ΧΗΕΜ523 Εξαμ #1                                                         Ναμε: _________________________________________

Σεχτιον 1. Φυν!

Ανσωερ τηε φολλοωινγ θυεστιονσ ωιτη τηε βεεστ ποσσιβλε ανσωερ γιϖεν τηε χηοιχεσ αϖαιλαβλε. Ιφ α θυεστιον χαν βε ανσωερεδ βψ μορε τηαν ονε ποσσιβλε χηοιχε, ιτ ωιλλ σαψ σο ιν τηε θυεστιον ιτσελφ.



1.
Hydrophobic interactions make important energetic contributions to:
A.
binding of a hormone to its receptor protein.
B.
enzyme-substrate interactions.
C.
membrane structure.
D.
three-dimensional folding of a polypeptide chain.
E.
All of the above are true.


2.
The aqueous solution with the lowest pH is:
A.
0.01 M HCl.
B.
0.1 M acetic acid (pKa = 4.86).
C.
0.1 M formic acid (pKa = 3.75).
D.
0.1 M HCl.
E.
10–12 M NaOH.


3.
A compound has a pKa of 7.4. To 100 mL of a 1.0 M solution of this compound at pH 8.0 is added 30 mL of 1.0 M hydrochloric acid. The resulting solution is pH:
A.
6.5.
B.
6.8.
C.
7.2.
D.
7.4.
E.
7.5.


4.
A compound is known to have a free amino group with a pKa of 8.8, and one other ionizable group with a pKa between 5 and 7. To 100 mL of a 0.2 M solution of this compound at pH 8.2 was added 40 mL of a solution of 0.2 M hydrochloric acid. The pH changed to 6.2. The pKa of the second ionizable group is:
A.
The pH cannot be determined from this information.
B.
5.4.
C.
5.6.
D.
6.0.
E.
6.2.


5.
The most important contribution to the stability of a protein's conformation appears to be the:
A.
entropy increase from the decrease in ordered water molecules forming a solvent shell around it.
B.
maximum entropy increase from ionic interactions between the ionized amino acids in a protein.
C.
sum of free energies of formation of many weak interactions among the hundreds of amino acids in a protein.
D.
sum of free energies of formation of many weak interactions between its polar amino acids and surrounding water.
E.
stabilizing effect of hydrogen bonding between the carbonyl group of one peptide bond and the amino group of another.


6.
In an aqueous solution, protein conformation is determined by two major factors. One is the formation of the maximum number of hydrogen bonds. The other is the:
A.
formation of the maximum number of hydrophilic interactions.
B.
maximization of ionic interactions.
C.
minimization of entropy by the formation of a water solvent shell around the protein.
D.
placement of hydrophobic amino acid residues within the interior of the protein.
E.
placement of polar amino acid residues around the exterior of the protein.


7.
Roughly how many amino acids are there in one turn of an α helix?
A.
1
B.
2.8
C.
3.6
D.
4.2
E.
10


8.
In the α helix the hydrogen bonds:
A.
are roughly parallel to the axis of the helix.
B.
are roughly perpendicular to the axis of the helix.
C.
occur mainly between electronegative atoms of the R groups.
D.
occur only between some of the amino acids of the helix.
E.
occur only near the amino and carboxyl termini of the helix.


9.
The major reason that antiparallel β-stranded protein structures are more stable than parallel β-stranded structures is that the latter:
A.
are in a slightly less extended configuration than antiparallel strands.
B.
do not have as many disulfide crosslinks between adjacent strands.
C.
do not stack in sheets as well as antiparallel strands.
D.
have fewer lateral hydrogen bonds than antiparallel strands.
E.
have weaker hydrogen bonds laterally between adjacent strands.


10.
Proteins often have regions that can fold and function as an independent entity from the whole protein. These regions are called:
A.
domains.
B.
oligomers.
C.
peptides.
D.
sites.
E.
subunits.


11.
An allosteric interaction between a ligand and a protein is one in which:
A.
binding of a molecule to a binding site affects binding of additional molecules to the same site.
B.
binding of a molecule to a binding site affects binding properties of another site on the protein.
C.
binding of the ligand to the protein is covalent.
D.
multiple molecules of the same ligand can bind to the same binding site.
E.
two different ligands can bind to the same binding site.


12.
In hemoglobin, the transition from T state to R state (low to high affinity) is triggered by:
A.
Fe2+ binding.
B.
heme binding.
C.
oxygen binding.
D.
subunit association.
E.
subunit dissociation.


13.
Michaelis and Menten assumed that the overall reaction for an enzyme-catalyzed reaction could be written as



Using this reaction, the rate of breakdown of the enzyme-substrate complex can be described by the expression:
A.
k1 ([Et] – [ES]).
B.
k1 ([Et] – [ES])[S].
C.
k2 [ES].
D.
k-1 [ES] + k2 [ES].
E.
k-1 [ES].


14.
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.


15.
The following data were obtained in a study of an enzyme known to follow Michaelis-Menten kinetics:


The Km for this enzyme is approximately:
A.
1 mM.
B.
1000 mM.
C.
2 mM.
D.
4 mM.
E.
6 mM.


Γαμεσ!
Ανσωερ τηε φολλοωινγ θυεστιονσ ασ χομπλετελψ ανδ τηορουγηλψ σα ποσσιβλε. Ψου μυστ βε χονχισε ανδ υσε χομπλετε σεντενχεσ.



