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The Structure of Protein Activity
INTRODUCTION
A protein molecule consists of one or more polypeptide chains. Chemists describe the structure of
proteins at 4 different levels.
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Primary structure sequence of amino acids in polypeptide chain or chains;
Secondary structure describes the repeating patterns in the of sections of the polypeptide
chains (example: fibrous proteins such as α-keratin in hair have helical chains of amino acids
held together by hydrogen bonds);
Tertiary structure describes the overall 3D folding and shape of a protein. This is held
together with hydrogen bonds and other wesk interactions between the R groups. Proteins
tend to fall into 2 groups:
o Fibrous proteins long molecules forming fibres e.g. muscle fibres
o Globular proteins compact, well folded molecules e.g. enzymes
Quaternary structure describes the linking between chains in proteins with 2 or more
polypeptide chains. E.g. Haemoglobin molecules consist of 4 chains fitting together tightly to
form compact globular assembly.
QUESTIONS
1. The structure below shows a short section of a protein molecule.
CH3
CH2OH
CH2COOH
CH2
a) Indentify the amino acids that are joined to make this section of a protein.
b) Which of these amino acids have side chains that:
i)are non-polar ii) are polar iii) can ionise?
2. Haemoglobin is a protein with a relative molecular mass of 66000.
a) Write one word to describe the tertiary structure of haemoglobin.
b) Assuming that the average mass of an amino acid unit in proteins is equal to that of an
aspartic acid unit, calculate the approximate number of amino acid molecules that are
needed to produce on molecule of haemoglobin.
3. How does the hydrogen bonding explain:
a) The solubility of many proteins in water.
b) The precise 3D structure of those enzymes which are proteins.
c) The eleasticity of natural fibres such as wool and silk?
4. Biochemists talk about enzymes being ‘denatured’ by strong acids, strong bases or by a rise
in temperature.
a) What do you think the term ‘denatured’ means?
b) Suggest a reason for the loss of catalytic activity when an enzyme is denatured by acids,
bases or a rise in temperature.