Enzyme, cleft membrane-bound

The conformation of membrane-bound enzymes is undoubtedly restricted by the membrane. However, the mechanism of action of these enzymes appears to be similar to that of soluble enzymes, so that the presence of clefts and conformational flexibility is to be expected. The mitochondrial coupling factor apparently contains both the ATP synthesizing enzyme and a proton channel conformational changes undoubtedly play a role in the function of this system. A large movement of polypeptide chains has been proposed in the functioning of this system (and for other membrane-bound enzymes), but no convincing experimental evidence is available to support such a hypothesis. 

FIGURE 6.10 The neuromuscular junction. The region of contact between one nerve and another nerve, or between a nerve and a muscle cell, is called a s)maptic cleft. The secretory vesicles are represented by circles in which acetylcholine is represented by dots. The nerve impulse provokes the entry of calcium ions (not shown) from the extracellular fluid into the nerve cell. Calcium ions act as a signal that stimulates the fusion of vesicles with the plcisma membrane, releasing acetylcholine into the extracellular fluid. Acetylcholine binds to membrane-bound proteins (acetylcholine receptors) on the plasma membrane of the muscle cell, resulting in stimulation of the muscle cell. Acetylcholinesterase of the neuromuscular junction catalyzes the destruction of acetylcholine in the moments after transmission of the nerve impulse. The enzyme is extracellular and is bound to proteoglycan, a molecule of extracellular matrix. 

Acetylcholine is synthesized from choline and acetyl coenzyme A through the action of the enzyme choline acetyltransferase and becomes packaged into membrane-bound vesicles. After the arrival of a nerve signal at the termination of an axon, the vesicles fuse with the cell membrane, causing the release of acetylcholine into the synaptic cleft. For the nerve signal to continue, acetylcholine must diffuse to another nearby neuron or muscle cell, where it will bind and activate a receptor protein. 

---