STAGE 4 OF 4

Weigh the protein

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THE BRIEF

Researchers routinely compute a protein's mass, for example to compare it with a result from a mass spectrometer. Each amino acid residue has its own mass, and a protein's mass is the sum of its residue masses plus one water molecule (18.01056), because the chain keeps one extra hydrogen at one end and one hydroxyl at the other.

Your task. Given a protein string, print its mass rounded to three decimal places. The residue mass table is provided in the starter code.

Input. One line: a protein as one-letter amino acid codes.

Output. One number with three decimals.

GIVEN INPUT

Your code reads this from standard input (input() or sys.stdin), both here and when it is graded.

MAMAPRTEINSTRING

EXPECTED OUTPUT FOR THIS INPUT

1760.866

SKILLS YOU'LL USE

Stuck? These bank problems practise the same techniques.

Protein Molecular Weight

Write your code

.ipynb

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Why do we add one water molecule to the sum of the residue masses?

Proteins are always dissolved in water, so it must be counted Each residue loses a water when peptide bonds form, but the whole chain keeps one H at one end and one OH at the other The mass table is wrong without it Because every protein contains exactly one water molecule inside it

What does a mass spectrometer measure?

The colour of the sample The mass-to-charge ratio of ions The number of genes in a cell The pH of the sample

Explain and submit

Explain how your program turns a protein string into one number, and describe one way this number could be used in a lab.

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