Levels of Protein Structure and Primary Structure – Peptides

00:01 Now that we have a basic understanding of peptide bonds and the geometry of proteins, let's now begin to understand protein structure at the four levels I talked about at the beginning of the lecture. The first of these is primary structure. The primary structure of a protein is the sequence of the amino acids within that protein. As we will see, the sequence of amino acids give a protein all of its characteristics. The second level of structure of a protein for us to understand is that of secondary structure. This involves interactions between amino acids that are fairly close together in primary sequence, but are not immediately adjacent to each other. On average these are interactions between amino acids that are about 10 or fewer amino acids away. Tertiary structure arises as a result of interactions between amino acids that are quite distant from each other in primary sequence, that is, greater than 10 amino acids from each other. These interactions happen because proteins can fold and bring regions that are not otherwise close together into close proximity. The last type of protein structure I want to discuss is that of quaternary structure, and this arises not for interactions within a protein, but actually interactions between separate protein units.

01:29 Now the primary structure of a protein, as I mentioned, is the sequence of amino acids. And you see on the screen a depiction of the sequence of the amino acids as if they were beads on a string. And we could imagine that for beads on a string, if we had 20 different beads, that we were stringing together to make a necklace, for example, that we would have a virtually infinite number of ways in which we could arrange the beads in terms of color, position and number within that necklace. The number of necklaces we could make, it would be quite varied. It’s for that reason that the primary sequence of amino acids gives rise to proteins that are so diverse in chemistry and biochemistry, and in function. Now as I noted, the primary structure of a protein arises from peptide bonds that are created in ribosomes that hold the individual amino acids together. The instructions for putting those amino acids together comes from information in the RNA using the genetic code, and the synthesis of a protein begins at the amino end, that we talked about before, and terminates at the carboxyl end. Ultimately all the properties of a protein, that is, the secondary structure, that tertiary structure, and possible quaternary structure, all of these properties are ultimately determined by the sequence of amino acids within the protein. Change the sequence of amino acids within a protein; we change the other properties of the protein.