 ## Protein Size & Molecular Weight Calculation

Protein molecules are important biomolecule in our cell system. The molecular weight of a typical protein molecule is comparable to the transfer RNA molecules. The average molecular weight of an amino acid is 110 Dalton. Molecular weights of typical polypeptide chains range from 15,000 to 70,000 and the length in its extended form would be 1000 to 5000 Å.  Some longer fibrous proteins, such as myosin (from muscle) and tropocollagen (from tendon) are in the range with length of 1600 Å and 2800 Å respectively. However, most of the proteins are highly folded and their longest dimension usually ranges from 40 to 80 Å.

## Molecular Weight calculation

Molecular weight of a peptide chain can easily be calculated by adding the molecular weight of an individual amino acids in the polypeptide and then subtracting the molecular weight of total water molecules released during the formation of all peptides.

For this there can be two ways to calculate the molecular weight of peptide:-

• Molecular weight of each amino acids can be considered as 128 dalton (average molecular weight of an amino acid).

Example: Molecular weight of a tripeptide

Solution: A tripeptide consists of three amino acids

Molecular weight of all amino acids = 128 x 3 = 384

Molecular weight of one H2O molecule = 18 x 2 = 36

Molecular weight of tripeptide = 384 – 36 = 348

The Number of peptide bonds in an amino acids is one less than the number of amino acids if the peptide is linear and equal to the number of amino acids if the peptide is cyclic.

• Molecular weight of amino acids residues (When two or more amino acids combine to form a peptide, the elements of water are removed, the remaining part of each amino acids is a residues) can be considered as 110 Dalton. In this case while calculating the weight of peptide the amino acids residue with molecular weight of 110 Dalton, we have already subtracted the water molecules released in peptide bond formation. But one molecule of water must be added due to presence of H on N amino and OH on carboxyl terminus terminus.

Molecular weight of all amino acids = 110 x 3 = 330

Molecular weight of one H2O molecule = 18 x 1 = 18

Molecular weight of tripeptide = 330 – 18 = 348

## MCQs

### Questions

1) If the molecular weight of an amino acid is 150 Da,  the molecular weight of a tripeptide will be

1. 15
2. 14
3. 30
4. 16

2) What is the molecular weight of circular peptide, if the peptide is made up of 10 amino acids and each free amino acids has a molecular weight of 128 Da.

1. 1100
2. 1280
3. 1000
4. 1200

3) If one lysine has a molecular weight of 160 Da, then what would be the molecular weight of linear and circular polymer of 20 lysines.

1. 3200 &   3200 Da
2. 3200 & 2850 Da
3. 2858 &   2840 Da
4. None of the above

4) The number of amino acid residues in a protein with molecular weight of 85kDa

1. 770
2. 7100
3. 8500
4. 85

5) What is the molecular weight of a protein which is encoded by a 2.7kb ORF

1. 990 kDa
2. 99 kDa
3. 90 Da
4. 90 kDa

6) Find out the molecular weight of peptide (in daltons) made up of 100 amino acids.

1. 11018
2. 10000
3. 11000
4. 1100

1) 4

2) 4 Explanation–>Amino acids forming a circular peptide then their molecular weight will be 128-18 = 110 (Water molecules will be lost from each). Thus the molecular weight of a circular peptide is 110 x 10 =1100.

3) 3

Explanation: For Linear polymer à 160 x 20= 3200

Polymerization in linear form will form 19 bonds and releases 19 water molecules 19 x 18 = 342

Molecular weight of linear polymer = 3200 – 342 = 2858 Da

For Circular polymer à 160 x 20= 3200

Polymerization in linear form will form 20 bonds and releases 20 water molecules 20 x 18 = 360

Molecular weight of linear polymer = 3200 – 360 = 2840 Da

4) 1 Explanation–>110 Da is the Average molecular weight of 1 residue in peptide. So, 85000 Da will contain 85000/110 = 772

5) 2 Explanation–> 1kb = 1000bp, Thus 2.7kb = 2700bp. 3bp encodes 1 AA. Therefore, 2700bp encodes 2700/3 = 900. 110 Da is the Average molecular weight of 1 residue in peptide. So, Total 900 a.a residue will generate total 900 x 110 = 99000 or 99kDa molecular weight protein.

6) 1