text: #!/usr/bin/env python3
  # -*- coding: utf-8 -*-
  """
  Created on Wed Dec 11 22:21:03 2019
  
  @author: emilia_chojak
  @e-mail: emilia.chojak@gmail.com
  """
  
  def amino_acid_content(protein_seq, amino_acid):
      amino_acid = amino_acid.upper()
      if amino_acid in protein_seq:
          return (protein_seq.count(amino_acid)/len(protein_seq))*100
      else:
          return 0
  
  assert amino_acid_content("MSRSLLLRFLLFLLLLPPLP", "M") == 5
  assert amino_acid_content("MSRSLLLRFLLFLLLLPPLP", "r") == 10
  assert amino_acid_content("MSRSLLLRFLLFLLLLPPLP", "L") == 50
  assert amino_acid_content("MSRSLLLRFLLFLLLLPPLP", "Y") == 0