Molecular Machines

Consider the humble mousetrap (Figure 20.1). The mousetraps that my family uses in our home to deal with unwelcome rodents consist of a num- ber of parts. There are: (1) a flat wooden platform to act as a base; (2) a metal hammer, which does the actual job of crushing the little mouse; (3) a wire spring with extended ends to press against the platform and the hammer when the trap is charged; (4) a sensitive catch which releases when slight pressure is applied; and (5) a metal bar which holds the hammer back when the trap is charged and connects to the catch. There are also assorted staples and screws to hold the system together.

If any one of the components of the mousetrap (the base, hammer, spring, catch, or holding bar) is removed, then the trap does not function. In other words, the simple little mousetrap has no ability to trap a mouse until sev- eral separate parts are all assembled.

Because the mousetrap is necessarily composed of several parts, it is irre- ducibly complex. Thus, irreducibly complex systems exist.

Now, are any biochemical systems irreducibly complex? Yes, it turns out that many are.

Earlier we discussed proteins. In many biological structures proteins are simply components of larger molecular machines. Like the picture tube, wires, metal bolts, and screws that comprise a television set, many proteins are part of structures that only function when virtually all of the compo- nents have been assembled.

A good example of this is a cilium. Cilia are hairlike organelles on the sur- faces of many animal and lower plant cells that serve to move fluid over the cell’s surface or to “row” single cells through a fluid. In humans, for example, epithelial cells lining the respiratory tract each have about 200 cilia that beat in synchrony to sweep mucus towards the throat for elimination.

A cilium consists of a membrane-coated bundle of fibers called an axoneme. An axoneme contains a ring of 9 double microtubules surrounding two central single microtubules. Each outer doublet consists of a ring of 13 filaments (subfiber A) fused to an assembly of 10 filaments (subfiber B). The filaments of the microtubules are composed of two proteins called alpha and beta tubulin. The 11 microtubules forming an axoneme are held together by three types of connectors: subfibers A are joined to the central microtubules by radial spokes; adjacent outer doublets are joined by linkers that consist of a highly elastic protein called nexin; and the central microtubules are joined by a connecting bridge. Finally, every subfiber A bears two arms, an inner arm and an outer arm, both containing the protein dynein.

But how does a cilium work? Experiments have indicated that ciliary mo- tion results from the chemically-powered “walking” of the dynein arms on one microtubule up the neighboring subfiber B of a second microtubule so that the two microtubules slide past each other. However, the protein cross-links between microtubules in an intact cilium prevent neighboring mi- crotubules from sliding past each other by more than a short distance. These cross-links, therefore, convert the dynein-induced sliding motion to a bending motion of the entire axoneme.