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In biochemistry, heat shock is the effect of subjecting a cell to a higher temperature than that of the ideal body temperature of the organism from which the cell line was derived. Heat shock refers to the cellular exposure to rapid changes in stressors such as temperature, toxins, oxidative stress, heavy metals, and pathogenic infections. Specifically temperature induced heat shock, even by a change of a few degrees, causes proteins to misfold, nonspecifically aggregate, and/or entangle. Other cellular damage induced by heat shock includes cytoskeleton rearrangement, changes in organelle localization, decreased ATP production, drop in cellular pH, decreased translation, and changes in RNA splicing. Introduction of heat shock to cells elicits the molecular response, the heat shock response (HSR), which repairs damages caused by stressors such as protein misfolding and protein aggregation.

Heat shock response
The cellular response to heat shock damage, the heat shock response, includes the transcriptional up-regulation of genes encoding heat shock proteins (HSPs) as part of the cell's internal repair mechanism. Stressors such as temperature changes, toxins, They are also called stress-proteins. and respond to heat, cold and oxygen deprivation by activating several cascade pathways. HSPs are also present in cells under perfectly normal conditions. Some HSPs, called chaperones, ensure that the cell’s proteins are in the right shape and in the right place at the right time. For example, HSPs help new or misfolded proteins to fold into their correct three-dimensional conformations, which is essential for their function. They also shuttle proteins from one compartment to another inside the cell, and target old or terminally misfolded proteins to proteases for degradation. Heat shock proteins are also believed to play a role in the presentation of pieces of proteins (or peptides) on the cell surface to help the immune system recognize diseased cells.

The up-regulation of HSP s during heat shock is generally controlled by a single transcription factor; in eukaryotes this regulation is performed by heat shock factor (HSF), while σ32 is the heat shock sigma factor in Escherichia coli. Under normal conditions HSF1 resides as a monomer, but when stress induces protein damage HSF1 is activated to trimerize. This trimer of HSF1 localizes to the nucleus, and here binds to the heat shock element in the promoter sequence of heat shock genes.

Inducing heat shock
In fish that survive at 0 °C, heat shock can be induced with temperatures as low as 5 °C, whereas thermophilic bacteria that proliferate at 50 °C will not express heat shock proteins until temperatures reach approximately 60 °C. The process of heat shocking can be done in a CO2 incubator, O2 incubator, or a hot water bath.