(a) Explain various levels of protein organization with suitable examples. (20 marks) (b) What is nerve impulse? How is the…

(a) Levels of protein organisation with examples (20 marks)

Proteins have four levels of organisation.

• Primary structure: the linear sequence of amino acids in the polypeptide chain, joined by covalent peptide bonds from the N-terminus to the C-terminus. The sequence is set by the gene and determines all higher levels. Example: insulin, with an A chain of 21 residues and a B chain of 30, joined by disulphide bonds, whose sequence was worked out by Sanger.
• Secondary structure: regular local folding of the backbone, held by hydrogen bonds between the C=O and N–H groups of the peptide bonds. The α-helix is a right-handed coil with 3.6 residues per turn, as in keratin and in myoglobin. The β-pleated sheet, parallel or antiparallel, is found in silk fibroin. Others are β-turns and the collagen triple helix (glycine at every third position).
• Tertiary structure: the overall three-dimensional folding of one polypeptide, held by interactions among the side chains: hydrophobic interactions (the main force, burying non-polar groups in the core), hydrogen bonds, ionic bonds (salt bridges), van der Waals forces and covalent disulphide bonds. Example: myoglobin, a compact globular chain of 153 residues, mostly α-helix, with one haem group, and lysozyme and ribonuclease. Supersecondary motifs and domains are units within it.
• Quaternary structure: the association of two or more polypeptide subunits, held by the same non-covalent forces (and sometimes disulphide bonds). Example: haemoglobin, a tetramer (α₂β₂) that binds oxygen cooperatively; other examples are lactate dehydrogenase and immunoglobulins.

Note. Heat, extreme pH or urea denature a protein by disrupting the higher levels, without breaking peptide bonds, and it loses its function. Anfinsen showed with ribonuclease that the primary structure carries the information for folding, and chaperones assist.

(b) Nerve impulse, and generation and propagation of the action potential (15 marks)

Nerve impulse. A nerve impulse is a wave of electrochemical change, a self-propagating depolarisation, that travels along the neuron membrane.

Resting potential. The inside of the axon is negative, at about −70 mV, because of the Na⁺–K⁺ pump (3 Na⁺ out and 2 K⁺ in) and the greater leak of K⁺ than Na⁺.

Generation of the action potential.

• A stimulus depolarises the membrane, and when the threshold (about −55 mV) is reached, voltage-gated Na⁺ channels open. Na⁺ rushes in, and depolarisation is rapid, with an overshoot to about +30 mV.
• The Na⁺ channels then inactivate, and the voltage-gated K⁺ channels open more slowly. K⁺ moves out and the membrane repolarises, with a brief hyperpolarisation, and the pump restores the ion distribution.
• It is all-or-none: a stimulus below threshold gives no impulse, and a larger one does not raise the size of the spike.
• In the absolute refractory period (Na⁺ channels inactivated) no impulse can be started, and in the relative refractory period only a strong stimulus can. This makes the impulse move in one direction.

Propagation. The depolarised region sets up local circuit currents to the neighbouring resting region, which is brought to threshold, and so the impulse travels along.

• In unmyelinated fibres the conduction is continuous, at about 0.5–2 m/s.
• In myelinated fibres it is saltatory: the impulse jumps between nodes of Ranvier, where Na⁺ channels are concentrated, at up to about 100 m/s, faster and using less energy.
• The velocity increases with axon diameter, myelination and temperature. Demyelination in multiple sclerosis slows conduction.

(c) Body axis formation in embryonic development (15 marks)

Concept. During early development the embryo establishes its anteroposterior (head–tail), dorsoventral (back–belly) and left–right axes. The axes are set by localised maternal determinants (mRNAs and proteins in the egg), by gradients of morphogens, and by organiser tissue that induces neighbouring cells. Axis information then switches on regional genes (the Hox genes).

Example: Drosophila.

• Anteroposterior axis: bicoid mRNA, placed at the anterior pole by nurse cells, is translated into a Bicoid protein gradient (highest anteriorly) that switches on hunchback and head genes. Nanos mRNA at the posterior end forms a Nanos gradient that blocks hunchback translation, so the posterior develops. Gap, pair-rule, segment-polarity and Hox (homeotic) genes then divide the embryo into segments.
• Dorsoventral axis: the Toll receptor is activated ventrally by Spätzle, so that a gradient of nuclear Dorsal protein forms (highest ventrally) and switches on twist and snail (mesoderm), with dpp dorsally.

Example: Xenopus.

• After fertilisation, cortical rotation moves the cortex about 30°, so the dorsal side forms opposite the sperm entry point. β-catenin (Wnt pathway) accumulates on the dorsal side and sets up the Nieuwkoop centre in dorsal vegetal cells.
• This induces the Spemann–Mangold organiser in the dorsal lip of the blastopore. The organiser secretes noggin, chordin and follistatin, which block BMP-4 and dorsalise the mesoderm and induce the neural plate. Spemann and Mangold (1924) showed that a grafted dorsal lip produced a second axis.
• The left–right axis arises from the leftward flow of fluid driven by cilia in the node, which switches on Nodal and Pitx2 on the left.