3 DIMENSIONAL STRUCTURE OF PROTEIN

The three dimensional (3D) structure of a protein is determined by its sequence and the function of a protein is determined by its 3D structure.

The relationship between the protein sequence and its 3D structure is still under investigation. Some similar sequences give completely different structures and some different sequences results in similar structures.

Four levels of structure:

1.    Primary 

·       Amino acid sequence, amino acids are COVALENTLY linked (peptide bonds!) and any disulfide bridges are included in the primary structure. 

 

 The following protein structures are dependent upon NON-COVALENT interactions:

2.   Secondary

·       Regular polypeptide folding patterns such as helices, sheets and turns that occurs within adjacent amino acids.

-Alpha helix

-Beta conformation

3.   Tertiary

·       Spatial relationship between all peptides in a protein. Protein folding brings sequentially distant amino acids close together.

·       Fibrous, long strands or sheets, structural proteins with a single secondary type of secondary structure.

·       Globular, spherical; complex structures with many types of secondary structure. 

·       Enzymes and polypeptide hormones are usually globular peptides.

4.   Quaternary structure

·       Arrangement of polypeptide chains (subunits) within a protein.

Note: Sometimes it is difficult to differentiate between secondary and tertiary structure