Approximating Protein Mass - Video Tutorials & Practice Problems
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Approximating Protein Mass
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Video transcript
In this video, we're gonna learn how to approximate the mass of a protein. So each amino acid actually has a unique mass, except for just 2 amino acids, and those two exceptions are leucine and isoleucine. And that's because they are isomers of one another, so they have the same exact chemical formula and the same exact mass. Now most proteins in nature are quite large and they range from having several 100 of amino acids up to several thousands of amino acids. And so calculating the exact mass of a protein with so many amino acids can be a pretty cumbersome long drawn out process, especially if we need to take into account the unique masses of the amino acids. And so because it is such a cumbersome process, approximate the mass of a protein. But luckily, the mass of a protein can be pretty easily protein. But luckily, the mass of a protein can be pretty easily approximated with only the total number of amino acid residues. And so when it comes to approximating the mass of a protein, we need to consider 2 different factors. The first is the average molecular weight of the 20 free alpha amino acids, which is a 128 grams per mole. And molecular weight can be abbreviated with m w or m r for relative molecular weight, which is the same value as the molecular weight and we're gonna treat them as being the same. Now the key to this first point here is actually the word free. And so recall that free amino acids are separate and independent from one another, and they're not linked via peptide bonds. And so this is the molecular weight, the average molecular weight of the free amino acids. Now with the second point here, we need to recall that a water molecule, an h two o molecule, is lost with the formation of each peptide bond via a dehydration synthesis reaction to link an amino acid. And so that means that we need to consider the molecular weight of a water molecule, which is 18 grams per mole. And so essentially, this makes the average molecular weight of an amino acid residue to be a 110 grams per mole. And, essentially, we get this number by taking the difference between the average molecular weight of the free amino acids minus the, molecular weight of a water molecule that's lost with the formation of each peptide bond. So a 128 minus 18 gives us a 110. And so down 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 a 110. And so down below in our example, it's asking us to determine the approximate molecular weight of a protein containing 200 amino acids. And so we know that the molecular weight is gonna 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 a 110 grams per mole. And so if you type this into your calculator, 200 times a 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. And so 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. So I'll see you guys there.
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What is the approximate M r of the enzyme glycogen phosphorylase, which has 842 amino acid residues?
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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?