Basically 2 characteristics of proteins that makes it not viable to centrifuge using density gradient.
This is not correct. Sucrose density gradient centrifugation is useful for purification of proteins, especially high molecular weight proteins and large complexes.
@Adam B Shapiro, the question I asked is basically a copy and paste from my 2nd year assessment, and I only asked because similar to your answer everything seems to suggest the question is incorrect! here is exactly how question is being asked in case I'm being silly: 1. In the practical you used differential centrifugation to isolate mitochondria, but as was discussed, density gradient centrifugation can provide much higher purity. Provide two reasons why biological membranes are amenable for equilibrium sucrose density centrifugation, while proteins are not.
Lets see if I can guess the answers to this question that were expected. (1) Many proteins are just too small to move through a sucrose gradient in the usual amount of time, and even if enough time were given they would not move very far. Large protein molecules (e.g. HSP70 or proteosomes) or ribonucleoprotein complexes (e.g. ribosomes), on the other hand, are big enough for this to be a useful technique. It was useful for the 400 kDa protein I studied in graduate school. (2) Sucrose gradient centrifugation takes a long time compared to chromatography. But since it runs unattended, it can run overnight. It's a very gentle technique, since it involves no surfaces or shearing forces that could damage a delicate high-molecular weight complex, and the high concentration of sucrose can also be stabilizing. It is the preferred method for purification of ribosomes.
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