Question

Figure 1. The hierarchical organisation of the protein tubulin. Individual molecular units (a) link together to form filaments (b), which assemble into microtubules (c).

Figure 1 illustrates the underlying structure of a microtubule, which was introduced in section 5.1 of the Block 3 book. The tubulin molecules provide binding sites for the energy carrying molecules GDP/GTP, which act in a similar way to ADP/ATP. The GDP/GTP molecules are shown in red on the diagram.

• a.Using examples based on the tubulin structure shown in Figure 1(a):
  • i.In a few sentences, explain what is meant by the terms ‘primary structure’ and ‘higher-order structure’ for a protein, and how they are related.

  • ii.Explain the importance of hydrogen bonding in the formation of higher-order structures. Include a simple diagram in your answer.

• b.The molecules GDP (guanosine diphosphate) and GTP (guanosine triphosphate) fulfil similar roles to ADP and ATP, as described in the Block 3 book.

  Explain the basic chemical difference between these molecules, and how this enables them to act as an energy transfer mechanism.

• c.Molecular motors carry cargo within living cells by ‘walking’ along microtubule tracks, at speeds of about 1-2 µm s^-1. If a small protein molecule has a diffusion constant of 0.5 x 10^-10 m² s^-1, estimate the time it would take to diffuse from one end of a 1 m long neuron to the other. Compare this with the time it would take a molecular motor walking along a microtubule track at 2 µm s^-1 to carry the protein the same distance. Express your answers in days (to the nearest day) and comment on your answer.

Answer 1