Amino acids and protein structureEdexcel International A Level Biology: Flashcards

Exam questions on Amino acids and protein structure

1. All amino acids share a common structure: a central carbon atom bonded to an amino group, a carboxyl group, a hydrogen atom and a variable R group. The simplest amino acid, glycine, has a hydrogen atom as its R group. Alanine has a methyl group, CH₃, as its R group.
   Describe how a dipeptide is formed from glycine and alanine.2 marks
2. Collagen and haemoglobin are both proteins. Collagen forms tough fibres in tendons: each molecule has three polypeptide chains wound round each other, and about one third of its amino acid residues are glycine. Haemoglobin is found inside red blood cells; each molecule has four polypeptide chains, each with a haem group that binds oxygen.
   Compare the structure and function of a fibrous protein and a globular protein, using collagen and haemoglobin as examples.2 marks
3. In sickle-cell disease, a single amino acid in the β polypeptide chain of haemoglobin is changed. A hydrophilic glutamic acid is replaced by a hydrophobic valine, at a position on the outside of the folded chain. The abnormal haemoglobin is much less soluble and, when oxygen concentration is low, its molecules stick together into long fibres.
   Describe the primary, secondary and tertiary structure of a protein.3 marks