Proteins are made of only 20 amino acids. Peptides can use any amino acid. Proteins are made of one or more polypeptides.
Peptides are distinguished from proteins on the basis of size. Peptides with fewer than about 10-20 residues may also be called oligopeptides; those with more, polypeptides. A protein contains at least one long polypeptide chain of a specific sequence of more than about 50 amino acid residues.
Nomenclature and Symbolism for Amino Acids and Peptides
3AA-11 to 3AA-13(http://www.sbcs.qmul.ac.uk/iupac/AminoAcid/A1113.html)
Proteins = polypeptides.
Peptides are up to 10 kDa, above this molecular mass it will be a protein.
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Protein is consisted of polypeptide chain. There is not a clear border between peptides and proteins. For peptides formed from fewer than 20 amino acid residues, we use in general the term of oligopeptide either we use the exact number of amino acid residue in order to characterise it (e.d, di-peptide, tri-peptide, tetra-, penta-, hexa-...etc for peptides consisted of 2, 3, 4, 5, 6-...amino acid residues respectively). Normally a polypeptide chain, consisted of more 20-30 amino residues, could be characterised as a protein as well.
also, an amino acid is called peptide,2 amino acids are dipeptide,3 amino acids are tripeptide, and so on, Peptides with fewer than about 10-20 residues may also be called oligopeptides. So, the polypeptide is composed of more than 10-20 amino acids till 50 amino acids .So, protein is composed of 50 and more than 50 amino acids and it has a function and specific shape, So usually it consists of one or more polypeptides.
PEPTIDES:- Compounds of aminoacids linked by peptide bonds.
DIPEPTIDES:- --------- 02 aminoacids.
TRIPEPTIDES---03, TETRAPEPTIDES----04, PENTAPEPTIDES-----05, HEXA------06, HEPTA----06, OCTA-----08, NONA-----09, DECA-------10 etc
OLIGOPEPTIDES :- -------11 to 20 amionacids.
POLYPEPTIDES:- ----------21 to 50 amionacids.
PROTEINS:- Compounds formed by more than 50 aminoacids linked by peptide bonds.
Dipeptides to decapeptides exists in biological fluids as such.
Oligopeptides, polypeptides, and proteines above classified exists as structural and functional compounds.
They are kind of similar, however Proteins are larger in size than Peptides
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