Question

It is often said that protein complexes are made from subunits (that is,individually synthesized proteins) rather than as one long protein because it is more likely to give a correct final structure.

a. Assuming an error rate of 1/10,000, what fraction of bacterial ribosomes would be constituted correctly if the proteins were synthesized from one large protein versus assembled from individual proteins? For sake of calculation assume that the ribosome is composed of 50 proteins, each 200 amino acids in length, and that the subunits – correct and incorrect – areassembled with equal likelihood into the complete ribosome. Note that the probability of a correct sequence, Pc, equals the fraction correct for each operation, fc, raised to a power equal to the number of operations, n:Pc=(fc)n. For an error rate 1/10,000, fC=0.9999.

b. Is the assumption that correct and incorrect subunits assemble equally well likely to be true? Why or why not? How would a change in that assumption affect the calculation in Part a?

Answer 1

Answer a:)

If 50 proteins each is 200 amino acids long make ribosomes, we get 10000 amino acids total in 50 fragments. If there is one fragment that is malfunctioned, that will not add with other polypeptides. Therefore, the

Typically, a bacterium has at least 10000 ribosomes. However, we calculate errors in 50 ribosomes:

In the translation of one type of 200 amino acids long protein, one among 50 units of the same protein is malfunctioned.

200 amino acids x 50 units = 10000 amino acids.

Fc = 0.9999

Therefore, the correct amino acids = 10000 x 0.9999

The correct amino acids = 9999

Incorrect amino acid = 10000 -9999

Incorrect amino acid = 1

There are 50 proteins and hence, each protein must get one malfunctioned protein.

Therefore, we will get 50 malfunctioned proteins. These malfunctioned proteins will not assemble, and the bacterium cell maximum loses 1 ribosome among 50 ribosomes.

Now, if one single protein makes a ribosome, then, each time one error will be made by the translation. This will cause the error in the proper functioning of the ribosome. Therefore, we cannot get a single proper functioning ribosome because, in the whole length of 10000 amino acids, at least one amino acid will be incorrect.

Answer b:)

If correct and incorrect sequence assembles equally, then we will not get any correct ribosome ever.