Approximating Protein Mass - Online Tutor, Practice Problems & Exam Prep

To approximate the mass of a protein, consider the average molecular weight of free alpha amino acids, which is 128 grams per mole. Each peptide bond formation loses a water molecule (18 grams per mole), leading to an average amino acid residue weight of 110 grams per mole. The equation for protein mass is: $M=N\mathrm{110}$, where N is the total number of amino acid residues. For example, a protein with 200 amino acids has an approximate mass of 22,000 grams per mole.

Approximating Protein Mass

In this video, we're going to learn how to approximate the mass of a protein. Each amino acid actually has a unique mass, except for just two amino acids, and those two exceptions are leucine and isoleucine. That's because they are isomers of one another, so they have the same exact chemical formula and the same exact mass. Most proteins in nature are quite large, ranging from having several hundred amino acids up to several thousands of amino acids. Calculating the exact mass of a protein with so many amino acids can be a pretty cumbersome and long-drawn-out process, especially if we need to take into account the unique masses of the amino acids. Because it is such a cumbersome process, we approximate the mass of a protein. Luckily, the mass of a protein can be easily approximated with only the total number of amino acid residues.

When it comes to approximating the mass of a protein, we need to consider two different factors. The first is the average molecular weight of the 20 free alpha amino acids, which is 128 grams per mole. Molecular weight can be abbreviated with MW or MR for relative molecular weight, which is the same value as the molecular weight, and we're going to treat them as being the same. The key to this first point here is actually the word 'free'. Recall that free amino acids are separate and independent from one another and are not linked via peptide bonds. This is the molecular weight of the free amino acids.

With the second point here, we need to recall that a water molecule, an H₂O molecule, is lost with the formation of each peptide bond via a dehydration synthesis reaction to link an amino acid. That means that we need to consider the molecular weight of a water molecule, which is 18 grams per mole. Essentially, this makes the average molecular weight of an amino acid residue to be 110 grams per mole. We get this number by taking the difference between the average molecular weight of the free amino acids (128 grams per mole) minus the molecular weight of a water molecule (18 grams per mole) that is lost with the formation of each peptide bond, so 128 minus 18 gives us 110.

Below, we have an equation for the approximate mass of a protein, which is equal to the total number of amino acid residues times the average molecular weight of each amino acid residue, which is 110 grams per mole. In our example, we are asked to determine the approximate molecular weight of a protein containing 200 amino acids. We know that the molecular weight is going to be equal to the total number of amino acids, which is given to us as 200, multiplied by the average molecular weight of an amino acid residue, which is 110 grams per mole. If you type this into your calculator, 200 times 110, you'll get an answer of 22,000 grams per mole, which is, of course, equivalent to answer option C here, so we can indicate that C here is correct. This concludes our lesson on approximating the mass of a protein, and we'll be able to get some practice in our next couple of videos. I'll see you guys there.

What is the approximate M _r of the enzyme glycogen phosphorylase, which has 842 amino acid residues?

So, this is a pretty straightforward practice problem asking, what is the approximate MR or molecular weight of the enzyme glycogen phosphorylase, which has 842 amino acid residues. And so down below, we have an image of the glycogen phosphorylase enzyme. Recall that our equation for calculating the approximate molecular weight of a protein is: Molecularweight=totalnumberofaminoacidresidues×averagemolecularweightperresidue =842×110g/mol And so if we type this into our calculators, what we get is 92620g/mol, which of course matches up with answer option B. So we can go ahead and give B here a check mark to indicate that B here is correct and cross off all the other ones which are incorrect. This concludes this practice problem and I'll see you guys in our next one.

Myoglobin is an oxygen-storage protein in muscle tissues. If its molecular weight is 16.7 kDa (1 Da = 1 g/mol), about how many amino acid residues does myoglobin have?

• A. 86
• B. 134
• C. 208
• D. 153

Here’s what students ask on this topic:

How do you approximate the mass of a protein?

To approximate the mass of a protein, you need to consider the average molecular weight of amino acid residues. The average molecular weight of free alpha amino acids is 128 grams per mole. However, each peptide bond formation loses a water molecule (18 grams per mole), resulting in an average amino acid residue weight of 110 grams per mole. The equation for protein mass is:

$M=N110$

where N is the total number of amino acid residues. For example, a protein with 200 amino acids has an approximate mass of 22,000 grams per mole.

What is the average molecular weight of free alpha amino acids?

The average molecular weight of free alpha amino acids is 128 grams per mole. This value is derived from the molecular weights of the 20 standard amino acids, excluding the effects of peptide bond formation. It is important to note that this value changes when amino acids are linked together in a protein due to the loss of water molecules during peptide bond formation.

Why is the average molecular weight of an amino acid residue 110 grams per mole?

The average molecular weight of an amino acid residue is 110 grams per mole because each peptide bond formation results in the loss of a water molecule (H₂O), which has a molecular weight of 18 grams per mole. The average molecular weight of free alpha amino acids is 128 grams per mole. Therefore, the weight of an amino acid residue in a protein is:

$128-18=110$

How do you calculate the approximate molecular weight of a protein with 300 amino acids?

To calculate the approximate molecular weight of a protein with 300 amino acids, you use the formula:

$M=N110$

where N is the total number of amino acid residues. For 300 amino acids, the calculation is:

$300110=\mathrm{33,000}$

Therefore, the approximate molecular weight of the protein is 33,000 grams per mole.

What is the significance of the water molecule in calculating protein mass?

The significance of the water molecule in calculating protein mass lies in the fact that each peptide bond formation during protein synthesis results in the loss of a water molecule (H₂O), which has a molecular weight of 18 grams per mole. This loss must be accounted for when determining the average molecular weight of amino acid residues in a protein. The average molecular weight of free alpha amino acids is 128 grams per mole, but after accounting for the loss of water, the average molecular weight of an amino acid residue becomes 110 grams per mole.