“Primary - where amino acids are in the correct order in the polypeptide chain.”
“Secondary - where small structures within the chain begin to form (including alpha helices and beta pleated sheets).”
“Tertiary - where more complex structures form due to more bonds forming. This is the final, three-dimensional structure of a protein with only one chain.”
“Quaternary - when the final protein consists of more than one polypeptide chain.”