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Components of Cells
The Macromolecules
Muscle Proteins

Types of Muscle

Animals move. This one characteristic, above all others, characterizes the animal way of life. In it's never ending search for food an animal must be capable of either walking, running, swimming or flying, all of which depend on specialized tissues rapidly contracting and then pulling on articulated bones.

In humans, these cells, often called muscle fibers, are responsible for rapid contraction. They are very large with many prominent nuclei, about 50 um in diameter and of varying lengths (several centimeters are possible!). These cells are in turn organized into larger structures called skeletal muscle.

Humans also have a different kind of muscle, cardiac and smooth muscle, which is not connected to bone, and is not usually under the conscious control of the animal. These muscles act automatically (and sometimes continuously) to pump blood around the body (the heart), or food through the intestines (peristalsis).

Two proteins and contraction

The basis of all muscle contraction is the ability of two proteins, actin and myosin to organize themselves into fibers that can slide past one another.

Actin filaments consist of long, long arrays (polymers) of smaller actin protein subunits. When seen under the electron microscope, an actin filament looks like two threads of actin polymers twisted around one another in a helical arrangement.

Each filament has a plus end, where cells can rapidly add more and more actin molecules, hence lengthening the filament, and a minus end which can add more subunits, but very slowly.

A myosin-II molecule consists of four polypeptide chains - two heavy chains and four light chains. These are complexed together in such a way that the myosin-II filament has a "head" at one end, (made up of part of the two heavy chains and all of the four light chains), and a "tail" made up of the rest of the heavy chains wrapped around each other in a double alpha-helix.

A large number of these myosin-II molecules come together to form a myosin-II thick filament, in which the "tail" regions of many myosin-II molecules aggregate with one another in such a way that their "head" regions stick out from the surface of the filament.

These myosin-II thick filaments are symmetrical, with regions of myosin "heads" at both ends, and a bare region in the middle where there are only the tail regions.

Myosin heads "walk"

Using ATP as a source of energy, the heads of the myosin-II molecules can "wag" back and forth. This simple action can then be used to drag, or pull, an actin filament past a myosin thick filament. Deceptively simple as this seems, it is the basis of all muscle contraction.

Muscle cells (fibers) are packed with tiny contractile units called myofibrils, which are in turn made up of even smaller contractile units called sarcomeres.

Each sarcomeres starts with a solid disc of material, the Z-disc, to which are attached the plus ends of many actin filaments.

In between the arrays of actin filaments are a set of myosin thick filaments, which point in both directions. On the other side of the myosin thick filaments are a second set of actin filaments, which are also attached to a second Z-disc.

It is the action and interaction of the myosin "heads" on the thick filaments with the nearby actin filaments which enables the two kinds of filament to be pulled past one another, shortening the sarcomeres, shortening the myofibrils, shortening the muscle fibers and hence shortening the muscle.

The sequence

A complete cycle of interaction (and movement) between the myosin heads and the actin filaments consists of about 5 recognizable steps or stages:

  1. Attached: the myosin head is firmly attached to the nearby actin filament and is "bent" in a way that it points to the longer, tail region which is buried in the main myosin thick filament.

  2. Released: a molecule of ATP binds to the myosin head in a special cleft region. This distorts the head of the myosin molecule just enough to cause it to be released from the actin filament.

  3. Set: the head of the myosin molecule tightly closes around the ATP molecule and in doing so it changes shape. This shape-change "wags" the head away from the tail region by about 5 nm, and "sets" it up by snapping the ATP into ADP and phosphate.

  4. Reattached: phosphate is released from the myosin head, allowing it to reattach to the actin filament (but now in a different place further back from its original attachment point!).

  5. Power stroke: the myosin head releases the ADP and "wags" again, bending back into its original position at the start of the cycle. This is the power stroke in which the actin filament (attached to the myosin head) is dragged 5 nm past the thicker myosin filament.

With each of these cycles of action (about 5 per second), the actin filament is pulled and dragged further and further past the myosin thick filament. The two filaments slide past one another at about 15 um/second, and whole sarcomere shortens and shortens. Multiplied many times over, these actions shorten the muscle and make all kinds of movement possible.

© 2003, Professor John Blamire