Question

Which of the following amino acids pairs would form the described hydrogen bond at pH 7? Explain your answer?

A. His Asp

B. Tyr His

C. Ser Arg

D. Glu Thr

A certain protein has a molecular weight of 250 kD. What is the approximate number of amino acids residues?

A. 2

B. 750

C. 1200

D. 2300

E. 27500

Answer 1

1.The side chains of the hydrophobic amino acids are nonpolar and cannot form hydrogen bonds with water or other polar entities. Within the hydrophobic amino acid group, side chains vary in size and shape. Four of the hydrophobic amino acids, alanine, valine, leucine, phenylalanine and isoleucine, have side chains that are branched or unbranched hydrocarbon chains.Methionine and tryptophan both contain an atom that can form a hydrogen bond, but the bulk of their side chains are nonpolar and therefore hydrophobic.Proline is unlike any other amino acid in that its nonpolar side chain doubles back to form a covalent bond with its amino group. This geometry causes a small kink in a polypeptide chain wherever a proline appears.

The side chains of polar amino acids readily form hydrogen bonds. Polar side chains are likely to end up on the surface of a water-soluble protein, where they participate in hydrogen bonding with water. The amino acids serine, threonine, and tyrosine all have hydroxyl groups, which are classic hydrogen-bond donors.Asparagine and glutamine each have an amide group, which can participate in hydrogen bonding.Histidine and cysteine are both versatile amino acids. Histidine has a polar imidazole ring, which can accept or donate a proton in acid-base catalysis

Therefore the answer is option A) & B)

2)If you take the average molecular weight of all 20 protein amino acids you get 128 daltons

Now…in making the protein, each amino acid is added to form the peptide bond through a dehydration reaction (removing a water, thus each amino acid is 18 daltons lighter) so the average molecular weight of the amino acid in the protein is 110 daltons (128 – 18 = 110).

So, the size of the protein is X amino acids x 110 daltons/amino acid = 250000 daltons.

X=2272.73 amino acids approximately= 2273 amino acids