10 Questions you want to ask about PROTEINASE K.

Posted by John Kim on Feb 19, 2013 11:00:00 AM

1. QUESTION: What is Proteinase K?

ANSWER: In molecular biology Proteinase K (also protease K or endopeptidase K) is a broad-spectrum serine protease. The enzyme was discovered in 1974 in extracts of the fungus Engyodontium album (formerly Tritirachium album). Proteinase K is able to digest native keratin (hair), hence, the name "Proteinase K". The predominant site of cleavage is the peptide bond adjacent to the carboxyl group of aliphatic and aromatic amino acids with blocked alpha amino groups. It is commonly used for its broad specificity. This enzyme belongs to Peptidase family S8. The molecular weight of Proteinase K is 28,900 daltons (28.9 kDa).
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2. QUESTION: What is the function of proteinase K in DNA extraction?

ANSWER: During the extraction of DNA (or nucleic acids in general), there is a lot of contaminating proteins present. These contaminants must be removed. Proteinase K, which is a broad spectrum serine protease, is used in many DNA extraction protocols to digest these contaminating proteins.

In addition, there may be nucleases (enzymes that degrade nucleic acids) present. The addition of proteinase K degrades these nucleases and protects the nucleic acids from nuclease attack. In addition, proteinase K is stable over a wide pH range and is well suited for use in DNA extraction.

 

3. QUESTION: What are proteinase K applications?

ANSWER: Proteinase K is commonly used in molecular biology to digest protein and remove contamination from preparations of nucleic acid. Addition of Proteinase K to nucleic acid preparations rapidly inactivates nucleases that might otherwise degrade the DNA or RNA during purification. It is highly-suited to this application since the enzyme is active in the presence of chemicals that denature proteins, such as SDS and urea, chelating agents such as EDTA, sulfhydryl reagents, as well as trypsin or chymotrypsin inhibitors. Proteinase K is used for the destruction of proteins in cell lysates (tissue, cell culture cells) and for the release of nucleic acids, since it very effectively inactivates DNases and RNases. Some examples for applications: Proteinase K is very useful in the isolation of highly native, undamaged DNAs or RNAs, since most microbial or mammalian DNases and RNases are rapidly inactivated by the enzyme, particularly in the presence of 0.5 - 1% SDS. Purification of genomic DNA from bacteria (miniprep): bacteria from a saturated liquid culture are lysed and proteins are removed by a digest with 100 μg/ml Proteinase K for 1 h at 37 °C. The enzyme's activity towards native proteins is stimulated by denaturants such as SDS. In contrast, when measured using peptide substrates, denaturants inhibit the enzyme. The reason for this result is that the denaturing agents unfold the protein substrates and make them more accessible to the protease.


4. QUESTION: Why is proteinase K digestion performed at 50°C?

ANSWER: Proteinase K activity is greatly increased by addition of denaturing agents like SDS or urea (Hilz et al., 2008), indicating that the denaturation of the substrates helps Proteinase K to degrade them. Increasing the temperature to 50°C will also unfold some proteins already, making it easier for the Proteinase K to degrade them. The proteinase K seems to be a pretty stable enzyme, and can still work at this temperature.

 

5. QUESTION: What are temperatures proteinase k inactivated?

ANSWER: Proteinase K is inactivated by heat, eg. Incubating at >55 °C.

 

 

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