3. Amino Acids

Question 1

What are the behaviours of alpha amino acids? (5)

Answer 1

1. Neurotransmitters
2. Hormones
3. Bacterial cell wall components
4. Metabolic intermediates
5. Precursor to heme

Question 2

Are amino acids hydrophilic or hydrophobic?

Answer 2

Mostly hydrophilic

Question 3

Peptides vs protein

Answer 3

PEPTIDE: short chains of up to 20 amino acids
PROTEIN: polymer of linear chain of amino acids

Question 4

Draw your amino acids structures along with their special features

Answer 4

see notes!

Question 5

What is a zwitterion?

Answer 5

A hybrid ion with NO net charge (0)

Question 6

What happens to acidic amino acids at pH = 7? (2)

Answer 6

1. Lose their proton from the side chain (COOH -> COO-)
2. Become anionic (negative charge)

Question 7

What happens at pH = 7 to basic amino acids? (2)

Answer 7

1. Bind a proton to their side chain (NH2 -> NH3+)
2. Become cationic (positive charge)

Question 8

What is molecular conformation?

Answer 8

Differences in spatial arrangement of groups joined by covalent bonds due to bond rotation

Question 9

What is an isomer? (3)

Answer 9

1. They have the same molecular formula but a different arrangement of their component groups
2. Ex) Leucine and Isoleucine
3. All amino acids except glycine have 4 different gorups attached to their alpha-carbon (asymmetrical/achiral) - these are handed molecules

Question 10

Explain the 2 possible arrangements of the 4 groups attached to carbon (3)

Answer 10

1. Called stereoisomers
2. Non-superimposable mirror images/enantiomers
3. D and L forms of amino acids

Question 11

D-form vs L-form (4)

Answer 11

D: hydrogen is first/left (+)
L: hydrogen is second/right (-)

These molecules are optically active: they rotate the plane of monochromatic plane-polarized light in opposite directions

All amino acids in protein are L, D form is found in bacteria

Question 12

Which amino acids have two chiral centres? How many stereoisomers do they have?

Answer 12

Threonine (Thr) and Isoleucine (Ile), 4

Question 13

How to calculate the number of steroisomers in an amino acid?

Answer 13

of stereoisomers = 2^C
(c= number of chiral centres)

Question 14

Explain the thalidomide L and D forms (3)

Answer 14

1. Thalidomide is a sedative.
2. The + form is therapeutic
3. Mirrored form (-) is teratogenic causing embryo malformation

Question 15

How many buffering regions of glycine mixed with NaOH

Answer 15

The titration curve will have 2 buffering regions: Amino pKa, Carboxyl pKa

Question 16

What groups ionize first?

Answer 16

Strong acids

Question 17

What is the isoelectric point (pI)?

Answer 17

1. The pH at which the concentration of the zwitterion of an amino acid is maximum
2. No net charge on the amino acid
3. Amino acid is stationary in an electric field

Question 18

Explain amino acid purification

Answer 18

Separating the molecules based on differences in net charge

Question 19

Explain electrophoresis

Answer 19

Electrophoresis separates molecules by applying an electric field. Ions move in the field according to their net charge

Question 20

Explain paper electrophoresis

Answer 20

The ions move through a buffer solution that permeates the paper

Question 21

Explain capillary electrophoresis (2)

Answer 21

1. Amino acids are separated in thin silica capillaries
2. Same as paper electrophoresis, except capillaries are used instead of paper

Question 22

Explain ion-exchange chromatography

Answer 22

Polystyrene or silica-based beads with anionic (+ charged to attract -) or cationic (- charged + attract +) functional groups attached to attract charged substances

Question 23

How does ion exchange chromatography work? (5)

Answer 23

1. Beads with an anionic or cationic functional groups are packed into a cylindrical column
2. A mixture of amino acids are applied in a buffer which bind to the beads
3. The amino acids are removed by washing the beads at a higher pH
4. When the pH reaches the pI of an amino acid, it unbinds from the beads and washes out of the column
5. The differences in pI of each amino acid cause them to dissociate at different pH’s

Question 24

Explains peptides (3)

Answer 24

1. The condensation of amino acids do not occur in neutral water as the OH is not a good leaving group
2. Polypeptide: more than 20 amino acids
3. Protein: one or more polypeptides

Question 25

Which part(s) of a polypeptide is/isn’t ionizable? (3)

Answer 25

Ionizable: the ends (amino/carboxyl terminus)
Non-ionizable: repeating backbones
Varies: side-chains

Question 26

Explain biologically active peptides (4)

Answer 26

1. Enkephalins are natural analgelsics - they bind to specific receptors in the brain and diminish the perception of pain
2. Many conformations because of rotation of single covalent bonds
3. Binding to a receptor can force it to only have one conformation
4. Aspartame is a synthetic dipeptide 150x sweeter than sugar