Histidine
Properties
| State | Solid (white crystalline powder) |
| Color | White |
| Solubility | Soluble in water (41 g/L at 25°C); slightly soluble in ethanol |
| Melting Point | 287°C (decomposes) |
| Boiling Point | Decomposes before boiling |
About Histidine
Histidine is the amino acid that does almost everything an enzyme needs done. Its imidazole side chain has a pKa right around 6.0 — the only standard side chain that flips between protonated and neutral forms across the physiological pH range. That seemingly modest property turns out to be the central trick behind a huge fraction of enzyme catalysis. The serine protease catalytic triad (His-Asp-Ser, found in chymotrypsin, trypsin, and the entire S1 family) uses His as the proton shuttle that activates serine for nucleophilic attack on the peptide bond. Carbonic anhydrase coordinates its catalytic Zn(2+) with three histidines in a perfect tetrahedral pocket, then uses a fourth histidine as a proton shuttle to regenerate the active enzyme — turnover number around 10^6 per second, one of the fastest enzymes known. Hemoglobin's proximal His F8 holds the heme iron in place, and the distal His E7 hydrogen-bonds to bound O2 to discriminate against CO. Histidine is also the precursor to histamine via histidine decarboxylase (the enzyme antihistamines exist to outcompete), and the dipeptide carnosine (beta-alanyl-L-histidine) buffers skeletal muscle pH during anaerobic exercise — concentrations in human soleus muscle hit 20 mM. Histidine is essential in human diet (we can't synthesize the imidazole ring de novo).
Where you'll encounter it
If you've ever purified a recombinant protein on a Ni-NTA column, taken an antihistamine for allergies, or felt your muscles burn during a sprint (carnosine doing pH buffering), you've relied on histidine chemistry. Walk into any molecular biology lab and the most common single workflow on the bench is His-tag purification — six histidines tacked onto your recombinant protein chelate Ni2+ on agarose beads, and a 250 mM imidazole elution gives you 90% pure protein in two hours. The Benadryl in your medicine cabinet exists specifically to outcompete histamine (made by decarboxylating histidine in mast cells) at the H1 receptor. And every time a sprinter pushes past the burn at 400 meters, intramuscular carnosine — beta-alanyl-L-histidine at 20 mM in soleus muscle — is buffering the pH crash from anaerobic glycolysis.
Common Uses
- Catalytic acid-base residue in serine proteases, ribonucleases, and phosphatases
- Zn(2+), Cu(2+), and Fe coordination in metalloenzyme active sites
- His6-tag affinity ligand for IMAC protein purification on Ni-NTA or Co-TALON resins
- Precursor for histamine via histidine decarboxylase in mast cells
- Component of carnosine, the intramuscular pH buffer (pKa 6.83)
- Parenteral nutrition component for infants (essential below age 1)
- Biochemistry teaching example for amino acid pKa and enzyme catalysis
Safety Information
Generally recognized as safe (GRAS, 21 CFR 172.320) for dietary use. No OSHA PEL established. Histidinemia, an autosomal recessive deficiency in histidase, raises blood histidine 5-10x normal but is now considered clinically benign (newborn screening for it has largely been discontinued). Excess oral doses above 24 g/day have been associated with elevated zinc excretion. Not classified as hazardous under GHS.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.