Tyrosine

Tyr · Y · 181.19 Da

Glucogenic and ketogenicAromaticHydroxyl-containing Non-essential

Phenol (aromatic -OH)

Phenylalanine plus a hydroxyl, which makes it polar and weakly acidic. Precursor of dopamine, adrenaline, melanin and thyroid hormone.

Mnemonic

tYrosine. The Y is in the name, and the letter looks like the phenol stub.

Ionization ladder

pKa	Group	Change	Charge
2.20	α-carboxyl	-COOH → -COO⁻	0 → −1
9.11	α-amino	-NH₃⁺ → -NH₂	+1 → 0
10.07	side chain	neutral → deprotonated	0 → −1

pI = (2.20 + 9.11) / 2 = 5.65, the midpoint of the two pKa values that bracket the neutral species.

Net charge across pH

pH 1+1

pH 40

pH 70

pH 10−1

pH 13−2

A group is deprotonated once the pH climbs past its pKa. Add the contributions and you have the net charge, with no memorization required.