Denatured proteins can exhibit a wide range of characteristics, from loss of
solubility to
protein aggregation.
Background Proteins or
polypeptides are polymers of
amino acids. A protein is created by
ribosomes that "read" RNA that is encoded by
codons in the gene and assemble the requisite amino acid combination from the
genetic instruction, in a process known as
translation. The newly created protein strand then undergoes
posttranslational modification, in which additional
atoms or
molecules are added, for example
copper,
zinc, or
iron. Once this post-translational modification process has been completed, the protein begins to fold (sometimes spontaneously and sometimes with
enzymatic assistance), curling up on itself so that
hydrophobic elements of the protein are buried deep inside the structure and
hydrophilic elements end up on the outside. The final shape of a protein determines how it interacts with its environment. Protein folding consists of a balance between a substantial amount of weak intra-molecular interactions within a protein (Hydrophobic,
electrostatic, and Van Der Waals Interactions) and protein-solvent interactions. As a result, this process is heavily reliant on environmental state that the protein resides in. When a protein is denatured, secondary and tertiary structures are altered but the
peptide bonds of the primary structure between the amino acids are left intact. Since all structural levels of the protein determine its function, the protein can no longer perform its function once it has been denatured. This is in contrast to
intrinsically unstructured proteins, which are unfolded in their
native state, but still functionally active and tend to fold upon binding to their biological target.
How denaturation occurs at levels of protein structure • In
quaternary structure denaturation, protein sub-units are dissociated and/or the spatial arrangement of protein subunits is disrupted. •
Tertiary structure denaturation involves the disruption of: •
Covalent interactions between amino acid
side-chains (such as
disulfide bridges between
cysteine groups) • Non-covalent
dipole-dipole interactions between polar amino acid side-chains (and the surrounding
solvent) •
Van der Waals (induced dipole) interactions between nonpolar amino acid side-chains. • In
secondary structure denaturation, proteins lose all regular repeating patterns such as
alpha-helices and
beta-pleated sheets, and adopt a
random coil configuration. •
Primary structure, such as the sequence of amino acids held together by covalent peptide bonds, is not disrupted by denaturation.
Loss of function Most biological substrates lose their biological function when denatured. For example,
enzymes lose their
activity, because the substrates can no longer bind to the
active site, and because amino acid residues involved in stabilizing substrates'
transition states are no longer positioned to be able to do so. The denaturing process and the associated loss of activity can be measured using techniques such as
dual-polarization interferometry,
CD,
QCM-D and
MP-SPR.
Loss of activity due to heavy metals and metalloids By targeting proteins, heavy metals have been known to disrupt the function and activity carried out by proteins. Heavy metals fall into categories consisting of transition metals as well as a select amount of
metalloid. This understanding has led to the notion that all the information needed for proteins to assume their native state was encoded in the primary structure of the protein, and hence in the
DNA that codes for the protein, the so-called "
Anfinsen's thermodynamic hypothesis". Denaturation can also be irreversible. This irreversibility is typically a kinetic, not thermodynamic irreversibility, as a folded protein generally has lower free energy than when it is unfolded. Through kinetic irreversibility, the fact that the protein is stuck in a local minimum can stop it from ever refolding after it has been irreversibly denatured.
Protein denaturation due to pH is the highest at this pH.
Significantly increasing or decreasing this reaction's pH can cause this enzyme to denature and, subsequently, decrease the reaction rate. Denaturation can also be caused by changes in the pH which can affect the chemistry of the amino acids and their residues. The ionizable groups in amino acids are able to become ionized when changes in pH occur. A pH change to more acidic or more basic conditions can induce unfolding. Acid-induced unfolding often occurs between pH 2 and 5, base-induced unfolding usually requires pH 10 or higher. == Consequences ==