Pyrrolysine Unit Converter
Pyrrolysine Unit Converter
Convert pyrrolysine between mg/dL and µmol/L at 255.318 Da. Read the next sentence before you use it: pyrrolysine does not occur in human protein and no clinical laboratory measures it, so this is arithmetic for a biochemistry audience and not a laboratory tool.
Pyrrolysine converter
mg/dL ⇄ µmol/LPyrrolysine 1.000 mg/dL, in a synthetic standard prepared for a mass spectrometry method
Formula and conversion factor
mg/dL = µmol/L ÷ 39.1668
- 39.1668
- derived from the formula mass of pyrrolysine, 255.318 Da, by the same rule every converter on this site uses: one mg/dL is 0.01 g/L, and 0.01 ÷ 255.318 × 10⁶ = 39.1668 µmol/L
- 255.318
- C12H21N3O3, summed from the IUPAC 2021 standard atomic weights: carbon 12 × 12.011 = 144.132, hydrogen 21 × 1.008 = 21.168, nitrogen 3 × 14.007 = 42.021, oxygen 3 × 15.999 = 47.997. PubChem CID 5460671 prints 255.31, which is the same number to its own precision
- Pyl, O
- the three-letter and one-letter codes. The single-letter code O is the one to watch: it is easy to read as a zero in a sequence alignment, and pyrrolysine is the only residue that uses it
- what this is not
- a clinical measurement. No reference interval is printed on this page and none exists. If a report in front of you names pyrrolysine, the likeliest explanations are a transcription error, a research assay, or a proteomics search space that included the residue
Worked example
Pyrrolysine 1.000 mg/dL, in a synthetic standard prepared for a mass spectrometry method
1.000 × 39.1668 = 39.17 µmol/L
There is no reference interval to read that against, and the page deliberately offers none
For orientation among residues that are measured: the same 1.000 mg/dL of lysine, at 146.19 Da, would be 68.4 µmol/L, and of glycine, at 75.07 Da, 133.2 µmol/L. Pyrrolysine is heavier than both, so a given mass is fewer moles
A plasma amino acid profile reports about twenty analytes and pyrrolysine is not among them. Nothing on a clinical report converts with this factor
Where pyrrolysine sits among the proteinogenic amino acids
| Residues | Codon | Occurs in human protein? | |
|---|---|---|---|
| The standard twenty | Ala to Val | The canonical genetic code | Yes |
| Selenocysteine (Sec, U) | 1 | UGA read through a SECIS element | Yes — about 25 human selenoproteins |
| Pyrrolysine (Pyl, O) | 1 | UAG reassigned by the pyl gene cluster | No |
An honest account of what this page is for
This is a unit converter for a molecule that no clinical laboratory measures. It exists because an older page on this site was indexed under this title, and removing it would break a link that people follow. What it should not do is imply a clinical use it does not have, so the standfirst says what it is and this section says why.
Pyrrolysine is the 22nd proteinogenic amino acid. It is a lysine residue whose epsilon-amino group carries a (4R,5R)-4-methylpyrroline-5-carboxylate ring, and it is inserted at a UAG codon that has been reassigned from its usual job as a stop signal. The reassignment is not a misreading: it requires a dedicated amino-acyl tRNA synthetase and tRNA, encoded by the pylTSBCD gene cluster, and it is genuinely a 22nd letter in those organisms’ genetic code rather than a post-translational modification. The best-studied setting is methylamine methyltransferase in Methanosarcina, where the pyrrolysine residue sits in the active site and does chemistry that lysine cannot.
None of that happens in a human cell. Humans have no pyl cluster, no pyrrolysyl-tRNA synthetase and no pyrrolysine in any protein. The residue has become a familiar reagent in synthetic biology precisely because it is foreign: the pyrrolysyl-tRNA synthetase and tRNA pair is orthogonal to the mammalian translation machinery, which is what makes it the workhorse for genetic code expansion and for site-specific incorporation of unnatural amino acids into proteins expressed in human cells. If pyrrolysine has reached you at all, that is by far the likeliest route.
