BS1030 Topic 1 Lecture 4 Ionisation of amino acids

What does pH measure?

The concentration of free hydrogen ions (H⁺) in a solution.

What does a high pH indicate?

Low hydrogen ion concentration and high alkalinity.

What does a low pH indicate?

High hydrogen ion concentration and high acidity.

What is the formula for pH?

pH = -log₁₀[H⁺].

What is an acid?

A proton donor.

What is a base?

A proton acceptor.

What is a strong acid?

An acid that dissociates completely in solution.

What is a weak acid?

An acid that dissociates only partially in solution.

What is the typical pH range of human blood?

7.35 to 7.45.

Which systems regulate blood pH in the body?

The blood (haemoglobin), renal, and pulmonary systems.

What is a buffer?

A system that resists changes in pH when small amounts of acid or base are added.

What does a buffer consist of?

A weak acid and its conjugate base.

How does a buffer respond to a rise in pH?

It donates protons (acts as an acid).

How does a buffer respond to a fall in pH?

It accepts protons (acts as a base).

What is the Henderson-Hasselbalch equation?

pH = pKa + log₁₀([A⁻]/[HA]).

What does [A⁻] represent in the Henderson-Hasselbalch equation?

The concentration of the conjugate base (unprotonated form).

What does [HA] represent in the Henderson-Hasselbalch equation?

The concentration of the acid (protonated form).

What does pKa represent?

The pH at which a weak acid is 50% dissociated.

How are pH and pKa related to acid strength?

The lower the pKa, the stronger the acid.

What does a titration curve show?

The relationship between pH and the degree of dissociation of an acid.

At what point on a titration curve does buffering occur?

Within one pH unit on either side of the pKa.

What are the two ionisable groups present in all amino acids?

The α-amino group and the α-carboxyl group.

What are the ionisation states of these groups at pH 7?

The α-amino group is -NH₃⁺ and the α-carboxyl group is -COO⁻.

What is a zwitterion?

A molecule with both positive and negative charges but an overall neutral charge.

What is the isoelectric point (pI)?

The pH at which an amino acid has no net charge.

How is the isoelectric point (pI) calculated for amino acids without ionisable side chains?

pI = (pK₁ + pK₂) / 2.

What happens to an amino acid at pH values above its pI?

It carries a net negative charge.

What happens to an amino acid at pH values below its pI?

It carries a net positive charge.

What are the pKa values of glycine's ionisable groups?

pKa₁ (-COOH) = 2.34 and pKa₂ (-NH₃⁺) = 9.60.

Which group in glycine is the stronger acid?

The -COOH group.

Would glycine be an effective buffer at physiological pH (7.4)?

No, because its pKa values are too far from 7.4.

What is the range of pKa values for α-amino groups in amino acids?

Approximately 8.8 to 11.

What is the range of pKa values for α-carboxyl groups in amino acids?

Approximately 1.8 to 2.4.

What is the charge of the α-amino group at pH 7?

Positively charged (-NH₃⁺).

What is the charge of the α-carboxyl group at pH 7?

Negatively charged (-COO⁻).

What determines whether an amino acid's side chain (R group) is ionised?

Its pKa and the surrounding pH.

What happens to an R group when the pH is below its pKa?

It remains protonated (keeps its H⁺).

What happens to an R group when the pH is above its pKa?

It loses its proton (deprotonated form).

What are the pKa values for histidine?

pKa₁ (-COOH) = 1.82, pKa₂ (R group) = 6.0, pKa₃ (-NH₃⁺) = 9.17.

Why is histidine an effective buffer in biological systems?

Its side chain has a pKa near physiological pH (~6), allowing it to gain or lose protons easily.

What role does histidine play in haemoglobin?

It helps buffer blood pH by accepting or donating protons.

Which amino acid's titration curve corresponds to three ionisation steps around pH 2, 6, and 9?

Histidine.

Which titration curve pattern represents glutamate?

The one showing three ionisation steps around pH 2, 4, and 9 (option C).

Which amino acid's side chains are charged at pH 7?

Aspartate, glutamate, lysine, arginine, and histidine.

What are the pKa values of tyrosine and cysteine side chains?

Tyrosine: 10.07; Cysteine: 8.18.

Are tyrosine and cysteine side chains charged at pH 7?

No, both are largely uncharged at physiological pH.

What are the pKa values of aspartate and glutamate side chains?

Aspartate: 3.65; Glutamate: 4.25.

Are aspartate and glutamate side chains charged at pH 7?

Yes, both are negatively charged (-COO⁻).

What are the pKa values of arginine, lysine, and histidine side chains?

Arginine: 12.48; Lysine: 10.53; Histidine: 6.0.

Are arginine, lysine, and histidine side chains charged at pH 7?

Yes, arginine and lysine are positively charged; histidine can be partially protonated.

Which five amino acids have ionised side chains at physiological pH?

Aspartate, glutamate, lysine, arginine, and histidine.

Which groups in a peptide cannot ionise?

α-amino and α-carboxyl groups that are part of peptide bonds.

Which groups determine the overall charge of a protein?

The ionisable R groups and the N- and C-termini.

Which amino acid's side chain can act as a physiological buffer?

Histidine.