged in the Golgi complex and transported along the axon to the nerve endings. Since peptide hormones must be transported out of the cell in vesicles, they are not likely to be synthesized by cytoplasmic ribosomes. These ribosomes are primarily involved in synthesizing cytoplasmic proteins that do not leave the cell.

The nucleus houses DNA and is the site of transcription. The nucleolus is the site of ribosomal submit synthesis. The smooth endoplasmic reticulum is responsible for certain waste disposal and other functions.

Example Question #1 : Understanding Proteins

The mRNA that encodes the proteins of eukaryotic ribosomes are synthesized in which of the following?

Possible Answers:

Mitochondria

Cytoplasm

Euchromatin

Nucleolus

Cell membrane

Correct answer:

Euchromatin

Explanation:

Euchromatin is the most active part of the genome and is almost always in active transcriptional mode. mRNA is formed via transcription from DNA in the nucleus, in the form of euchromatin, as opposed to heterochromatin. Heterochromatin is characterized by tight winding of DNA around histones, such that it cannot easily be accessed by RNA polymerase and transcription proteins. Most DNA resembles euchromatin during interphase, when transcription is most active, and condenses to heterochromatin during mitosis.

Note that assembly of eukaryotic ribosomal subunits takes place in the nucleolus, but mRNA is always transcribed directly from DNA within the greater nuclear structure.

Example Question #42 : Biochemical Concepts

Which of the following structures remains constant when a protein is in its denatured form?

Possible Answers:

Secondary structure

Quaternary structure

Primary structure

Tertiary structure

Correct answer:

Primary structure

Explanation:

Denaturation of a protein involves the breakdown of noncovalent bonds between amino acid residues. The formation of noncovalent bonds, such as hydrogen bonding and van der Waals forces, lead to higher order structures such as secondary, tertiary, and quaternary structure. Upon denaturation these noncovalent bonds in the protein chain are broken and the protein reverts back to its primary structure. The primary structure of a protein consists of the amino acid sequence joined together by peptide bonds (covalent bond). Covalent bonds are much stronger and more permanent that hydrogen bonds and other intermolecular forces, and can endure denaturation. Environmental conditions such as temperature and pH contribute to denaturation of a protein.

Example Question #4 : Understanding Proteins

If a solution has a pH of 13, a nonpolar amino acid in solution will contain which of the following?

Possible Answers:

A deprotonated carboxylic acid and an overall charge of

A protonated carboxylic acid and an overall charge of

A protonated amine and an overall charge of

A deprotonated amine and an overall charge of

Correct answer:

A deprotonated carboxylic acid and an overall charge of

Explanation:

Recall that all amino acids have a carboxylic acid, amino group, hydrogen, and a functional group attached to their central carbon. In nonpolar amino acids the functional group will not contain any groups capable of protonation or deprotonation. The changes due to the pH in a nonpolar amino acid will only involve the amino and carboxylic acid groups.

A solution with pH of 13 is very basic. The carboxylic acid will be deprotonated and the amine will be intact (neither protonated nor deprotonated). The deprotonated carboxylic acid will give an overall charge of  to the molecule.

Example Question #3 : Macromolecules

A reaction between an alpha-carboxylic acid and an alpha-amino group creates a peptide bond. Which of the following describes this process?

Possible Answers:

Hydrogenation

Dehydration synthesis

Hydrolysis

Esterification

Correct answer:

Dehydration synthesis

Explanation:

Hydrolysis reactions involve breakdown of molecules (lysis) in the presence of water. Water is a reactant in hydrolysis reactions. Dehydration synthesis reactions involve formation of bonds between molecules (synthesis) and removal of water at the end of the reaction (dehydration). Water is a product in dehydration synthesis reactions.

During the formation of peptide bonds a hydroxyl group from carboxylic acid and a hydrogen atom from the amino group are released and form water. Formation of peptide bonds is a dehydration synthesis reaction because bonds are synthesized and water is released.

Formation and destruction of bonds within macromolecules always involve a hydrolysis or a dehydration synthesis reaction. Esterification and hydrogenation reaction refer to other organic chemistry processes.

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