Short answers, pulled from the story.
The primary structure refers to the specific sequence of amino acids held together by peptide bonds made during protein biosynthesis. Counting residues always starts at the N-terminal end where the amino group remains free from a peptide bond.
Linus Pauling suggested two main types of secondary structure in 1951: the alpha-helix and the beta-strand. These structures are defined by patterns of hydrogen bonds between the main-chain peptide groups.
Hemoglobin serves as an example of a heterotetramer composed of two alpha and two beta chains. Complexes containing four subunits are termed tetramers while those with two subunits are called dimers.
Around 90% of the protein structures available in the Protein Data Bank have been determined by X-ray crystallography. This method allows scientists to measure the three-dimensional density distribution of electrons in the crystallized state.
Some less stable variants exist as intrinsically disordered proteins lacking a stable tertiary structure. Proteins populate ensembles of conformational states rather than existing as static objects.