The function of a protein is directly related to its structure.
Structural similarities can point to functional and evolutional relationships between proteins.
Finding the structure of a proteins is, however, a long and costly process. Theory and experiments indicate that protein structure is fully defined by its amino-acid sequence. Unlike the 3D structure, the amino-acid sequence of a protein is easily available. Protein folding problem is given a protein amino-acid sequence to compute it's 3D structure. This is an outstanding open problem in molecular biology.