16.
Give the general Henderson-Hasselbalch equation and sketch the plot it describes (pH against amount of NaOH added to a weak acid). On your curve, label the pKa for the weak acid and indicate the region in which the buffering capacity of the system is greatest.

Answer:


17.
The artificial sweetener NutraSweet®, also called aspartame, is a simple dipeptide, aspartylphenylalanine methyl ester, on which the free carboxyl of the dipeptide is esterified to methyl alcohol. Draw the structure of aspartame, showing the ionizable groups in the form they have at pH 7. (The ionizable group in the side chain of aspartate has a pKa of 3.96.)

Answer:


18.
Why is silk fibroin so strong, but at the same time so soft and flexible?

Answer:


19.
Ανσωερ τηε φολλοωινγ θυεστιονσ αβουτ προτειν φολδινγ:

ι) Ωηατ ωασ τηε Ανφινσεν εξπεριμεντ? Ωηατ διδ ηισ ωορκ σηοω?





ιι) Ωηατ ισ Λεϖιντηαλ∋σ Παραδοξ?





ιιι) Δεσχριβε τηε χηρονολογψ οφ τηε φολδινγ οφ α γλοβυλαρ προτειν. Ψου μυστ υσε τηε τερμσ ανδ χονχεπτσ ωε δισχυσσεδ ωηεν δεσχριβινγ ηοω α προτειν γοεσ φρομ α λινεαρ σεθυενχε οφ αμινο αχιδσ το α χομπαχτ, φυνχτιοναλ μολεχυλε.




ιϖ) Ωηατ ισ μεαντ βψ τηε τερμσ ∀Μοτιφσ∀ ανδ ∀Δομαινσ∀? Ωηατ αρε τηε διφφερενχεσ βετωεεν τηε τωο?

Answer:


20.
(α) Ωηατ ισ τηε εφφεχτ οφ πΗ ον τηε βινδινγ οφ οξψγεν το ηεμογλοβιν (τηε Βοηρ Εφφεχτ)? (β) Βριεφλψ δεσχριβε τηε μεχηανισμ οφ τηισ εφφεχτ υσινγ σπεχιφιχ δεταιλσ, αμινο αχιδσ ανδ χηεμιχαλ ρεαχτιονσ.

Answer:


21.
Ανσωερ τηε φολλοωινγ θυεστιονσ αβουτ ενζψμε–χαταλψζεδ ρεαχτιονσ:

ι) Ωριτε ουτ τηε ενζψμε–χαταλψζεδ χηεμιχαλ ρεαχτιον ωιτη αλλ τερμσ (Ε, Σ ανδ Π), τηε χορρεχτ αρροωσ ανδ αλλ ρατε χονσταντσ οϖερ τηειρ ρεσπεχτιϖε ερρορσ.



ιι) Δραω ανδ λαβελ α ρεαχτιον χοορδινατε διαγραμ φορ αν ενζψμε–χαταλψζεδ ρεαχτιον, Σ → Π.





ιιι) Ηοω δοεσ αν ενζψμε δεχρεασε τηε ΔΓ∗ (τρανσιτιον στατε φρεε ενεργψ) φορ τηε χηεμιχαλ ρεαχτιον? Ιν οτηερ ωορδσ, ωηατ τραιτσ οφ ενζψμεσ αλλοω τηεμ το βε εφφιχιεντ χαταλψστσ?




Answer:


22.
Ανσωερ τηε φολλοωινγ θυεστιονσ αβουτ ενζψμε κινετιχσ.

ι) Τηε Μιχηαελισ–Μεντεν χονσταντ, Κμ, ισ αχτυαλλψ α συμμαρψ οφ τηρεε τερμσ. Ωηατ ισ τηε ματηεματιχαλ ρεπρεσεντατιον οφ τηε Μιχηαελισ χονσταντ ανδ ωηατ δοεσ ιτ ρεπρεσεντ φορ μοστ ενζψμε χαταλψζεδ ρεαχτιονσ?

ιι) Ωριτε ουτ τηε Μιχηαελισ–Μεντον εθυατιον ανδ σηοω ωηατ α πλοτ οφ ινιτιαλ ϖελοχιτψ ασ α φυνχτιον οφ συβστρατε χονχεντρατιον ωουλδ λοοκ λικε. Λαβελ τηε ςμ ανδ τηε Κμ ον ψουρ πλοτ.


ιιι) Ωηατ ισ τηε Λινεωεαϖερ Βυρκ αππροξιματιον οφ τηε Μιχηαελισ–Μεντον εθυατιον? Υσινγ τηισ εθυατιον, ηοω ωουλδ ψου δετερμινε ςμ ανδ Κμ φορ α γιϖεν ενζψμε?

Answer:


Χαρναϖαλε!

Ψου∋ρε τιρεδ, ιτσ λατε, βυτ νοω ισ ωηεν ψου νεεδ το συμμον αλλ οφ ψουρ μιγητ ανδ χρυση τηισ τηινγ! Χ∋μον! Λετ∋σ δο τηισ! Ανσωερ τηε φολλοωινγ θυεστιονσ λεαϖινγ νο ροομ φορ δουβτ.



STOP This is the end of the test. When you have completed all the questions and reviewed your answers, press the button below to grade the test.