Reference
How to read a peptide sequence
A peptide is written as a string of letters, and the string carries more information than it looks like it does — including which end is which, and what has been done to the molecule.
Direction comes first
A peptide chain has two different ends. One finishes with a free amine group and is called the N-terminus; the other finishes with a free carboxylic acid group and is the C-terminus.
By universal convention a sequence is written N-terminus first, left to right. That is not decoration. The same residues in the opposite order are a different molecule, in the same way that two words with the same letters are different words.
Two notations for the same thing
Sequences are written either in three-letter codes separated by hyphens, or in single letters run together. They mean exactly the same:
Gly-His-LysandGHK— the same tripeptide.Lys-Pro-ValandKPV— likewise.
Which is why two compounds in our range have names that are not names at all but their own sequences. GHK is glycine-histidine-lysine and KPV is lysine-proline-valine. Three letters, three residues, nothing hidden.
What the extra notation means
A sequence rarely appears on its own. The symbols around it record modifications, and they change the molecule:
-
Ac-at the start. An acetyl group on the N-terminus. Common, because it removes the free amine that enzymes recognise as a starting point. -
-NH2at the end. The C-terminus has been amidated — converted from an acid to an amide. Also a stability modification. -
D-before a residue. The mirror-image form of that amino acid. Natural proteins are built almost entirely from L-amino acids, so a D-residue is a deliberate choice — FOXO4-DRI is built entirely from them. -
cyclo(…). The chain is joined into a ring rather than left with two ends, which makes it considerably harder to break down. - A bracket or bridge between two cysteines. A disulfide bond, closing part of the chain into a loop. Oxytocin has one.
Numbers in a name
A fragment is often written with the residue range it came from. GHRH(1–29)
means the first twenty-nine residues of growth hormone-releasing hormone — not the
whole hormone, and not a different molecule that happens to resemble it.
This is worth reading carefully, because a fragment and its parent are routinely discussed as though they were the same substance. TB-500 and thymosin beta-4 are the clearest example in this range: one is a short synthetic piece of the other, and they are frequently sold and written about interchangeably.
What a sequence does not tell you
It gives you the identity of the molecule and nothing else. It does not tell you the purity of a particular batch, how much peptide is in a vial, or whether what you have been sent matches what the label says. Those are questions for a certificate, not for a sequence.
