The 20 AA

Question 1

Glycine

Answer 1

G - Gly
Achiral
Nonpolar, aliphatic R group
Can be on surface loops because of their small size
alpha helix breaker
Flexible bc have 2 H

Question 2

Alanine

Answer 2

A - Ala
Nonpolar, aliphatic R group
Can be on surface loops because of their small size

Question 3

Proline

Answer 3

P - Pro
Nonpolar, aliphatic R group
Special among the 20 as an imino acid – imines defined as secondary amines (note: N bonded to two C inside of group, not one C)
alpha helix breaker

MCAT note: 2degree a amino group

Question 4

Valine

Answer 4

V - Val
Nonpolar, aliphatic R group

Question 5

Leucine

Answer 5

L - Leu
Nonpolar, aliphatic R group

Question 6

Isoleucine

Answer 6

I - Ile
Nonpolar, aliphatic R group

Question 7

Methionine

Answer 7

M - Met
Nonpolar, aliphatic R group

Question 8

Phenylalanine

Answer 8

F - Phe
Aromatic R group
Non-polar
Delocalized electrons in rings (aromaticity) allows aromatic rings to be detected by UV due to the ability of these delocalized electrons to absorb UV light around 280nm wavelength; this is because the electrons can readily transition between energy levels within the aromatic ring system

Question 9

Tyrosine

Answer 9

Y - Try
Aromatic R group
Non-Polar
Polar atoms in tyrosine and tryptophan, but most of sidechains are hydrophobic
Delocalized electrons in rings (aromaticity) allows aromatic rings to be detected by UV due to the ability of these delocalized electrons to absorb UV light around 280nm wavelength; this is because the electrons can readily transition between energy levels within the aromatic ring system
Increases pI making more basic

Question 10

Tryptophan

Answer 10

W - Try
Aromatic R group
Non-polar
Polar atoms in tyrosine and tryptophan, but most of sidechains are hydrophobic
Delocalized electrons in rings (aromaticity) allows aromatic rings to be detected by UV due to the ability of these delocalized electrons to absorb UV light around 280nm wavelength; this is because the electrons can readily transition between energy levels within the aromatic ring system

W absorbs more strongly, then Y and F

Question 11

Serine

Answer 11

S - Ser
Polar, uncharged “neutral” R group

Question 12

Threonine

Answer 12

T - Thr
Polar, uncharged “neutral” R group

Question 13

Cysteine

Answer 13

C - Cys
Polar, uncharged “neutral” R group
Sidechain can form disulfide bonds within a protein or between chains (MCAT note) in an oxidized state (extracellular) and it’s referred to at Cystine in this state.
Intracellular: Cysteine, thiol group, reduced form

Question 14

Asparagine

Answer 14

N - Asn
Polar, uncharged “neutral” R group

Question 15

Glutamine

Answer 15

Q - Gln
Polar, uncharged “neutral” R group

Question 16

Lysine

Answer 16

K - Lys
Polar
Positively charged R group
0 or +1 charge
Increases pI making more basic

Question 17

Arginine

Answer 17

R - Arg
Polar
Positively Charged R group
0 or +1 charge
Increases pI making more basic

Question 18

Histidine

Answer 18

H - His
Polar
Positively Charged R group
0 or +1 charge
MCAT note:
* pKa~6.5~physiological pH so can exist in both protonated and deprotonated form.
* pH less than aa pKa= protonated
pH more than aa pKa= deprotonated
* useful at active site of protein where it can stabilize or destabilize a protein

Question 19

Aspartate

Answer 19

D - Asp
Polar
Negatively Charged R group
0 or -1 charge
In protonated form aka Aspartic Acid
Decreases pI making more acidic

Question 20

Glutamate

Answer 20

E - Glu
Polar
Negatively Charged R group
0 or -1 charge
In protonated form aka Glutamic Acid
Decreases pI making more acidic