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  Stability of Proteins, Biology tutorial

Introduction:

Noncovalent forces play the extremely important role in stability of proteins. Whenever such forces are compromised, native protein denatures. These forces are weak forces with strengths (1-7 kcalmol-1) far less than those of covalent bonds (≥ 50 kcalmol-1).

Protein Folding:

Main structure of protein is capable to fold impulsively to its native conformation. Native conformation is functional, folded conformations of proteins. Protein's internal residues direct the folding and are driven mainly by noncovalent hydrophobic interactions among nonpolar residues. In the beginning, folding is begun by those short-range noncovalent interactions between the side chain and its nearest neighbors which create secondary structures in small regions of polypeptide. This is followed by long-range interaction between, say, 2 α-helices which come together to create stable supersecondary structures. Procedures, then, continues until complete domains form and complete polypeptide is folded.

Protein folding doesn't continue impulsively all the time. They are in numerous examples, helped by the class of proteins known as molecular chaperones. These proteins interact with partly folded and inappropriately folded polypeptides, helping accurate folding by preventing protein aggregation previous to folding. Noncovalent forces don't only help in folding of polypeptide chains in unique native conformation, they also assist to stabilize native structure against denaturation- a procedure which engages lose of native conformation.

Forces which Stabilize Protein:

Folding of polypeptides in their exclusive native conformation is mainly governed by weak noncovalent forces that also assist to stabilize proteins. Stability here signifies tendency to keep native conformation. Major noncovalent forces comprise: hydrogen bonding, hydrophobic interaction, electrostatic forces, and van der Waals-London Dispersion Forces.

i) Hydrophobic Interaction:

Hydrophobic interaction is a force which causes nonpolar substances and nonpolar portions of amphipathic molecules (molecules with both polar and nonpolar regions like proteins) to minimize their contact with water. Amphipathic substances can form, in aqueous solution, the stable structure called as micelle in which hydrophobic portions cluster together so as to provide lowest interaction with aqueous solvent, while polar/hydrophilic portions of proteins are set to support or maximize contact with water environments.

ii) Hydrogen Bonding:

Hydrogen bonding is created when the hydrogen atom, covalently bonded to the electronegative atom, is shared with the second electronegative atom. In proteins, hydrogen bonding is found mainly between side chains of hydrophilic group of amino acids. Functional groups of peptide backbone can also create hydrogen bonds. The majority of the hydrogen bonds in proteins are local, meaning that they engage hydrogen bonding mates which are close together in sequence. Secondary structures of proteins are stabilized by hydrogen bonding. α-helix and β-conformations are widely hydrogen -bonded.

iii) Electrostatic Interaction:

Electrostatic interactions (ionic or salt linkages) are strongest of all noncovalent forces. In proteins, they can happen between positive charges on His, Lys, Arg and α-amino groups and negative charges of Asp, Glu, and α-carbonyl group. Depending on whether interacting charges are same or opposite, electrostatic forces could be repulsive or attractive.

Intramolecular ionic bonds are rarely utilized in stabilisation of protein structure. When they are utilized, it is frequently with great effect. For instance, ionized groups stabilize interactions between proteins and other molecules (like cofactors, prosthetic groups e.t.c.).

iv) Van der Waals-London Dispersion Forces:

These are the weakest of noncovalent forces. Van der Waals force is transient dipole. Transient dipole can interrelate with charged groups and with lasting dipoles. Likewise, they can make dipole.

Interaction between the transient dipole and dipole they induce are the most significant contribution to attractive forces between neutral atoms. They are called as London, Heitler or dispersion forces. Dispersion forces may be among the most significant in deciding stability of the tertiary structure of protein as all atoms of protein are engaged. Though individual dispersion forces are of very short range, general resultant force of the extremely large number of such dipoles is of long-range interaction.

Protein Denaturation:

Ribosomes are cellular organelles which are known for protein synthesis. Initially, they generate polypeptide chains of amino acids residues which should fold in functional native conformation. Stability of native conformation can be lost. Denaturation is loss of native secondary, tertiary, and /or quaternary structure of protein. Main structure is not usually changed by denaturation. This means that denaturating agents usually affect weak interactions, leaving covalent bonds of polypeptide intact. At times, denatured state is related with loss of protein's function though this is not always case.

Denaturating Agents:

Proteins are denatured by the diversity of situation and substances:

i) Heating- when protein is heated, its conformationally sensitive properties like optical rotation, viscosity and UV absorption, change abruptly over the narrow range of temperature.

Abruptness of change recommends that unfolding is cooperative process. Any incomplete unfolding of structure destabilizes remaining structure that should concurrently collapse into random coil.

ii) Change in pH- variations in pH modify ionization states of amino acid side chains. This leads to alteration in net charge on protein and changes in H bonding requirements.

iii) Treatment with Detergents- Such substances influence protein structures by tempering with hydrophobic interactions liable for protein's native structure.

iv) High Concentration of Organic Solvents- organic solvents like alcohol and acetone also interfere with hydrophobic interactions thus stabilizing protein structure.

v) Certain solutes such as urea and guanidine hydrochloride also denature proteins. Also, different salts exhibit more variable influences on protein structure.

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