The chirality of an amino acid results from the fact that its α carbon:
A) has no net charge.
B) is a carboxylic acid.
C) is bonded to four different
... [Show More] chemical groups.
D) is in the L absolute configuration in naturally occurring proteins.
E) is symmetric.
C) is bonded to four different chemical groups.
Of the 20 standard amino acids, only ___________ is not optically active. The reason is that its side chain ___________.
A) alanine; is a simple methyl group
B) glycine; is a hydrogen atom
C) glycine; is unbranched
D) lysine; contains only nitrogen
E) proline; forms a covalent bond with the amino group
B) glycine; is a hydrogen atom
Two amino acids of the standard 20 contain sulfur atoms. They are:
A) cysteine and serine.
B) cysteine and threonine.
C) methionine and cysteine
D) methionine and serine
E) threonine and serine.
C) methionine and cysteine
All of the amino acids that are found in proteins, except for proline, contain a(n):
A) amino group.
B) carbonyl group.
C) carboxyl group.
D) ester group.
E) thiol group.
A) amino group.
Which of the following statements about aromatic amino acids is correct?
A) All are strongly hydrophilic.
B) Histidine's ring structure results in its being categorized as aromatic or basic, depending on pH.
C) On a molar basis, tryptophan absorbs more ultraviolet light than tyrosine.
D) The major contribution to the characteristic absorption of light at 280 nm by proteins is the phenylalanine R group.
E) The presence of a ring structure in its R group determines whether or not an amino acid is aromatic.
C) On a molar basis, tryptophan absorbs more ultraviolet light than tyrosine.
Which of the following statements about cystine is correct?
A) Cystine forms when the —CH2—SH R group is oxidized to form a —CH2—S—S—CH2— disulfide bridge between two cysteines.
B) Cystine is an example of a nonstandard amino acid, derived by linking two standard amino acids.
C) Cystine is formed by the oxidation of the carboxylic acid group on cysteine.
D) Cystine is formed through a peptide linkage between two cysteines.
E) Two cystines are released when a —CH2—S—S—CH2— disulfide bridge is reduced to —CH2—SH.
A) Cystine forms when the —CH2—SH R group is oxidized to form a —CH2—S—S—CH2— disulfide bridge between two cysteines.
The uncommon amino acid selenocysteine has an R group with the structure —CH2—SeH (pKa ≈ 5). In an aqueous solution, pH = 7.0, selenocysteine would:
A) be a fully ionized zwitterion with no net charge.
B) be found in proteins as D-selenocysteine.
C) never be found in a protein.
D) be nonionic.
E) not be optically active.
A) be a fully ionized zwitterion with no net charge.
Amino acids are ampholytes because they can function as either a(n):
A) acid or a base.
B) neutral molecule or an ion.
C) polar or a nonpolar molecule.
D) standard or a nonstandard monomer in proteins.
E) transparent or a light-absorbing compound.
A) acid or a base.
Titration of valine by a strong base, for example NaOH, reveals two pK's. The titration reaction occurring at pK2 (pK2 = 9.62) is:
A) —COOH + OH− → —COO− + H2O.
B) —COOH + —NH2 → —COO− + —NH2+.
C) —COO− + —NH2+ → —COOH + —NH2.
D) —NH3+ + OH− → —NH2 + H2O.
E) —NH2 + OH− → —NH− + H2O.
D) —NH3+ + OH− → —NH2 + H2O.
In a highly basic solution, pH = 13, the dominant form of glycine is:
A) NH2—CH2—COOH.
B) NH2—CH2—COO−.
C) NH2—CH3+—COO−.
D) NH3+—CH2—COOH.
E) NH3+—CH2—COO−.
C) NH2—CH3+—COO−.
For amino acids with neutral R groups, at any pH below the pI of the amino acid, the population of amino acids in solution will have:
A) a net negative charge.
B) a net positive charge.
C) no charged groups.
D) no net charge.
E) positive and negative charges in equal concentration.
B) a net positive charge.
At pH 7.0, converting a glutamic acid to γ-carboxyglutamate, will have what effect on the overall charge of the protein containing it?
A) it will become more negative
B) it will become more positive.
C) it will stay the same.
D) there is not enough information to answer the question.
E) the answer depends on the salt concentration.
A) it will become more negative
At pH 7.0, converting a proline to hydroxyproline, will have what effect on the overall charge of the protein containing it?
A) it will become more negative
B) it will become more positive.
C) it will stay the same.
D) there is not enough information to answer the question.
E) the answer depends on the salt concentration.
C) it will stay the same.
What is the approximate charge difference between glutamic acid and α-ketoglutarate at pH 9.5?
A) 0
B) ½
C) 1
D) 1½
E) 2
B) ½
The formation of a peptide bond between two amino acids is an example of a(n) ______________ reaction.
A) cleavage
B) condensation
C) group transfer
D) isomerization
E) oxidation reduction
B) condensation
The peptide alanylglutamylglycylalanylleucine has:
A) a disulfide bridge.
B) five peptide bonds.
C) four peptide bonds.
D) no free carboxyl group.
E) two free amino groups.
C) four peptide bonds.
An octapeptide composed of four repeating glycylalanyl units has:
A) one free amino group on an alanyl residue.
B) one free amino group on an alanyl residue and one free carboxyl group on a glycyl residue.
C) one free amino group on a glycyl residue and one free carboxyl group on an alanyl residue.
D) two free amino and two free carboxyl groups.
E) two free carboxyl groups, both on glycyl residues.
C) one free amino group on a glycyl residue and one free carboxyl group on an alanyl residue.
At the isoelectric pH of a tetrapeptide:
A) only the amino and carboxyl termini contribute charge.
B) the amino and carboxyl termini are not charged.