So the arithmetic on this page is correct and the factor is re-derived from the formula mass like every other converter here, but the page carries no reference interval, no bar and no verdict, because supplying any of those would be a fabrication. If you want the 21st amino acid, which is a different matter entirely and is measured, the neighbouring page is the selenocysteine unit converter. If you want the residue pyrrolysine is built from, the lysine converter is the clinical page.
Frequently asked questions
Is pyrrolysine measured in a clinical laboratory?
No. There is no routine or specialist clinical assay for pyrrolysine, no reference interval, and no disease state defined by its concentration. It does not appear in a plasma or urine amino acid profile, which report the standard residues. This page converts units and deliberately prints no interval, because there is nothing to print.
What is the molecular weight of pyrrolysine?
255.318 Da, for the free amino acid C12H21N3O3. That is the sum of the IUPAC 2021 standard atomic weights: 12 carbons at 12.011, 21 hydrogens at 1.008, three nitrogens at 14.007 and three oxygens at 15.999. PubChem CID 5460671 prints 255.31. The residue mass inside a peptide is 237.30, which is the free amino acid less the water lost in forming the peptide bond — use the residue mass for a proteomics calculation and the free mass, above, for a solution concentration.
Why is pyrrolysine called the 22nd amino acid?
Because it is the twenty-second residue for which a dedicated biosynthetic and translational apparatus exists, rather than a modification applied after translation. The standard twenty are followed by selenocysteine, the 21st, and pyrrolysine, the 22nd. Both are inserted at codons that otherwise mean stop — selenocysteine at UGA and pyrrolysine at UAG — and both need their own synthetase and tRNA. Hydroxyproline and phosphoserine, by contrast, are made by modifying a residue that translation already placed, and are not counted.
Which organisms use pyrrolysine?
Certain methanogenic archaea, most notably the genus Methanosarcina, and a small number of bacteria. In those organisms it is found in the methylamine methyltransferases of the methanogenesis pathway, where the pyrroline ring participates directly in catalysis. It has not been found in any eukaryote.
Then why does this converter exist?
For two reasons, and neither of them is clinical. An older page on this site is indexed under this title and the link should keep working. And the conversion is genuinely wanted by people preparing standards or working on genetic code expansion, where pyrrolysine is a common reagent rather than an analyte. The page says all of this rather than dressing itself up as a laboratory tool.
Related calculators
References
- Srinivasan G, James CM, Krzycki JA. Pyrrolysine encoded by UAG in Archaea: charging of a UAG-decoding specialized tRNA. Science. 2002;296(5572):1459–1462. doi:10.1126/science.1069588
- Hao B, Gong W, Ferguson TK, James CM, Krzycki JA, Chan MK. A new UAG-encoded residue in the structure of a methanogen methyltransferase. Science. 2002;296(5572):1462–1466. doi:10.1126/science.1069556
- Gaston MA, Jiang R, Krzycki JA. Functional context, biosynthesis, and genetic encoding of pyrrolysine. Curr Opin Microbiol. 2011;14(3):342–349. doi:10.1016/j.mib.2011.04.001
- Wan W, Tharp JM, Liu WR. Pyrrolysyl-tRNA synthetase: an ordinary enzyme but an outstanding genetic code expansion tool. Biochim Biophys Acta. 2014;1844(6):1059–1070. doi:10.1016/j.bbapap.2014.03.002
- PubChem Compound Summary CID 5460671, Pyrrolysine. National Center for Biotechnology Information. Molecular formula C12H21N3O3, molecular weight 255.31. Accessed 2026.
- IUPAC Commission on Isotopic Abundances and Atomic Weights. Standard Atomic Weights 2021: C 12.011, H 1.008, N 14.007, O 15.999. The 255.318 used on this page is summed from these.
- data/_factors.json, analyte
pyrrolysine: mw 255.318; "It does not occur in human protein and no clinical assay measures it."
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.