Irreversible egg protein denaturation and loss of solubility, caused by the high temperature (while
cooking it)
Denaturation is the alteration of a protein or nucleic
acid's shape through some form of external stress (for example, by applying heat,
acid or alkali),
in such a way that it will no longer be able to carry out its cellular function. In biology, the shape and form of biological
compounds such as proteins critically determine the function of the protein. Under stress, the shape and form (protein structure and protein folding) of the protein warp,
and the protein changes (denatures) due to its inability to retain its old shape.
Common examples
When food is cooked, some of its proteins become denatured. This is why boiled eggs become hard and cooked meat becomes
firm.
A classic example of denaturing in proteins comes from egg whites, which are largely egg
albumins in water. Fresh from the eggs, egg whites are transparent and liquid. But by cooking they are turned opaque and white, and form an interconnected solid mass. The same transformation can be effected with a denaturing chemical. Pouring egg whites into a beaker
of acetone will also turn egg whites opaque and solid. The skin which forms on curdled milk is another common example of denatured protein. And the traditional Peruvian cold appetizer known as
ceviche is prepared by chemically "cooking" raw fish and shellfish in an acidic citrus marinade,
without heat.
Although denaturation can be irreversible, an example of reversible denaturing in proteins is the modern permanent wave technique for curling or straightening hair.
Protein denaturation
Denatured proteins can exhibit a wide range of characteristics, from loss of solubility to
communal aggregation.
Background
Proteins are very long strands of amino acids linked
together in specific sequences. A protein is created by ribosomes that "read" mRNA 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 (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.
When a protein is denatured, the secondary and tertiary structures are altered but the peptide bonds between
the amino acids are left intact. Since the structure of the protein determines 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.
How denaturation occurs at levels of protein structure
- See also: 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:
-
Loss of function
Most biological proteins lose their biological function when denatured. For example, enzymes
lose their catalytic 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.
Reversibility and irreversibility
In many proteins (unlike egg whites), denaturation is reversible (the proteins can regain their native state when the
denaturing influence is removed). This was important historically, as it 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.
Nucleic acid denaturation
The denaturation of nucleic acids such as DNA due to high
temperatures, is the separation of a double strand into two single strands, which occurs when the hydrogen bonds between the strands are broken. This may occur during polymerase chain reaction. Nucleic acid strands realign when "normal" conditions are restored
during annealing. If the conditions are restored too quickly, the nucleic acid
strands may realign imperfectly.
See also
External Links
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