Question 1

Matching

-In SDS-PAGE,disulfide-linked polypeptides can be separated after reacting the protein first with ______.

Accepted Answer

Disulfide bonds are strong covalent bonds between two cysteine residues that can cause polypeptides to remain bound together during SDS-PAGE, hindering their separation by size. Treatment of proteins with reducing agents, such as 2-mercaptoethanol, breaks these disulfide bonds and reduces the protein to its monomeric form, allowing it to be separated by size during electrophoresis.

Question 2

Since there are 20 standard amino acids,the number of possible linear polypeptides of length N can be expressed as:

Accepted Answer

The number of possible linear polypeptides of length N, given 20 standard amino acids, is calculated as 20^n. This is because for each position in the polypeptide chain, there are 20 possible amino acids that can be placed, and this choice is independent for each of the N positions.

Question 3

Which of the following has the most dramatic influence on the characteristics of an individual protein?

Accepted Answer

The amino-acid sequence determines the primary structure of a protein, which ultimately determines its secondary, tertiary, and quaternary structures as well as its function. The amino-acid composition, location of its encoding gene within the genome, stereochemistry at the F1F1F1S1F1F1F10-carbon, and sequence of tRNA molecules involved in its translation can also affect the protein, but not as dramatically as the amino-acid sequence.

Question 4

Proteins are synthesized in vivo by the translation of

Accepted Answer

Proteins are synthesized in vivo by the translation of mRNA, which carries the genetic information from DNA to the ribosome where it is used to synthesize a specific protein. cDNA is a synthesized copy of mRNA, but it is not involved in protein synthesis. tRNA and rRNA are also involved in protein synthesis, but they do not carry the genetic information from DNA to the ribosome like mRNA does. Exons are part of DNA but are not directly involved in the translation of proteins.

Question 5

Matching

-In order for DEAE to act as an anion exchanger,it must have a ______.

Accepted Answer

DEAE (Diethylaminoethyl) acts as an anion exchanger by attracting and binding negatively charged ions (anions), which it can do because it carries a positive charge.

Question 6

Matching

-One of the reasons the primary structure is important for a protein is that it determines the ______ the molecule adopts in aqueous solutions.

Accepted Answer

The primary structure, or sequence of amino acids, determines the folding and shape of the protein in aqueous solutions.

Answer: N
 The pI, or isoelectric point, is the pH at which a protein has no net charge and is an important factor in determining its solubility and behavior in solution.

Answer: Q
 Sodium dodecyl sulfate (SDS) is a detergent that is commonly used in protein electrophoresis to denature proteins and coat them with a uniform negative charge, allowing for separation and analysis based on size.

Question 7

Matching

-In general,proteins are least soluble in water when the pH is close to the ______.

Accepted Answer

The isoelectric point (pI) of a protein is the pH at which the protein has a net charge of zero. This is the pH at which there is equal numbers of positively and negatively charged amino acid residues. Proteins are least soluble in water when the pH is close to their pI because they are not charged and cannot interact favorably with the charged molecules of water.

Question 8

Matching

-To help prevent denaturation of proteins in solution,steps are taken to avoid _________ and adsorption to surfaces.

Accepted Answer

Foaming can introduce air bubbles and increase the surface area for proteins to adsorb, leading to denaturation.

Question 9

Matching

-The endoprotease ______ cleaves polypeptides on the C-terminal side of certain bulky hydrophobic amino acid residues.

Accepted Answer

Chymotrypsin is a type of endoprotease that cleaves polypeptides on the C-terminal side of certain bulky hydrophobic amino acid residues, such as phenylalanine, tryptophan, and tyrosine.

Question 10

The vast majority of polypeptides contain between ______ amino acid residues.

Accepted Answer

The vast majority of polypeptides contain between 100 and 1000 amino acid residues. While there are exceptions to this, such as insulin which consists of only 51 amino acids, the majority of proteins fall within this size range.

Question 11

Matching

-Molecules that contain a(n)______ are capable of absorbing light.

Accepted Answer

Molecules that contain a chromophore are responsible for absorption and emission of light.

Answer: N 
 pI (isoelectric point) is the pH at which a molecule has no net charge and can be used for separating proteins based on their charge.

Answer: O 
 Chymotrypsin is a protease enzyme that cleaves peptide bonds and is commonly used in protein sequencing.

Answer: Q 
 Sodium dodecyl sulfate (SDS) is a detergent commonly used in protein electrophoresis to denature proteins and give them a negative charge, allowing them to be separated by size.

Question 12

Matching

-Either dansyl chloride or Edman's reagent can be used to identify the ______ of a protein.

Accepted Answer

Dansyl chloride and Edman's reagent are both used for N-terminal sequencing of a protein, identifying the first (N-terminal) amino acid of the peptide chain.