C) the total net charge is zero.
D) there are four ionic charges.
E) two internal amino acids of the tetrapeptide cannot have ionizable R groups.
C) the total net charge is zero.
Which of the following is correct with respect to the amino acid composition of proteins?
A) Larger proteins have a more uniform distribution of amino acids than smaller proteins.
B) Proteins contain at least one each of the 20 different standard amino acids.
C) Proteins with different functions usually differ significantly in their amino acid composition.
D) Proteins with the same molecular weight have the same amino acid composition.
E) The average molecular weight of an amino acid in a protein increases with the size of the protein.
C) Proteins with different functions usually differ significantly in their amino acid composition.
The average molecular weight of the 20 standard amino acids is 138, but biochemists use 110 when estimating the number of amino acids in a protein of known molecular weight. Why?
A) The number 110 is based on the fact that the average molecular weight of a protein is 110,000 with an average of 1,000 amino acids.
B) The number 110 reflects the higher proportion of small amino acids in proteins, as well as the loss of water when the peptide bond forms.
C) The number 110 reflects the number of amino acids found in the typical small protein, and only small proteins have their molecular weight estimated this way.
D) The number 110 takes into account the relatively small size of nonstandard amino acids.
E) The number 138 represents the molecular weight of conjugated amino acids.
B) The number 110 reflects the higher proportion of small amino acids in proteins, as well as the loss of water when the peptide bond forms.
In a conjugated protein, a prosthetic group is:
A) a fibrous region of a globular protein.
B) a nonidentical subunit of a protein with many identical subunits.
C) a part of the protein that is not composed of amino acids.
D) a subunit of an oligomeric protein.
E) synonymous with "protomer."
C) a part of the protein that is not composed of amino acids.
Prosthetic groups in the class of proteins known as glycoproteins are composed of:
A) carbohydrates.
B) flavin nucleotides.
C) lipids.
D) metals .
E) phosphates.
A) carbohydrates.
For the study of a protein in detail, an effort is usually made to first:
A) conjugate the protein to a known molecule.
B) determine its amino acid composition.
C) determine its amino acid sequence.
D) determine its molecular weight.
E) purify the protein.
A) conjugate the protein to a known molecule.
In a mixture of the five proteins listed below, which should elute second in size-exclusion (gel- filtration) chromatography?
A) cytochrome c Mr = 13,000
B) immunoglobulin G Mr = 145,000
C) ribonuclease A Mr = 13,700
D) RNA polymerase Mr = 450,000
E) serum albumin Mr = 68,500
B) immunoglobulin G Mr = 145,000
By adding SDS (sodium dodecyl sulfate) during the electrophoresis of proteins, it is possible to:
A) determine a protein's isoelectric point.
B) determine an enzyme's specific activity.
C) determine the amino acid composition of the protein.
D) preserve a protein's native structure and biological activity.
E) separate proteins exclusively on the basis of molecular weight.
E) separate proteins exclusively on the basis of molecular weight.
To determine the isoelectric point of a protein, first establish that a gel:
A) contains a denaturing detergent that can distribute uniform negative charges over the protein's surface.
B) exhibits a stable pH gradient when ampholytes become distributed in an electric field.
C) is washed with an antibody specific to the protein of interest.
D) neutralizes all ionic groups on a protein by titrating them with strong bases.
E) relates the unknown protein to a series of protein markers with known molecular weights, Mr.
B) exhibits a stable pH gradient when ampholytes become distributed in an electric field.
The first step in two-dimensional gel electrophoresis generates a series of protein bands by isoelectric focusing. In a second step, a strip of this gel is turned 90 degrees, placed on another gel containing SDS, and electric current is again applied. In this second step:
A) proteins with similar isoelectric points become further separated according to their molecular weights.
B) the individual bands become stained so that the isoelectric focus pattern can be visualized.
C) the individual bands become visualized by interacting with protein-specific antibodies in the second gel.
D) the individual bands undergo a second, more intense isoelectric focusing.
E) the proteins in the bands separate more completely because the second electric current is in the opposite polarity to the first current.
A) proteins with similar isoelectric points become further separated according to their molecular weights.
The term specific activity differs from the term activity in that specific activity:
A) is measured only under optimal conditions.
B) is the activity (enzyme units) in a milligram of protein.
C) is the activity (enzyme units) of a specific protein.
D) refers only to a purified protein.
E) refers to proteins other than enzymes.
B) is the activity (enzyme units) in a milligram of protein.
Which of the following refers to particularly stable arrangements of amino acid residues in a protein that give rise to recurring patterns?
A) Primary structure
B) Secondary structure
C) Tertiary structure
D) Quaternary structure
E) None of the above
B) Secondary structure
Which of the following describes the overall three-dimensional folding of a polypeptide?
A) Primary structure
B) Secondary structure
C) Tertiary structure
D) Quaternary structure
E) None of the above
D) Quaternary structure
The functional differences, as well as differences in three-dimensional structures, between two different enzymes from E. coli result directly from their different:
A) affinities for ATP.
B) amino acid sequences.
C) roles in DNA metabolism.
D) roles in the metabolism of E. coli.
E) secondary structures.
B) amino acid sequences
One method used to prevent disulfide bond interference with protein sequencing procedures is:
A) cleaving proteins with proteases that specifically recognize disulfide bonds.
B) protecting the disulfide bridge against spontaneous reduction to cysteinyl sulfhydryl groups.
C) reducing disulfide bridges and preventing their re-formation by further modifying the —SH groups.
D) removing cystines from protein sequences by proteolytic cleavage.
E) sequencing proteins that do not contain cysteinyl residues.
C) reducing disulfide bridges and preventing their re-formation by further modifying the —SH groups. [Show Less]