Structure function Relationship
The reason why protein can work systematically is partly because each protein has its own 3D structures. Thus, I think the first step to know the nature of protein, we should start to understand the relation among sequence-structure-function relationship in proteins.
Protein function can be classified into biochemical function and biological function, or molecular function and cellular function. The former function is realized by a single protein or a single protein complex, and the latter function is achieved through the interaction network of proteins.