What is meant by primary, secondary, and tertiary structure of protein?

Question

Is one of these more important than the others? Why, or why not?

Thanks!

Answer 1

Proteins are polymers of amino acids joined by peptide bonds.

The primary structure usually refers to the actual order of amino acids in the sequence making the polymer.  These are always listed from the N-terminal amino acid (which has a free -NH2 group) on the left to the C-terminal amino acid (which has a free -COOH group) on the right.

The sulfur bridges between appropriate amino acids is usually included in the primary structure because they are covalent bonds.  Some people, however, find this confusing so they say that these -S-S- bonds are part of the secondary structure.

The secondary structure refers to the way that the primary chain in the polymers interacts with itself because of hydrogen bonding.  Some the regions form helices while others may form sheets.  Each helix is called an alpha-helix to denote a clock-wise spiral.  The sheets are called beta-pleated because they tend to be folded.

The tertiary structure is the overall 3-dimensional shape of the protein including all hydrogen bonds, polar and ionic interactions and van der Waals dispersion forces.

As to importance of these structures, there are two different schools of thought.  The function of the protein (especially enzymes) is due to the final tertiary shape.  Some say that this makes the tertiary structure the most important.

But since the tertiary structure depends upon the secondary structure, which in turn depends upon the primary structure, then all of the structures are equally important.  Change one amino acid in the primary chain and all of the structures can be changed.

Answer 2

Primary - the order of the amino acids in sequence.

secondary - the interactions among the side chains (mainly alpha helixes and beta sheets

tertiary - the overall structure of the protein, how the alpha helices and beta sheets are folded to create the overall protein structure.

quarternary - is also a structure, it is the interaction of multiple protein domains to form one large protein. (like hemoglobin)