Proteins and Enzymes

ID #2181

Do all polypeptides have tertiary structure? I am confused, are they proteins that are three-dimensional making them tertiary, but have no function so they are not considered a protein, or are they able to just be even primary structure since it is just repeating nucleotides?


Protein folding can be confusing, but try to focus on what you have at each stage of the folding process. 

First, proteins and polypeptides are both made up of amino acids, not nucleotides. Multiple amino acids are joined to each other through peptide bonds to create a chain of amino acids (called a polypeptide). When that polypeptide has a purpose and is functional we call it a protein. 

Polypeptides and proteins both have primary structure because they both have a linear arrangement of amino acids formed into a chain through peptide bonding, and the chain can be read N-C to see the sequence of the polypeptide or protein. 

Unless they are denatured, polypeptides and proteins have secondary structure. Secondary structure is folding of that single chain into alpha helices and beta strands, structures which are formed by hydrogen bonding between backbone N-H and C=O groups. So, that's why we say secondary structure is independent of the R-group. IT'S OUTSIDE the scope of this class, but some polypeptides are functional proteins at the secondary structure level. 

If polypeptides and proteins have interactions between their R-groups or between their R-groups and backbone N-H and C=O groups, they have tertiary structure. ANY PROTEIN OR POLYPEPTIDE WITH THREE DIMENSIONAL SHAPE HAS TERTIARY STRUCTURE. 

At this point, if the polypeptide with tertiary structure is functional, we would call it a protein (and that type of protein wouldn't have quaternary structure). However, if the polypeptide with tertiary structure is not functional (ie. it can't do its job without help from additional polypeptide subunits), then it has to associate with the other polypeptides creating quaternary structure in order to become a functional protein. Again, not every protein would have quaternary structure, because not every protein is made up of more than one polypeptide to be functional. 

You'll want to review your lecture notes to understand the hydrophobic, ionic, hydrogen bonding, and other interactions that occur during the tertiary and quaternary levels of protein folding. 

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