Enzymes, action upon each other

A simplified view of metabolism is to consider a cell as a "bag of enzymes." Indeed, much of metabolism can be explained by the action of several thousand enzymes promoting specific reactions of their substrates. These reactions are based upon the natural chemical reactivities of the substrates. However, the enzymes, through the specificity of their actions and through association with each other,96 221 223 channel the reactions into a selected series of metabolic pathways. The reactions are often organized as cycles which are inherently stable. We have seen that biosynthesis often involves ATP-dependent reductive reactions. 

As for the first question cytoskeletal interactions are apparently involved in keeping the proteinases and their transmembrane substrates apart, keeping them in distinct domains of the plasma membrane. See also ref. 15. Upon activation, the cytoskeletal attachments seem to be loosened so that the proteinases and their substrates can approach each other and interact. How spatial restriction and compartmentalization of enzymes and substrates controls their action is of more general importance. We shall come back to that (Chapter 7). 

Lipases are produced by microorganisms such as bacteria and molds are produced by plants are present in animals, especially in the pancreas and are present in milk. Lipases may cause spoilage of food because the free fatty acids formed cause rancidity. In other cases, the action of lipases is desirable and is produced intentionally. The boundary between flavor and off-flavor is often a very narrow range. For instance, hydrolysis of milk fat in milk leads to very unpleasant off-flavors at very low free fatty acid concentration. The hydrolysis of milk fat in cheese contributes to the desirable flavor. These differences are probably related to the background upon which these fatty acids are superimposed and to the specificity for particular groups of fatty acids of each enzyme. 

Thus, the individual pharmacological effects exerted by each calcium channel blocker may depend upon the extent to which the drug affects other intracellular systems as well as its potency at the calcium channel. Many of the effects described here would tend to increase the vasodilator action of the drugs, such as inhibition of calmodulin-dependent enzymes. However, these other effects are subsidiary to blockade of the calcium channel, as they occur mainly at concentrations higher than those required to block the channels therefore, at low concentrations the actions of the calcium channel blocking drugs are relatively specific. 

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