The Cross-bridge cycle

Much of our understanding of the mechanism of muscle contraction has come from excellent biochemical studies performed from the 1950s to the mid-1970s (Webb and Trentham, 83). It was during this period that methods for isolating specific muscle proteins were developed as well as the methods for measuring their physicochemical and biochemical properties.

In its simplest form, biochemical experiments on muscle contractile proteins have shown that, during the cross-bridge cycle, actin (A) combines with myosin (M) and ATP to produce force, adenosine diphosphate (ADP) and inorganic phosphate, Pi This can be represented as a chemical reaction in the form
A + M + ATP -> A + M + ADP + Pi + Force (equation 1)
However, we also know that upon the death of a muscle, a rigor state is entered whereby actin and myosin interact to form a very stiff connection. This can be represented as
A + M -> A.M "rigor" complex (equation 2)
If actin and myosin can interact by themselves, where does ATP come into the picture during contraction? Experiments have demonstrated that the myosin molecule can hydrolyze ATP into ADP and Pi. In other words,
M + ATP -> M + ADP + Pi (equation 3)
Scientists now agree that ATP serves at least two functions in skeletal muscle systems: First, ATP disconnects actin from myosin, and second, ATP is hydrolyzed by the myosin molecule to produce the energy required for muscle contraction. This description of the different biochemical steps involved in muscle contraction is referred to as the Lymn-Taylor actomyosin ATPase hydrolysis mechanism. (Webb and Trentham, 83)

The relationship between the Lymn-Taylor kinetic scheme and the mechanical cross-bridge cycle is not fully known. However, Lymn and Taylor proposed that their biochemical data could be incorporated into a four-step cross-bridge cycle that could be envisioned thus:

Cross-bridge cycle. Download PostScript

  1. The actin-myosin bridge very rapidly dissociates due to ATP binding to myosin.
  2. The free myosin bridge moves into position to attach to actin, during which ATP is hydrolyzed. (Eq. 3)
  3. The free myosin bridge along with its hydrolysis products rebinds to the actin filament. (Eq. 2)
  4. The cross-bridge generates force, and actin displaces the reaction products (ADP and Pi) from the myosin cross-bridge. This is the rate-limiting step of contraction. The actin-myosin cross-bridge is now ready for the ATP binding of step 1.
It might be appreciated that confirmation of this mechanism would be very difficult indeed! This is currently an active area of muscle biophysical research (Webb and Trentham, 83). One might imagine the difficulty in confirming these elementary mechanical and biophysical reactions. However, a recent advance in biochemistry has allowed direct testing and manipulation of this scheme. The advance involves the development of "caged" compounds--compounds which are inactive in their caged form and become active when the cage is instantaneously removed by a pulse of high-energy laser light (McCray et al., 80). Using caged ATP, single muscle fibers have been subjected to experiments such as those described above and found to behave much as predicted based on the biochemical data (Goldman, 87).

More details, including some nice animations, can be found on Michael A. Ferenczi's page at the National Institute for Medical Research