4.3. Super secondary structure
They are also called as motif, it is a folding pattern involving two or more elements of secondary structure. Motif provides function to domain. They are of different types simplest among them with specific function consists of two alpha-helices joined by a loop region.
Examples of motif are :
4.3.1. DNA binding motif :–
A motif specific for calcium bending and is present in paravalbumin, calmodulin, troponin c, and the proteins that bind calcium and thereby regulate cellular activities.
4.4. Tertiary structure of protein -
Tertiary structure of protein is a three dimensional structure of protein. Tertiary structure is formed during protein folding. Protein folding involves the interaction of diverse R group (side chain) of amino acids. Various forces are involve in formation of tertiary structure.
1. Hydrogen bonding
2. Disulphide bond
3. Electrostatic or Ionic bond
4. Hydrophobic bonding
The protein can be classified broadly based upon the tertiary structure.
(A) Globular proteins have a core of hydrophobic amino acid residue and a water exposed, polar, charged hydrophilic residue on surface. This arrangement makes protein soluble in water and stable in conformation. The disulphide bond increase the stability of protein. The protein folding process in achieved with help of chaperons
4.4.1. Importance of tertiary structure
Protein specific function depends on tertiary structure. If this is disrupted, the protein is said to be denatured and show loss its activity. eg : Denature enzymes loss its catalytic power with denatured antibodies can no longer bind with antigen. A mutation in the DNA may alter the amino acid in protein which resulted into improper folding. Misfolding protein fail to perform its function.
4.4.2. Some of examples includes :
Cystic fibrosis is caused because of failure of the mutant CFTR protein to its target place in plasma membrane.
Diabetes is caused by improper folding of mutant versions of : V2 the vasopressin (ADH) receptor and aquaporin.
Hypercholesterolemia is caused by failure of mutant low density lipoprotein receptors to reach the plasma membrane.
Osteogenesis imperfecta is caused by failure of mutant type one collagen molecules to assemble correctly.
Mutant protein form inclusion bodies that is aggregation of insoluble protein caused by nonfunctional deposits.
4.5. Quaternary structure :
Quaternary structure is formed by more than one polypeptide. A protein with multiple polypeptide chains is multimeric in nature . Multimeric protein exhibit quaternary structure.
One Subunit Monomer
Two Subunit Dimer
Three Subunit Trimer
Four Subunit Tetramer
More than four–Multimers Changes in conformation within individual subunits can cause change in quaternary structure.