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Proteins: from one amino acid to a working shape questions
The general structure of an amino acid, the peptide bond made by condensation, the four levels of protein structure and the bonds that hold each one, globular against fibrous proteins with haemoglobin and collagen as the examples, denaturation, and the biuret test.
7 original questions · 21 marks · the proteins: from one amino acid to a working shape notes · Biological molecules, water and inorganic ions
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Describe the four levels of protein structure, naming in each case the type of bond that holds that level together.
Mark scheme
- B1 primary structure is the sequence of amino acids in the polypeptide, held by peptide bonds
- B1 secondary structure is the regular coiling into an α-helix or pleating into a β-pleated sheet, held by hydrogen bonds between groups in the backbone
- B1 tertiary structure is the overall three-dimensional fold of one polypeptide, held by hydrogen bonds, ionic bonds, disulfide bridges and hydrophobic interactions between side chains
- B1 quaternary structure is the arrangement of two or more polypeptides, together with any prosthetic groups, in the finished protein
Compare the structure of haemoglobin with the structure of collagen, and relate the difference in each case to what the protein does.
Mark scheme
- B1 haemoglobin is globular and compact, whereas collagen is fibrous and forms long strands
- B1 haemoglobin has an irregular amino acid sequence, whereas collagen has a repetitive one in which about every third amino acid is glycine
- B1 haemoglobin carries hydrophilic side chains on the outside so it is soluble and can be transported in solution, whereas collagen is insoluble, which suits it to a structural role
- B1 haemoglobin is four polypeptides each holding one haem prosthetic group that binds one oxygen molecule, whereas collagen is three chains wound round each other and cross-linked, giving strength in tension
While one molecule of a polypeptide was being assembled, water of total relative mass 3582 was released. Taking the relative molecular mass of water as 18, calculate the number of amino acids in that polypeptide.
Mark scheme
- M1 number of water molecules released = 3582 divided by 18
- M1 = 199 molecules of water, which is also the number of peptide bonds formed
- A1 200 amino acids, because n amino acids give n − 1 peptide bonds
A patient's body temperature rises to 43 °C and several of her proteins stop working. Explain what has happened to those proteins, referring to the bonds involved.
A protein taken from a bacterium living in a hot spring contains far more cysteine than the equivalent human protein does. Suggest why this helps the bacterial protein keep its shape at high temperature.
State the three groups that every amino acid carries on its central carbon, and state what varies between the twenty amino acids.
Name the bond formed when two amino acids are joined together, and name the molecule released as it forms.
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