Chapter 3 Practice Problems

Question 1

The term specific activity differs from the term activity in that specific activity:

-is the activity (enzyme units) of a specific protein
-is measured only under optimal conditions
-refers to proteins other than enzymes
-refers only to a purified protein
-is the activity (enzyme units) in a milligram of protein

Answer 1

-is the activity (enzyme units) in a milligram of protein

Question 2

Which two amino acids differ from each other by only one atom?

Ser and Thr
Ser and Cys
Leu and Ile
Ala and Ser
Asp and Asn

Answer 2

Ser and Cys

Question 3

Of the 20 standard amino acids, only ___ is not optically active. The reason is that its side chain ___.

Lysine: contains only nitrogen
Proline: forms a covalent bond with the amino group
Glycine: is unbranched
Glycine: is a hydrogen atom
Alanine: is a simple methyl group

Answer 3

Glycine: is a hydrogen atom

Question 4

Which of the following statements about aromatic amino acids is correct?

-The presence of a ring structure in its R group determines whether or not an amino acid is aromatic
-On a molar bases, tryptophan absorbs more ultraviolet light than tyrosine
-All are strongly hydrophilic
-The major contribution to the characteristic absorption of light at 280 nm by proteins is the phenylalanine R group
-Histidine’s ring structure results in its being categorized as aromatic or basic, depending on pH

Answer 4

-On a molar bases, tryptophan absorbs more ultraviolet light than tyrosine

Question 5

The formation of a peptide bond between two amino acids is an example of a(n) ___ reaction

condensation
isomerization
cleavage
oxidation-reduction
group transfer

Answer 5

condensation

Question 6

Two amino acids of the standard 20 contain sulfur atoms. They are:

methionine and cysteine
cysteine and threonine
threonine and serine
methionine and serine
cysteine and serine

Answer 6

methionine and cysteine

Question 7

At the isoelectric pH of a tetrapeptide:

there are four ionic charges
the amino and carboxyl termini are not charged
two internal amino acids of the tetrapeptide cannot have ionizable R groups
only the amino and carboxyl termini contribute charge
the total net charge is zero

Answer 7

the total net charge is zero

Question 8

By adding SDS (sodium dodecyl sulfate) during the electrophoresis of proteins, is it possible to:

determine a protein’s isoelectric point
determine the amino acid composition of the protein
separate proteins exclusively on the basis of molecular weight
preserve a protein’s native structure and biological activity
determine an enzyme’s specific activity

Answer 8

separate proteins exclusively on the basis of molecular weight

Question 9

At pH 7.0 converting a proline to hydroxyproline, will have what effect on the overall charge of the protein containing it?

It will stay the same
The answer depends on the salt concentration
It will become more positive
It will become more negative
There is not enough information to answer the question

Answer 9

It will stay the same

Question 10

Amino acids are ampholytes because they can function as either a(n):

neutral molecule or an ion
standard or nonstandard monomer in proteins
polar or a nonpolar molecule
acid or base
transparent or light-absorbing compound

Answer 10

acid or base

Question 11

Titration of valine by a strong base, for example, NaOH, reveals two pKs. The titration reaction occurring at pK2 (pK2= 9.62) is:

-COOH + OH- –> -COO- + H2O
-NH3+ + OH- –> -NH2 + H2O
-COOH + -NH2 –> -COO- + -NH2+
-COO- + -NH2+ –> -COOH + -NH2
-NH2 + OH- –> -NH- + H2O

Answer 11

-NH3+ + OH- –> -NH2 + H2O

Question 12

The functional differences, as well as differences in three-dimensional structures, between two different enzymes from E. coli, result directly from their different:

roles in the metabolism of E. coli
secondary structures
roles in DNA metabolism
amino acid sequences
affinities for ATP

Answer 12

amino acid sequences

Question 13

In a mixture of the five proteins listed below, which should elute second in size-exclusion (gel-filtration) chromatography>

1. cytochrome c Mr= 13,000
2. immunoglobulin G Mr= 145,000
3. ribonuclease A Mr= 13,700
   4, RNA polymerase Mr= 450,000
4. serum albumin Mr= 68,500

Answer 13

2. immunoglobulin G Mr= 145,000

Question 14

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:

be found in proteins as D-selenocysteine
never be found in a protein
not be optically active
be nonionic
be a fully ionized zwitterion with no net charge

Answer 14

be a fully ionized zwitterion with no net charge

Question 15

In a highly basic solution, pH= 13, the dominant form of glycine is:

NH2-CH2-COOH
NH3+ - CH2-COOH
NH2-CH3+–COO-
NH3+ –CH2-COO-
NH2-CH2-COO-

Answer 15

NH2-CH2-COO-