Extracellular enzymes role

Extracellular functions of calcium are better understood than intracellular functions, partly because of the difficulties associated with the analysis of calcium inside the cell. Extracellular roles are those of (a) trigger, (b) an activator and stabilizer of extracellular enzymes, and (c) in bone and teeth (Section 62.1.3.9). Intracellular activities are mainly trigger effects, and are intimately associated with problems of transport and binding. 

The general picture of muscle contraction in the heart resembles that of skeletal muscle. Cardiac muscle, like skeletal muscle, is striated and uses the actin-myosin-tropomyosin-troponin system described above. Unlike skeletal muscle, cardiac muscle exhibits intrinsic rhyth-micity, and individual myocytes communicate with each other because of its syncytial nature. The T tubular system is more developed in cardiac muscle, whereas the sarcoplasmic reticulum is less extensive and consequently the intracellular supply of Ca for contraction is less. Cardiac muscle thus relies on extracellular Ca for contraction if isolated cardiac muscle is deprived of Ca, it ceases to beat within approximately 1 minute, whereas skeletal muscle can continue to contract without an extraceUular source of Ca +. Cyclic AMP plays a more prominent role in cardiac than in skeletal muscle. It modulates intracellular levels of Ca through the activation of protein kinases these enzymes phosphorylate various transport proteins in the sarcolemma and sarcoplasmic reticulum and also in the troponin-tropomyosin regulatory complex, affecting intracellular levels of Ca or responses to it. There is a rough correlation between the phosphorylation of Tpl and the increased contraction of cardiac muscle induced by catecholamines. This may account for the inotropic effects (increased contractility) of P-adrenergic compounds on the heart. Some differences among skeletal, cardiac, and smooth muscle are summarized in... 

A biochemical catalyst is called an enzyme. Enzymes are specialized proteins that catalyze specific biochemical reactions. Some enzymes are found in extracellular fluids such as saliva and gastric juices, but most are found inside cells. Each type of cell has a different array of enzymes that act together to determine what role the cell plays in the overall biochemistry of the organism. Enzymes are complicated molecules. Biochemists have determined the molecular structures of some enzymes, but the structures of many enzymes are not yet known. 

The matrix metalloprotease (MMP) family of zinc hydrolases are thought to play important roles in extracellular tissue remodeling in angiogenesis and other normal physiological processes, in some inflammatory processes and in metastatic processes in cancer. Like the zinc carboxypeptidases, the MMPs also utilize a zinc-coordinated water molecule to initiate attack on the scissile amide bond of protein substrates. These enzymes are synthesized by the ribosome in a latent form composed of a catalytic domain and an N-terminal extension, referred to as the prodomain the latent, or inactive form of the enzyme is referred to as a zymogen or... 

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