how protease stops its own degradation while lysing proteins,when protease is also a protein?

Question

How protease stops its own degradation while lysing protein when it is also a protein,why protease cannot get lysed itself also?

Answer 1

There are many kinds of proteases, each of them acts on a specific site(s)--a particular amino acid sequence(s), to lyse a protein(s). Through evolution, each protease has evloved to not having these sites. Therefore, it can preserve itself while lysing the others.

Answer 2

First of all the activity of any enzyme depends on a lot of factors, so I guess your question is how come under conditions when the protease is at its maximal activity it doesn't degrade itself.

There are different types of proteases.

Some are endopeptidases which means that they recognise specific patterns in the sequence of the protein and they will cut at those sites. Usually such proteases don't have these patterns on themselves.

However there are exceptions where a protease is formed as an inactive precursor and cleavage of a small part of it results in an active protease which can activate more precursor molecules. For example trypsin, which is one of the proteases secreted by the pancreas, is in the form of trypsinogen. Another protease cleaves a 6 amino acid long peptide and forms active trypsin which can also catalyse the reaction trypsinogen->trypsin.

There are also exopeptidases. They start from one end of the protein and chop it up. They don't recognise specific sequences. However usually they are in complex with other proteins. These recognise specific degradation signals on a protein and will "present" to the protease for degradation only proteins having such signals. These signals can be some type of modification like ubiquitination or special sequence at the N-terminus (see N-terminal rule) or the C-terminus (like the SsrA pathway).

Finally in the case of in vitro experiments, where you could have purified protease and another protein, it depends on the relative amounts of the two, on the specificity of the protease and the time period you allow the reaction to take place. Usually other proteins are in excess and that's why you don't see any significant degradation of the protease. However if the ratio of substrate/protease is not very big you can have protease degrading itself. This also happens if you let the reaction occur for a long time. Also in such cases (excess of protease) even proteases which recognise certain patterns can start chopping up the substrate at more sites (usually these are close but not exact matches of the recognition sequence).

Answer 3

Great question. It sounds like an oxymoron doesn't it?

First of all, the protease enzymes in the small intestine are at a pretty neutral pH so they function fine and there is little to worry about degradation. But then comes pepsin in the stomach, where the pH is around 1.5 or so.

In most reactions in the gastrointestinal tract an enzyme lowers the energy of activation and allows the reaction to proceed faster than it would under normal conditions. The rate of reaction with a catalyst (specifically pepsin is a catalyst) is affect by temperature, pH, amount of reactants, amount of catalyst, etc. So each enzyme is specifically designed to function optimally at a given pH. For instance, amylase in the mouth works optimally at a pH of 7, but when it gets to the stomach amylase is inactivated and denatured by the acidic conditions of the stomach. Pepsin works optimally at about a pH of 2 and then inactivates once it reaches the alkaline conditions of the duodenum of the small intestine.

To put it simply, the pH of the stomach is low to denature proteins that enter it. The change in pH itself, technically, doesn't degrade the protein, but rather "unwinds" it so that the pepsin (which is designed to work fine in low pH) can easily cleave the proteins, into oligopeptides. So depending on how a protein is conformed in its secondary or tertiary structure it is "allowed" (can't think of a better word) to work optimally at a given pH.

Take a look at this website regarding rates of reactions:

http://www.gcsechemistry.com/rc14.htm

Answer 4

The same applies to Restriction endonucleases which cut DNA at specific sequences. Orginisms possessing such a R.E. protect their own DNA by methylating it.

Answer 5

generally the proteins to be degraded will have ubiquitins bound t o them, signaling them for degradation. Other wise, every protein would be up for degradation