Yes, in most of the sources something is missing in defining protein.

Here, something is shared, which emerges on a discussion over the topic with my coauthor during preparation of the manuscript on the chapter on protein in a book on Biochemistry which is at present at the stage of editing.

It is tried to define protein as:

"Proteins are biomacromolecules with conformation dependent functional specificity, formed as linear heteropolymer of alpha amino acids linked by peptide bonds in specific sequences, translated in the ribosome under the modulation of transcribed polyribonucleotides from segments of polydeoxyribonucleotide based information pool forming the set of nano tools for maintaining the structural and functional integrity of the living system through metabolism."

In the functional aspect, inclusion of the most fundamental function of protein as nano tool in metabolism is providing one missing component in definitions for protein and mentioning about the conformation dependent functional specificity in protein is appreciated.