There are numerous examples in nature. In the course of evolution, protein sequences change more rapidly than protein structures. Many enzymes therefore strikingly similar structures, but are very different in their primary sequences. An example i just dug out from an old lecture, are the enzymes benzoylformate decarboxylase and pyruvate decarboxylase. Both enzymes share the same fold (to a high degree), but just 21% of their sequences are identical.
Protein structure is the three-dimensional arrangement of atoms in a protein molecule. Proteins are polymers — specifically polypeptides — formed from sequences of monomer amino acids. By convention, a chain under 40 amino acids is often identified as a peptide, rather than a protein. To be able to perform their biological function, proteins fold into one or more specific spatial conformations driven by a number of non-covalent interactions such as hydrogen bonding, ionic interactions, Van der Waals forces, and hydrophobic packing. To understand the functions of proteins at a molecular level, it is often necessary to determine their three-dimensional structure. This is the topic of the scientific field of structural biology, which employs techniques such as X-ray crystallography, NMR spectroscopy, and dual polarisation interferometry to determine the structure of proteins.