Question 13

The salting in of proteins can be explained by:

Accepted Answer

Salt counter-ions can reduce the electrostatic attractions between protein molecules, thus leading to the salting in of proteins.

Question 14

Natural proteins most commonly contain linear polypeptides between 100 and 1000 residues in length.One of the reasons polypeptides outside this range may be disfavored is that

Accepted Answer

Smaller polypeptides may not have enough residues to form the stable folded structures needed for protein function. Larger polypeptides are not disfavored due to insolubility or inability to be synthesized by ribosomes, but rather due to the difficulty in maintaining precise folding and stability over a long chain. Amide linkages are strong enough to keep larger polypeptides intact. Prion-like aggregates are a potential issue for any polypeptide, regardless of size.

Question 15

A fast and common method for determining the protein concentration in column effluent is

Accepted Answer

Measuring the absorbance of light at 280 nm is a common and fast method for determining protein concentration in column effluent. This method takes advantage of the fact that proteins absorb light at this wavelength due to the presence of aromatic amino acids such as tryptophan and tyrosine.

Question 16

Matching

-In ______ chromatography,a protein mixture must be applied to the column at a low pH so that the proteins will have a net positive charge and bind to the column.

Accepted Answer

Cation exchange chromatography works by using a column with an anionic resin. At low pH, proteins will have a net positive charge and will bind to the column. As the pH is raised, the proteins will be eluted off the column based on their pI values.

Question 17

Matching

-______ chromatography is a method of fractionating a protein mixture according to differences in polarity.

Accepted Answer

Hydrophobic interaction chromatography separates proteins based on their hydrophobicity, which is related to their polarity. Proteins with more hydrophobic surfaces will interact more strongly with the hydrophobic stationary phase and thus can be fractionated according to their differences in polarity.

Question 18

Matching

-If antibodies to the protein being assayed are available,a(n)______ can be carried out.

Accepted Answer

Enzyme-linked immunosorbent assay (ELISA) uses antibodies to specifically detect and quantify antigens or proteins of interest in a sample.

Answer: N 
 The isoelectric point (pI) is the pH at which a protein has no net charge and can be used to separate proteins based on their charge using techniques such as isoelectric focusing.

Answer: Q 
 Sodium dodecyl sulfate (SDS) is a detergent that can denature proteins and coat them with a negative charge, allowing for separation by size using techniques such as SDS-PAGE.

Question 19

Matching

-If the cDNA for a protein has been cloned,it may be possible to obtain large quantities of the protein by _________________ in bacteria.

Accepted Answer

High level expression in bacteria is the most efficient method for obtaining large quantities of a cloned protein.

Question 20

Which statement about insulin is correct?

Accepted Answer

All of the statements are correct. Insulin is composed of two polypeptides, the A chain and the B chain, which are connected by interchain disulfide bonds. The A chain contains an intrachain disulfide bond. The A chain and the B chain are encoded by separate genes, but they are transcribed as a single mRNA molecule and are then cleaved during translation.

Quiz 5: Proteins: Primary Structure

Matching -In SDS-PAGE,disulfide-linked polypeptides can be separated after reacting the protein first with ______.

Since there are 20 standard amino acids,the number of possible linear polypeptides of length N can be expressed as:

Which of the following has the most dramatic influence on the characteristics of an individual protein?

Proteins are synthesized in vivo by the translation of

Matching -In order for DEAE to act as an anion exchanger,it must have a ______.

Matching -One of the reasons the primary structure is important for a protein is that it determines the ______ the molecule adopts in aqueous solutions.

Matching -In general,proteins are least soluble in water when the pH is close to the ______.

Matching -To help prevent denaturation of proteins in solution,steps are taken to avoid _________ and adsorption to surfaces.

Matching -The endoprotease ______ cleaves polypeptides on the C-terminal side of certain bulky hydrophobic amino acid residues.

The vast majority of polypeptides contain between ______ amino acid residues.

Matching -Molecules that contain a(n) ______ are capable of absorbing light.

Matching -Either dansyl chloride or Edman's reagent can be used to identify the ______ of a protein.

The salting in of proteins can be explained by:

Natural proteins most commonly contain linear polypeptides between 100 and 1000 residues in length.One of the reasons polypeptides outside this range may be disfavored is that

A fast and common method for determining the protein concentration in column effluent is

Matching -In ______ chromatography,a protein mixture must be applied to the column at a low pH so that the proteins will have a net positive charge and bind to the column.

Matching -______ chromatography is a method of fractionating a protein mixture according to differences in polarity.

Matching -If antibodies to the protein being assayed are available,a(n) ______ can be carried out.

Matching -If the cDNA for a protein has been cloned,it may be possible to obtain large quantities of the protein by _________________ in bacteria.

Which statement about insulin is correct?