Ch 4 pH Buffer and Amino Acids Flashcards
What does K_a represent?
A numerical value of acid strength
K_a is a measure of the dissociation of an acid into hydrogen ion and conjugate base when dissolved in water.
What is the formula for K_a?
K_a = [H⁺][A⁻]/[HA]
This formula represents the concentrations of hydrogen ions, conjugate base, and undissociated acid.
How does the K_a value relate to acid strength?
The larger the K_a value, the stronger the acid.
What is pK_a?
pK_a = -log K_a
pK_a is often used to express the strength of an acid.
What does a lower pK_a value indicate?
The stronger the acid.
What is the relationship between pH and pK_a?
pH can be altered by adding a base or acid.
What is a buffer?
A weak acid whose pH resists change upon addition of either more acid or more base.
What components make up a buffer?
A weak acid and its conjugate base.
What is the common buffer found in cells?
H₂PO₄⁻/HPO₄²⁻
This buffer is critical for maintaining the pH of biological fluids.
What happens when acid is added to a buffer?
The buffer binds H⁺ ions.
What happens when a base is added to a buffer?
The buffer releases H⁺ ions.
What is buffer capacity?
Buffers do not have unlimited ability to withstand pH change if large amounts of acid or base are added.
When is a buffer’s maximum capacity reached?
When pK_a is equal to the pH, the acid is 50% dissociated.
How does pH relate to pK_a in the Henderson-Hasselbalch equation?
If [A⁻] > [HA], then pH > pK_a; if [A⁻] < [HA], then pH < pK_a.
What is the effective buffer range?
A buffer is effective in a range of about 1 pH unit above or below the pK_a.
What is the principal buffer in blood?
H₂CO₃/HCO₃⁻.
What can hyperventilation result in?
Increased blood pH.
What can hypoventilation result in?
Decreased blood pH.
What is the structure of an amino acid?
Contains an α-carboxyl group, an α-amino group, and an α-carbon.
What distinguishes one amino acid from another?
The R group.
How many different R groups are commonly found in proteins?
20.
What is unique about glycine?
It has no side chain and is the only achiral amino acid.
What type of amino acid is proline?
An imino acid.
What are the classifications of amino acids based on their side chains?
- Non-polar
- Polar
- Charged
- Uncharged
- Acidic
- Basic
- Aromatic
List the non-polar amino acids.
- Ala (Alanine)
- Val (Valine)
- Leu (Leucine)
- Ile (Isoleucine)
- Pro (Proline)
- Phe (Phenylalanine)
- Trp (Tryptophan)
- Met (Methionine)
What are polar amino acids?
- Gly (Glycine)
- Asn (Asparagine)
- Gln (Glutamine)
- Ser (Serine)
- Thr (Threonine)
- Cys (Cysteine)
- Tyr (Tyrosine)
What characterizes acidic amino acids?
They donate protons and become negatively charged at physiological pH.
What characterizes basic amino acids?
They can accept a proton and are positively charged at physiological pH.
What is the representation of acidic amino acids?
- Asp (Aspartic Acid)
- Glu (Glutamic Acid)
What is the representation of basic amino acids?
- Arg (Arginine)
- Lys (Lysine)
- His (Histidine)
What are the acidic amino acids that are negatively charged at physiological pH?
- Asp
- Glu
- Cys (somewhat)
- Tyr (somewhat)
Acidic amino acids have a carboxyl group that can donate a proton, leading to a negative charge.
Which amino acids are classified as basic amino acids that are positively charged at physiological pH?
- Arg
- Lys
- His
Basic amino acids have side chains that can accept protons, resulting in a positive charge.
What group in amino acids has a neutral charge?
The amide group
The side chain amide has O=C bonded to an amine, resulting in no net charge.
Why aren’t Asparagine or Glutamine considered basic amino acids?
They do not have a positively charged side chain at physiological pH.
How many pK_a values do all amino acids have at least?
Two pK_a values
This includes the pK_a of the amino group and the carboxylic acid group.
What is the pK_a of the amino group in amino acids?
About 9.5
What is the pK_a of the carboxylic acid group in amino acids?
About 2
At neutral pH, what happens to the amino group and the carboxyl group?
- Amino group gains a proton
- Carboxyl group loses its proton
What does the titration curve of an amino acid represent?
The reaction of each functional group with hydroxide ions.
What is the Zwitterion?
The state of an amino acid where the positive NH3+ is balanced by the negative COO-
This occurs at the isoelectric point (pI).
What are the two types of peptide bond formation reactions?
Condensation reaction
A condensation reaction occurs between the carboxylate ion of one amino acid and the amino group of another.
What is released during the formation of a peptide bond?
Water
Why is there little rotation around the peptide bond?
Due to its partial double bond character
Define ‘Peptide’.
A relatively short string of amino acids.
Define ‘Polypeptide or protein’.
A longer string of amino acids, often 100 or more.
What is the N-terminus of a peptide?
The start or left end of a peptide or protein, free amino terminus.
What is the C-terminus of a peptide?
The end or right end of a peptide or protein, free carboxyl terminus.
Why are the side chains of amino acids important?
They contribute most to localized charges on proteins.
At physiological pH, what is the charge of the side chains of Asp and Glu?
Normally deprotonated (COO-)
At physiological pH, what is the charge of the side chains of Lys and Arg?
Normally protonated (NH3+)
What happens to the charge of Cysteine at pH 9.0?
Cysteine deprotonates from its neutral form.
What are essential amino acids?
- Phenylalanine
- Valine
- Tryptophan
- Threonine
- Isoleucine
- Methionine
- Histidine
- Arginine
- Lysine
- Leucine
Essential amino acids must be supplied by the diet.
What are non-essential amino acids?
- Alanine
- Asparagine
- Aspartic acid
- Cysteine
- Glutamic acid
- Glutamine
- Glycine
- Proline
- Serine
- Tyrosine
Non-essential amino acids can be synthesized by the body.
What is the net charge of a peptide at physiological pH if the pK values are less than 7.4?
They tend to deprotonate.
What is the net charge of a peptide at physiological pH if the pK values are greater than 7.4?
They tend to protonate.
What is the net charge on the peptide ‘ASP-GLY-ALA-VAL-ILE-THR-MET-TRP-LYS-PRO-ASN-GLN-PHE-LEU-LYS-GLU-SER-ARG’ at neutral pH?
8
What is the total net charge of the peptide ‘ASP-GLY-ALA-VAL-ILE-THR-MET-TRP-LYS-PRO-ASN-GLN-PHE-LEU-LYS-GLU-SER-ARG’ at neutral pH?
-2
