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Chapter 5: Q5-5P (page 127)

You are using ammonium sulphate to purify protein Q (pI = 5.0) by salting out from a solution at pH 7.0. How should you adjust the pH of the mixture to maximize the amount of protein Q that precipitates?

Short Answer

Expert verified

By lowering the pH of the mixture it will maximize the amount of protein Q that gets precipitated.

Step by step solution

01

Definition of protein

Huge molecules known as proteins are composed of basic building pieces known as amino acids.

02

How to adjust the pH to maximize the protein

The pI and pH will be equal to 5.0 if the pH is reduced from 7.0 to 5.0. If they both equalise the protein, then it will be the least soluble since the net charge will be zero (and it will promote precipitation).

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Most popular questions from this chapter

You wish to determine the sequence of a polypeptide that has the following amino acid composition.

1 Ala 4 Arg 2 Asn 3 Asp 4 Cys 3 Gly 1 Gln 4 Glu 1 His 1 Lys 1 Met 1 Phe 2 Pro 4 Ser 2 Tyr 1 Trp

(a) What is the maximum number of peptides you can expect if you cleave the polypeptide with cyanogen bromide?

(b) What is the maximum number of peptides you can expect if you cleave the polypeptide with chymotrypsin?

(c) Analysis of the intact polypeptide reveals that there are no free sulfhydryl groups. How many disulfide bonds are likely to be present?

(d) How many different arrangements of disulfide bonds are possible?

Question: Sketch a phylogenetic tree for the family of homologous proteins

whose partial sequences are given below.

Protein A T L A D K A I S L H D S

Protein B T L G D K A V S I H E S

Protein C T L A D K A I S V H D S

Treatment of a polypeptide with 2-mercaptoethanol yields two polypeptides:

1. Ala–Val–Cys–Arg–Thr–Gly–Cys–Lys–Asn–Phe–Leu

2. Tyr–Lys–Cys–Phe–Arg–His–Thr–Lys–Cys–Ser

Treatment of the intact polypeptide with trypsin yields fragments with the following amino acid compositions:

3. (Ala, Arg, Cys2, Ser, Val)

4. (Arg, Cys2, Gly, Lys, Thr, Phe)

5. (Asn, Leu, Phe)

6. (His, Lys, Thr)

7. (Lys, Tyr)

Indicate the positions of the disulfide bonds in the intact polypeptide.

A pentapeptide has the sequenceNNKNN(using one-letter symbols for amino acids). Calculate the mass of this peptide, as determined by mass spectrometry, to three significant figures.

Describe how a protein may be quantified by an assay or by absorbance spectroscopy.
See all solutions

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