Enzymes Substances that catalyze biological reactions

Enzyme A substance that catalyzes biological reactions. 

Amino Acids and Proteins. Proteins are the molecules that perform the functions of life. They can be enzymes that catalyze biological reactions, or they can be the receptor site on a membrane that binds a specific substance. Proteins are important parts of both bones—the so-called hard biologies—and the soft biologies such as muscle and skin. Any discussion of the structure of living organisms must begin with the structure of proteins. 

We will examine in detail enzymes and enzyme inhibitors, enzymes being the proteinaceous substances that catalyze the myriad biochemical reactions or processes that occur in the body. Almost without exception, there is one enzyme specific to each biochemical reaction. (What may be called supporting reactions, side reactions, or simultaneous reactions may also be involved and, in fact, may be a necessity.) In turn, there are other proteinaceous substances, or still other chemical substances, that will inhibit, block, modulate, or control the action of enzymes, and on rare occasion, even accelerate or promote the action. Known as enzyme inhibitors, such substances may affect more than one enzyme, or many different enzymes that is, they may have side effects, or in other words, are nonselective. These inhibitors may be generated internally, or more likely may originate from external sources. That is, they may be designated as medicines. In fact, modem medicine is more and more viewed as encompassing enzymes and enzyme inhibitors. Broadly, this may be viewed as the utilization of biologically active or biochemically active substances, that is, bioactive substances, either natural or synthetic. 

One final difference between laboratory and biological reactions is in their specificity. A catalyst might be used in the laboratory to catalyze the reaction of thousands of different substances, but an enzyme, because it can bind only a specific substrate molecule having a specific shape, will catalyze only a specific reaction, it s this exquisite specificity that makes biological chemistry so remarkable and that makes life possible. Table 5.4 summarizes some of the differences between laboratory and biological reactions. 

What is most remarkable is that cells can produce proteins with strikingly different properties and activities by joining the same 20 amino acids in many different combinations and sequences. From these building blocks different organisms can make such widely diverse products as enzymes, hormones, antibodies, transporters, muscle fibers, the lens protein of the eye, feathers, spider webs, rhinoceros horn, milk proteins, antibiotics, mushroom poisons, and myriad other substances having distinct biological activities (Fig. 3-1). Among these protein products, the enzymes are the most varied and specialized. Virtually all cellular reactions are catalyzed by enzymes. 

Whereas a major function of biological membranes is to maintain the status quo by preventing loss of vital materials and entry of harmful substances, membranes must also engage in selective transport processes. Living cells depend on an influx of phosphate and other ions, and of nutrients such as carbohydrates and amino acids. They extrude certain ions, such as Na+, and rid themselves of metabolic end products. How do these ionic or polar species traverse the phospholipid bilayer of the plasma membrane How do pyruvate, malate, the tricarboxylic acid citrate and even ATP move between the cytosol and the mitochondrial matrix (see figs. 13.15 and 14.1) The answer is that biological membranes contain proteins that act as specific transporters, or permeases. These proteins behave much like conventional enzymes They bind substrates and they release products. Their primary function, however, is not to catalyze chemical reactions but to move materials from one side of a membrane to the other. In this section we discuss the general features of membrane transport and examine the structures and activities of several transport proteins. 

In Section 3.2 we introduced the basic processes of advection, diffusion, and drift, by which material is transported in biophysical systems. In this chapter we focus on a specialized class of transport transport across biological membranes. Transport of a substance across a membrane may be driven by passive permeation, as described by Equation (3.60), or it may be facilitated by a carrier protein or transporter that is embedded in the membrane. Thus transport of substances across membranes mediated by transporters is termed carrier-mediated transport. The most basic way to think about carrier proteins or transporters is as enzymes that catalyze reactions that involve transport. 

A substrate is the substance upon which an enzyme acts in an enzymatic reaction. Enzymes are biological catalysts that increase the rate of chemical reactions by decreasing the activation energy required for that reaction. An enzyme catalyzes a chemical reaction converting a snbstrate reactant to a product. An individnal enzyme generally has more than one snbstrate and may be specific to several reaction intermediates that are part of an overall reaction. 

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