©THEBRIGHT EXAM STUDY SOLUTIONS 8/22/2024 12:54 PM
MCAT Biochemistry Review Exam
Questions And Correct Answers
Amino Acid - answer✔✔Dipolar compound containing an amino group and a carboxyl group
Amino Group - answer✔✔-NH2
Carboxyl Group - answer✔✔-COOH
Alpha Carbon - answer✔✔Central carbon atom in amino acid
Attached to amino & carboxyl groups, H atom, and side chain
Side Chain - answer✔✔The variable component of an amino acid that gives the amino acid its
identity and chemical properties
also called R-group
Chiral - answer✔✔A molecule with a nonsuperimposable mirror image
All amino acids (except glycine) are chiral
(S) Absolute Configuration - answer✔✔All amino acids (except cysteine) have an (S) absolute
configuration
L-amino Acid - answer✔✔All amino acids found in eukaryotes
Nonpolar, Nonaromatic Amino Acids - answer✔✔Glycine, alanine, valine, leucine, isoleucine,
methionine, proline
Glycine - answer✔✔Single H atom as side chain
Achiral
Smallest amino acid
Alanine - answer✔✔Alkyl side chain (1C)
Valine - answer✔✔Alkyl side chain (3C)
Leucine - answer✔✔Alkyl side chain (4C)
Isoleucine - answer✔✔Alkyl side chain (4C)
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Methionine - answer✔✔Methyl side chain (-CH3)
Contains S atom in side chain
Proline - answer✔✔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 - answer✔✔Tryptophan, phenylalanine, tyrosine
Aromaticity - answer✔✔The ability of a molecule to delocalize pi electrons around a conjucated
ring, creating exceptional stability
Tryptophan - answer✔✔Double-ring system
Contains N atom in one ring
Largest of aromatic amino acids
Phenylalanine - answer✔✔Benzyl side chain (benzene ring + -CH2 group)
Smallest aromatic amino acid
Relatively nonpolar
Tyrosine - answer✔✔Phenylalanine + -OH group
Relatively polar
Polar Amino Acids - answer✔✔Serine, threonine, asparagine, glutamine, cysteine
Polarity - answer✔✔An uneven sharing of electrons in a molecule, creating a slightly positive
side and a slightly negative side
Serine - answer✔✔-OH group in side chain
Highly polar - participate in H-bonding
Threonine - answer✔✔-OH group in side chain
Highly polar - participate in H-bonding
Asparagine - answer✔✔Amide (-NH2) side chain
Amide N do not gain or lose protons with changes in pH - do not become charged
Glutamine - answer✔✔Amide (-NH2) side chain
Amide N do not gain or lose protons with changes in pH - do not become charged
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Cysteine - answer✔✔Thiol (-SH) side chain - weaker than OH bond
Prone to oxidation
Negatively-charged (Acidic) Amino Acids - answer✔✔Aspartic acid (aspartate), glutamic acid
(glutamate)
Aspartic Acid (Aspartate) - answer✔✔Carboxylate (-COO(-)) group in side chain
Deprotonated form of asparagine
Glutamic Acid (Glutamate) - answer✔✔Carboxylate (-COO(-)) group in side chain
Deprotonated form of glutamine
Positively-charged (Basic) Amino Acids - answer✔✔Lysine, arginine, histidine
Lysine - answer✔✔Terminal primary amino group
Arginine - answer✔✔Has 3 N atoms in side chain
Charge delocalized over all three N atoms
Histidine - answer✔✔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 - answer✔✔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 - answer✔✔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 - answer✔✔Cysteine, threonine, serine, tyrosine,
tryptophan, proline, methionine, glycine
Amphoteric - answer✔✔Ability to act as an acid or a base
Can either accept or donate a proton
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For ionizable groups: tend to be protonated at low pH; deprotonated at high pH
pKa - answer✔✔The pH at which half of the species are deprotonated
[HA] = [A(-)]
pKa1 - answer✔✔pKa for carboxyl group
Usually around 2
pKa2 - answer✔✔pKa for amino group
Usually between 9 and 10
pKa3 - answer✔✔For amino acids with ionizable side chains
Amino Acids in Acidic Conditions - answer✔✔Amino acid is fully protonated
i.e. (-NH3+) and (-COOH)
Amino Acids in Neutral Conditions - answer✔✔Form zwitterions
pH is near pI of amino acid
i.e. (-COO(-)) and (-NH3(+))
Zwitterion - answer✔✔A molecule that contains charges, but is neutral overall
Amino Acids in Basic Conditions - answer✔✔Amino acid is fully deprotonated
i.e. (-COO(-)) and (-NH2)
Isoelectric Point (pI) - answer✔✔The pH at which every molecule in solution is electrically
neutral
Predominantly in zwitterion form
pI(neutral amino acid) = - answer✔✔(pKa(NH3+) + pKa(COOH))/2
For amino acids with neutral side chains
Have relatively neutral pI values (~6)
Buffer - answer✔✔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) = - answer✔✔(pKa(R group) + pKa(COOH))/2
For amino acids with negatively charged side chains
Have relatively low pI values (~3.2)
