Protein Folding and Structure
To understand how the protein gets its final shape or conformation, we need to understand the four levels of protein structure: primary, secondary, tertiary, and quaternary. For a short (4 minutes) introduction video on protein structure click here.
The unique sequence of amino acids in a polypeptide chain is its primary structure. The linear sequence of amino acids in the polypeptide chain are held together by peptide bonds and result in the N-C-C-N-C-C patterned backbone. The primary structure is coded for in the DNA, a process you will learn about in the Transcription and Translation modules.
The local folding of the polypeptide in some regions gives rise to the secondary structure of the protein. The most common shapes created by secondary folding are the α-helix and β-pleated sheet structures. These secondary structures are held together by hydrogen bonds forming between the backbones of amino acids in close proximity to one another. More specifically, the oxygen atom in the carboxyl group from one amino acid can form a hydrogen bond with a hydrogen atom bound to the nitrogen in the amino group of another amino acid that is four amino acids farther along the chain.
The unique three-dimensional structure of a polypeptide is its tertiary structure. This structure is in part due to chemical interactions at work on the polypeptide chain. Primarily, the interactions among R groups creates the complex three-dimensional tertiary structure of a protein. The nature of the R groups found in the amino acids involved can counteract the formation of the hydrogen bonds described for standard secondary structures. For example, R groups with like charges are repelled by each other and those with unlike charges are attracted to each other (ionic bonds). When protein folding takes place, the hydrophobic R groups of nonpolar amino acids lay in the interior of the protein, whereas the hydrophilic R groups lay on the outside. These types of interactions are also known as hydrophobic interactions. Interaction between cysteine side chains forms disulfide linkages in the presence of oxygen, the only covalent bond forming during protein folding.
All of these interactions, weak and strong, determine the final three-dimensional shape of the protein. When a protein loses its three-dimensional shape, it may no longer be functional.
In nature, some proteins are formed from several polypeptides, also known as subunits, and the interaction of these subunits forms the quaternary structure. Weak interactions between the subunits help to stabilize the overall structure. For example, a multisubunit protein called insulin (a globular protein) has a combination of hydrogen bonds and disulfide bonds that hold the multiple subunits together. Each of these subunits went through primary, secondary and tertiary folding independently of one another.
Denaturation and Protein Folding
Each protein has its own unique sequence and shape that are held together by chemical interactions. If the protein is subject to changes in temperature, pH, or exposure to chemicals, the protein structure may change, losing its shape without losing its primary sequence. This process is known as denaturation. Denaturation is often reversible because the primary structure of the polypeptide is conserved in the process if the denaturing agent is removed, allowing the protein to resume its function. Sometimes denaturation is irreversible, leading to loss of function. One example of irreversible protein denaturation is when an egg is fried. The albumin protein in the liquid egg white is denatured when placed in a hot pan when heat causes the strands of protein present to unravel and stick together in less ordered blobs. Not all proteins are denatured at high temperatures; for instance, bacteria that survive in hot springs have proteins that function at temperatures close to boiling. The stomach is also very acidic, has a low pH, and denatures proteins as part of the digestion process; however, the digestive enzymes of the stomach retain their activity under these conditions.
Protein folding is critical to its function. It was originally thought that the proteins themselves were responsible for the folding process. Only recently was it found that often they receive assistance in the folding process from protein helpers known as chaperones (or chaperonins) that associate with the target protein during the folding process. They act by preventing aggregation of polypeptides that make up the complete protein structure, and they disassociate from the protein once the target protein is folded.