UPDATED Protein Structure Series: • Protein Structure (Par...
Moof's Medical Biochemistry Video Course: moof-university...
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The four levels of protein structure EXPLAINED in this video!
Primary structure of proteins describes the actual sequence of amino acids in a polypeptide; namely, it describes the order in which the amino acids are connection reading from the N (amino) terminus to the C (carboxy) terminus of a polypeptide. Two polypeptides (or proteins) can have the same amino acid contents, but if they are connected in different orders in the two polypeptides, the two polypeptides will end up folding in different ways and yield different structures. It's often said that, because of this idea, the primary structure of a protein determines its overall 3D structure.
Secondary structure of proteins describes the ordered patterns of 3D arrangements on localized regions of the BACKBONE of the polypeptide. The word BACKBONE is particularly important, as it implies the repeating N-C-C units of each amino acid, and specifically the interactions between the components of said backbone, the carbonyl and amide. It is often mentioned that the interactions that hold secondary structure together are hydrogen bonds, which is, of course, true. It is also mentioned that secondary structure consists of alpha helices and beta pleated sheets are the actual secondary structures held together by said hydrogen bonds. That's also true. It must be specifically noted, though, that the hydrogen bonds that hold together the alpha helices and beta pleated sheets are the hydrogen bonds between the carbonyl oxygen and the amide hydrogen of the backbone. This does NOT include any other hydrogen bonds (like those that hold together side chains), which brings me to ....
Tertiary structure of proteins is often describe simply as the overall 3D arrangement of a polypeptide after folding. That's hardly it. Tertiary structure further includes the interactions between and among the side chains of the amino acids, as well as the interactions between and among secondary structures. Prosthetic groups, which are non amino acid components of proteins, that are a part of a protein's overall structure, are included in tertiary structure as well. The different types of interactions between side chains include the noncovalent interactions (hydrophobic interactions, hydrogen bonds, and electrostatic interactions) as well as a specific type of covalent interaction (disulfide bonds / disulfide bridges). All are important components of a protein's tertiary structure.
Quaternary structure of proteins simply describes the idea that multiple polypeptide chains can come together and form a multi-subunit protein. If and when this happens, it is classified as quaternary structure.
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28 авг 2024
