Practice Quiz 2 Chapters 6-8

Question 1

The steady-state assumption, as applied to enzyme kinetics, implies:

a) Km=Ks
b) the enzyme is regulated
c) the ES complex is formed and broken down at equivalent rates
d) the Km is equivalent to the cellular substrate concentration
e) the maximum velocity occurs when the enzyme is saturated

Answer 1

c

Question 2

which of the following statements about allosteric control of enzymatic activity is false?

a) allosteric effectors give rise to sigmoidal V0 vs [S] kinetic plots
b) allosteric proteins are generally composed of several subunits
c) an effector may either inhibit or activate an enzyme
d) binding of the effector changes the conformation of the enzyme molecule
e) heterotropic allosteric effectors compete with substrate for binding sites

Answer 2

e

Question 3

An enzyme follows Michaelis-Menten kinetics. Indicate which of the kinetic parameters would be altered by the following factors. Give only one answer for each.

K_m, V_max, Neither, Both

a) a competitive inhibitor
b) a mixed inhibitor
c) 6M urea
d) doubling the [S]

Answer 3

a) K_m
b) both
c) both
d) neither

Question 4

Starch and glycogen are both polymers of:

a) fructose
b) glucose 1-phosphate
c) sucrose
d) a-D-glucose
e) b-D-glucose

Answer 4

d

Question 5

In glycoproteins, the carbohydrate moiety is always attached through the amino acid residues:

a) asparagine, serine, or threonine
b) aspartate or glutamate
c) glutamine or arginine
d) glycine, alanine, or aspartate
e) tryptophan, aspartate, or cysteine

Answer 5

a

Question 6

Which of the following monosaccharides is not an aldose?

a) maltose
b) fructose
c) glucose
d) glyceraldehyde
e) ribose

Answer 6

b

Question 7

The reference compound for naming D and L isomers of sugars is

a) fructose
b) glucose
c) glyceraldehyde
d) ribose
e) sucrose

Answer 7

c

Question 8

When two carbohydrates are epimers:

a) one is a pyranose, the other a furanose
b) one is an aldose, the other a ketose
c) they differ in length by one carbon
d) they differ only in the configuration around one carbon atom
e) they rotate plane-polarized light in the same addition

Answer 8

d

Question 9

When the linear form of glucose cyclizes, the product is a(n):

a) anhydride
b) glycoside
c) hemiacetal
d) lactone
e) oligosaccharide

Answer 9

c

Question 10

Which of the following is not a reducing sugar?

a) fructose
b) glucose
c) glyceraldehyde
d) ribose
e) sucrose

Answer 10

e

Question 11

From the abbreviated name of the compound Gal(ß1 → 4)Glc, we know that:

a) C-4 of glucose is joined to C-1 of galactose by a glycosidic bond
b) the compound is a D-enantiomer
c) the galactose residue is at the reducing end
d) the glucose is in its pyranose form
e) the glucose residue is the ß anomer

Answer 11

a

Question 12

The biochemical property of lectins that is the basis for most of their biological effects is their ability to bind to:

a) amphipathic molecules
b) hydrophobic molecules
c) specific lipids
d) specific oligosaccharides
e) specific peptides

Answer 12

d

Question 13

The difference between thymine and uracil is:

a) one methylene group on the pyrimidine ring
b) one methyl group on the pyrimidine ring
c) one hydroxyl group on the ribose ring
d) one amine group on the pyrimidine ring
e) one methyl group on the purine ring

Answer 13

b

Question 14

For the helix in double-stranded B-form DNA, the majority of the stability can be attributed to:

a) base-pairing interactions via H-bonds
b) interactions along the phosphate backbone
c) base-stacking interactions via van-der-Waals interactions
d) covalent bonds between adjacent bases in one strand
e) ionic interactions with metal ions

Answer 14

c

Question 15

Which deoxyoligonucleotides will hybridize with a DNA containing the sequence (5’)AGACTGGTC(3’)?

Answer 15

(5’)GACCAGTCT(3’)

Question 16

The number of structurally different polysaccharides that can be made with 20 different monosaccharides is far greater than the number of different polypeptides that can be made with 20 different amino acids if both polymers contain an equal number of total residues. Explain why.

Answer 16

Because virtually all peptides are linear (i.e., are formed with peptide bonds between the a-carboxyl and a-amino groups), the variability of peptides is limited by the number of different subunits.

Polysaccharides can be linear or branched, can be alpha or beta linked, can be joined 1→4, 1→3, 1→6. The number of different ways to arrange 20 different sugars in a branched oligosaccharide is therefore much larger than the number of different ways a peptide could be made with an equal number of residues

Question 17

Describe one biological advantage of storing glucose units in branched polymers (glycogen, amylopectin) rather than in linear polymers.

Answer 17

The enzymes that act on these polymers to mobilize glucose for metabolism act only on nonreducing ends. With extensive branching, there are more such ends for enzymatic attach than would be present in the same quantity of glucose stored in a linear polymer (one nonreducing end). In effect, branched polymers increase the substrate concentration for these enzymes.

Question 18

Explain how it is possible that a polysaccharide molecule, such as glycogen, may have only one reducing end, and yet have many nonreducing ends.

Answer 18

The molecule is branched, with each branch ending in a nonreducing end.

Question 19

Explain how a biochemist might discover that a certain enzyme is allosterically regulated.

Answer 19

The enzyme would show kinetics that does not fit the Michaelis-Menten equation; the plot of V vs [S] would be sigmoidal, not hyperbolic.

The enzyme kinetics would be affected by molecules other than the substrate(s).

Question 20

Why does pH affect the activity of an enzyme and why?

Answer 20

The state of ionization of several amino acid side chains is affected by pH, and the activity of many enzymes requires that certain of the amino acid residue side chains be in a specific ionization state.