Chapter 5, Problem 1P

Answers to all problems are at the end of this book. Detailed solutions are available in the Student Solutions Manual, Study Guide, and Problems Book.

Calculating the Molecular Weight and Subunit Organization of a Protein From Its Metal Content The element molybdenum {atomic weight 95.95) constitutes 0.0K if the weight of nitrate reductase. If the molecular weight of nitrate reductase is 240,000, what is its likely quaternary structure"?

Interpretation Introduction

Interpretation:

The quaternary structure of the nitrate reductase needs to be determined based on its molecular weight and percentage of element molybdenum with atomic weight 95.95 present in it.

Concept introduction:

Nitrate reductase’s quaternary structure can be determined by calculating molecular weight of the molybdenum and its number of moles in nitrate reductase.

From the number of moles of a species and its molar mass, mass can be calculated as follows:

  $m=n\times M$

Here, n is number of moles and M is molar mass.

Answer to Problem 1P

Molecular weight of molybdenum=192 g/mol

Number of moles of molybdenum= 2.00 mol

Nitrate reductase’s quaternary structure consists of two peptide chain and each peptide has one mol of molybdenum. Thus, it is a dimer.

Explanation of Solution

The Nitrate reductase’s quaternary structure can be determined by calculating number of moles of the molybdenum in a nitrate reductase where one mol of molybdenum is constituted by each peptide.

Molecular weight of molybdenum = nitrate reductase’s molecular weight × molybdenum percentage in nitrate reductase

Therefore,

  $\text{Molecular weight of molybdenum}=\text{240000 g×}\frac{\text{0}\text{.8}}{\text{100}}=192\text{ g}$

Molecular weight of molybdenum is used to determine the number of moles of molybdenum in nitrate reductase as follows:

  $n=\frac{m}{M}$

Here, m is molecular weight of molybdenum present in nitrate reductase and M is atomic weight or molar mass of the molybdenum.

Putting the values,

  $n=\frac{1\text{92 g}}{\text{95}\text{.95 g/mol}}=2\text{ mol}$

Thus, the number of moles of molybdenum in nitrate reductase is 2.00 mol.

As we know that, one mol of molybdenum is constituted by each peptide. Therefore, nitrate reductase is dimer which is made up of two equivalent peptide chains. And each peptide chain consists of one mole of the molybdenum.

Conclusion

Molecular weight of molybdenum is calculated by using molecular weight of nitrate reductase and molybdenum percentage. The value of molecular weight of molybdenum is needed in finding the number of moles of molybdenum in nitrate reductase. From these calculations, it is determined that the quaternary structure of nitrate reductase constitutes two peptide chains and each chain has one mol of molybdenum.