October 13, 2007

Molecular Motors

There are miniature machines in the cells of our bodies, which carry molecules on their 'legs' and walk on their 'heads', as they move from one part of the cell to the other. This way, they transport synaptic vesicles, small bags filled with neurotransmitters, from the nerve cell-body to the end of the neuron, the synaptic knobs. They also pull the chromosomes apart during cell division: mitosis and meiosis. In can also transport molecules from the synaptic knobs towards its cell body (retrograde transport). In this mode of transport, the nerve terminals pick up molecules (by endocytosis) such as nerve growth factors and send them to the cell-body. By this mechanism, they also pick up numerous viruses (polio virus that cause poliomyelitis etc.), toxins (toxins of Clostridium tetani; of tetanus) and send them to the interior as well, to its own detriment.

Broadly speaking, the molecular motors are of two types: those which move over actin (a filamentous molecule) and those which walk on microtubules. Kinesin is one such molecule which walks on microtubules. It has two heads. One head binds to ATP molecules (the energy currency of the cells) and hydrolyzes it to derive energy; while the other head bends and 'swings' forward. This way they produce a seemingly continuous motion. The cargo is bound to the other end (so lets we call them legs). Kinesins, with some exceptions, typically transport molecules towards the + end (polymerizing end) of microtubules. Dyneins, are quite like kinesins, but they carry molecules towards the -ve end of microtubules. It is of two types: cytoplasmic dynein and axonemal dynein. Axonemal dyneins are found in the cilia or flagella of cells, allowing the cell to move about by 'beating'.

Myosin on the other hand, walks along actin microfilaments. When we move our muscles, one of its heads hydrolyzes ATP molecules, and derives energy from it. It harnesses this energy to 'bend its other head', in what we call a power stroke. Thus, in doing so, it brings actin microfilaments closer (via this power stroke). Your muscle contracts, as a result. This is known as the 'ratchet theory' or walk along theory (of muscle contraction).

This video clip beautifully illustrates how kinesin does a perfect 10:

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