For DNA one can distinguish between
protein-coding DNA sequences, i.e. nucleic acid sequences inside DNA (vs. non-coding sequences)
DNA sequences that do not code for proteins but are transcribed into functional RNA
non-transcribed DNA sequences that have other (e.g. regulatory or structural) functions
non-transcribed and non-functional DNA sequences (‘junk DNA’)
The relative proportions of these seem to depend heavily on the organism and are only roughly known.
What I wonder is whether there are protein sequences (i.e. amino acid sequences inside a protein) that have no functional purpose.
A non-functional protein sequence might be a sequence that is neither effective to the outside (e.g. as a site for post-translational modification) nor to the inside (e.g. in folding). But even those parts of the protein might contribute structurally by their sheer presence and mass which is important when considering proteins not only as chemical but also as somehow mechanical devices. This might be called its structural purpose.
Assuming that each part of a protein has either a functional or only a structural purpose the question arises what the relative proportions of functional vs. only structural parts are.