MCAT Biochemistry Review With Correct
Solutions
Amino Acid CORRECT ANSWERS Dipolar compound containing an amino group and a
carboxyl group
Amino Group CORRECT ANSWERS -NH2
Carboxyl Group CORRECT ANSWERS -COOH
Alpha Carbon CORRECT ANSWERS Central carbon atom in amino acid
Attached to amino & carboxyl groups, H atom, and side chain
Side Chain CORRECT ANSWERS The variable component of an amino acid that gives
the amino acid its identity and chemical properties
also called R-group
Chiral CORRECT ANSWERS A molecule with a nonsuperimposable mirror image
All amino acids (except glycine) are chiral
(S) Absolute Configuration CORRECT ANSWERS All amino acids (except cysteine)
have an (S) absolute configuration
L-amino Acid CORRECT ANSWERS All amino acids found in eukaryotes
Nonpolar, Nonaromatic Amino Acids CORRECT ANSWERS Glycine, alanine, valine,
leucine, isoleucine, methionine, proline
Glycine CORRECT ANSWERS Single H atom as side chain
Achiral
Smallest amino acid
Alanine CORRECT ANSWERS Alkyl side chain (1C)
Valine CORRECT ANSWERS Alkyl side chain (3C)
Leucine CORRECT ANSWERS Alkyl side chain (4C)
Isoleucine CORRECT ANSWERS Alkyl side chain (4C)
Methionine CORRECT ANSWERS Methyl side chain (-CH3)
Contains S atom in side chain
Proline CORRECT ANSWERS Cyclic amino acid
,N from amino group becomes part of side chain, forming five-part ring, limiting where is
can appear on a protein
Rigid/constraints on flexibility
Aromatic Amino Acids CORRECT ANSWERS Tryptophan, phenylalanine, tyrosine
Aromaticity CORRECT ANSWERS The ability of a molecule to delocalize pi electrons
around a conjucated ring, creating exceptional stability
Tryptophan CORRECT ANSWERS Double-ring system
Contains N atom in one ring
Largest of aromatic amino acids
Phenylalanine CORRECT ANSWERS Benzyl side chain (benzene ring + -CH2 group)
Smallest aromatic amino acid
Relatively nonpolar
Tyrosine CORRECT ANSWERS Phenylalanine + -OH group
Relatively polar
Polar Amino Acids CORRECT ANSWERS Serine, threonine, asparagine, glutamine,
cysteine
Polarity CORRECT ANSWERS An uneven sharing of electrons in a molecule, creating
a slightly positive side and a slightly negative side
Serine CORRECT ANSWERS -OH group in side chain
Highly polar - participate in H-bonding
Threonine CORRECT ANSWERS -OH group in side chain
Highly polar - participate in H-bonding
Asparagine CORRECT ANSWERS Amide (-NH2) side chain
Amide N do not gain or lose protons with changes in pH - do not become charged
Glutamine CORRECT ANSWERS Amide (-NH2) side chain
Amide N do not gain or lose protons with changes in pH - do not become charged
Cysteine CORRECT ANSWERS Thiol (-SH) side chain - weaker than OH bond
Prone to oxidation
Negatively-charged (Acidic) Amino Acids CORRECT ANSWERS Aspartic acid
(aspartate), glutamic acid (glutamate)
Aspartic Acid (Aspartate) CORRECT ANSWERS Carboxylate (-COO(-)) group in side
chain
,Deprotonated form of asparagine
Glutamic Acid (Glutamate) CORRECT ANSWERS Carboxylate (-COO(-)) group in side
chain
Deprotonated form of glutamine
Positively-charged (Basic) Amino Acids CORRECT ANSWERS Lysine, arginine,
histidine
Lysine CORRECT ANSWERS Terminal primary amino group
Arginine CORRECT ANSWERS Has 3 N atoms in side chain
Charge delocalized over all three N atoms
Histidine CORRECT ANSWERS Has aromatic ring with 2 N atoms (ring is called an
imidazole)
At pH 7.4, one N is protonated and the other isn't
Under acidic conditions, the 2nd N becomes protonated, making it positively charged
Hydrophobic Amino Acids CORRECT ANSWERS Being repelled by water
Alanine, isoleucine, leucine, valine, phenylalanine
More likely to be found in center of protein
Nonpolar, uncharged compounds
Long alkyl chains
Hydrophilic Amino Acids CORRECT ANSWERS Being attracted to water
Histidine, arginine, lysine, glutamate, aspartate, asparagine, glutamine
Polar and charged compounds and those that participate in H-bonding
Neither Really Hydrophobic/-philic CORRECT ANSWERS Cysteine, threonine, serine,
tyrosine, tryptophan, proline, methionine, glycine
Amphoteric CORRECT ANSWERS Ability to act as an acid or a base
Can either accept or donate a proton
For ionizable groups: tend to be protonated at low pH; deprotonated at high pH
pKa CORRECT ANSWERS The pH at which half of the species are deprotonated
[HA] = [A(-)]
pKa1 CORRECT ANSWERS pKa for carboxyl group
Usually around 2
pKa2 CORRECT ANSWERS pKa for amino group
Usually between 9 and 10
pKa3 CORRECT ANSWERS For amino acids with ionizable side chains
, Amino Acids in Acidic Conditions CORRECT ANSWERS Amino acid is fully protonated
i.e. (-NH3+) and (-COOH)
Amino Acids in Neutral Conditions CORRECT ANSWERS Form zwitterions
pH is near pI of amino acid
i.e. (-COO(-)) and (-NH3(+))
Zwitterion CORRECT ANSWERS A molecule that contains charges, but is neutral
overall
Amino Acids in Basic Conditions CORRECT ANSWERS Amino acid is fully
deprotonated
i.e. (-COO(-)) and (-NH2)
Isoelectric Point (pI) CORRECT ANSWERS The pH at which every molecule in solution
is electrically neutral
Predominantly in zwitterion form
pI(neutral amino acid) = CORRECT ANSWERS (pKa(NH3+) + pKa(COOH))/2
For amino acids with neutral side chains
Have relatively neutral pI values (~6)
Buffer CORRECT ANSWERS When pH of a solution is approximately = pKa of solute
The pH doesn't change very much, even when acid or base are added to solution
pI(acidic amino acid) = CORRECT ANSWERS (pKa(R group) + pKa(COOH))/2
For amino acids with negatively charged side chains
Have relatively low pI values (~3.2)
pI(basic amino acid) = CORRECT ANSWERS (pKa(NH3+) + pKa(R group))/2
For amino acids with positively charged side chains
Have relatively high pI values (~9.75)
Titration CORRECT ANSWERS A laboratory technique in which a solution of unknown
concentration is mixed with a solution of known concentration to determine the unknown
concentration
Titration Curves for Amino Acids CORRECT ANSWERS Curve is nearly flat at pKa
values of amino acids
Nearly vertical at pI of amino acid
Peptides CORRECT ANSWERS A molecule composed of more than one amino acid
Can be subdivided into dipeptides, tripeptides, oligopeptides, and polypeptides
Dipeptide CORRECT ANSWERS Two amino acid residues