Proteins

Question 1

What is the acidic group in amino acids?

Answer 1

Carboxyl group (–COOH)

Question 2

What is the basic group in amino acids?

Answer 2

Amino group (–NH2)

Question 3

What group differentiates one amino acid from another, giving it its distinctive properties such as melting point and boiling point?

Answer 3

R group

Question 4

Do amino acids exhibit optical activity?

Answer 4

YES

d (lower case) isomer can rotate plane-polarized light (light in one direction only) to the right and l (lower case) isomer can rotate plane-polarized light to the left

Question 5

Which amino acid enantiomer (D or L) is more predominant in nature?

Answer 5

L isomer

Question 6

Any relationship between enantiomerism and optical activity?

Answer 6

NONE. You can have D, d or D, l isomers.

Question 7

Why are amino acids classified as amphoteric substances?

Answer 7

Because they have BOTH acidic and basic groups. Thus they can serve as both acids and bases.

Question 8

What would make an amino acid nonpolar?

Answer 8

If their sidechains are composed of hydrocarbon groups.

ex. alanine (which has a –CH3 sidechain, or tryptophan, which has a very bulky sidechain composed of only carbon and H)

Question 9

What would make an amino acid polar?

Answer 9

Polar groups in their sidechains such –OH, –COOH or –NH2

ex. serine (has –CH2OH sidechain) or aspartic acid (has –CH2COOH sidechain)

Question 10

What would make an amino acid acidic?

Answer 10

Carboxylic groups in their sidechains. These amino acids are NEGATIVELY CHARGED at physiological pH (pH ~7).

ex. aspartic acid (has –CH2COOH sidechain)

Question 11

What would make an amino acid basic?

Answer 11

Amino groups in their sidechains. These amino acids are POSTIVELY CHARGED at physiological pH (pH ~7).

ex. arginine (has guanidino side chain with amino group in it)

Question 12

What are essential amino acids?

Answer 12

Amino acids that they body cannot synthesize, therefore they must be obtained from the diet.

Mnemonic: PVT TIM HALL (phenylalanine, valine, tryptophan, threonine, isoleucine, methionine, histidine, arginine, leucine and lysine)

Question 13

What is the bond formed between 2 amino acids?

Answer 13

Peptide bond

Question 14

Can the peptide bond rotate?

Answer 14

No, it has a double bond character, which makes it slightly rigid (more rigid than a single bond).

Question 15

In globular proteins, which amino acids are exposed? Which are tucked away in the core of the protein?

Answer 15

Exposed amino acids on the surface must be able to interact with the aqueous environment, thus they must be hydrophilic, or polar. Those tucked away in the core of the protein are those that cannot interact with the aqueous environment, thus they must be the hydrophobic, or nonpolar amino acids.

Question 16

What is the usual function of fibrous proteins like collagen and keratin?

Answer 16

Structural support

Question 17

What are the 4 levels of organization in proteins?

Answer 17

Primary, secondary, tertiary and quaternary

Question 18

What is the structure found at the pimary level of organization?

Answer 18

Amino acid sequence

Question 19

What stabilizes the structure at the pimary level of organization?

Answer 19

Peptide bonds

Question 20

What is the structure found at the secondary level of organization?

Answer 20

Alpha helixes and beta-pleated sheets

Question 21

What stabilizes the structures at the secondary level of organization?

Answer 21

Hydrogen bonds (inter and intra)

Question 22

What is the structure found at the tertiary level of organization?

Answer 22

3D conformation of proteins

ex. structure of myoglobin

Question 23

What stabilizes the structures at the tertiary level of organization?

Answer 23

Disulfide bonds (bond between 2 cysteine molecules), salt bridges (between charged amino acids, example aspartic acid and histidine), hydrophobic interactions (between hydrophobic amino acids)

Question 24

What is the structure found at the quaternary level of organization?

Answer 24

Interactions of several subunits (dimers, tetramers)

ex. structure of hemoglobin (a tetramer)

Question 25

What stabilizes the structures at the tertiary level of organization?

Answer 25

Disulfide bonds (bond between 2 cysteine molecules), salt bridges (between charged amino acids, example aspartic acid and histidine), hydrophobic interactions (between hydrophobic amino acids)

Question 26

Which provides the high rigidity of collagen?

Answer 26

Proline and hydroxyproline

Question 27

Which amino acid occurs in every 3rd amino acid in the sequence of collagen?

Answer 27

Glycine

forms cross-links

Question 28

What amino acids mostly make up elastin?

Answer 28

Glycine, alanine and lysine (few proline)

Question 29

Which unusual amino acid is found in elastin?

Answer 29

Desmosine

Question 30

What is present in alpha-keratin that makes it very tough?

Answer 30

Disulfide bonds

(As opposed to beta-keratin, which does not have any cysteine residues. This makes beta-keratin more soft and supple like silk.)

Question 31

Between myoglobin and hemoglobin, which exhibits a hyperbolic curve, and which one exhibits a sigmoidal curve?

Answer 31

Myoglobin: hyperbolic
Hemoglobin: sigmoidal

Question 32

Which is more ideal for transport and which is more ideal for storage?

Answer 32

Hemoglobin: transport
Myoglobin: storage

Hemoglobin binds O2 only at high concentrations (such as in the lungs) and releases it at low concentrations (peripheral tissues). Myoglobin releases O2 only at very very low O2 concentrations.

Question 33

Sickle cell anemia is caused by a single mutation in amino acid # 6 of the beta subunit of hemoglobin. Which amino acid is it and what is it mutated to?

Answer 33

Glutamate to valine.

Why does it make a difference? Glutamate is POLAR, while valine is NONPOLAR. This would affect the structure because glutamate is found outside hemoglobin while valine would be found inside hemoglobin.

Question 34

How does sickle cell anemia cause problems? Does the disease have any benefit?

Answer 34

The sickle cell structure allows for the polymerization of Hb, causing RBCs to clump together and potentially block blood vessels. The structure isn’t also ideal for O2 binding.

Yes, sickle cell anemia has benefit in malaria-infested regions. The parasite responsible for malaria spends a part of its life cycle inside the RBC. With defective Hb, however, the RBC ruptures more easily, making the parasite unable to reproduce.