Amino acid side chains basic

It is interesting to note that the amino acid side chains may be either neutral as in valine, acidic as in glutamic acid or basic as in lysine. The presence of both acidic and basic side chains leads to proteins such as casein acting as amphoteric electrolytes and their physical behaviour will depend on the pH of the environment in which the molecules exist. This is indicated by Figure 30.2, showing a simplified protein molecule with just one acidic and one basic side group. 

Some amino acids have side chains that bear- acidic or basic groups. As Table 27.3 indicates, these amino acids are characterized by three pK values. The third pK reflects the nature of the side chain. Acidic amino acids (aspartic and glutfflnic acid) have acidic side chains basic amino acids (lysine, arginine, and histidine) have basic side chains. 

Milk contains a range of groups which are effective in buffering over a wide pH range. The principal buffering compounds in milk are its salts (particularly soluble calcium phosphate, citrate and bicarbonate) and acidic and basic amino acid side-chains on proteins (particularly the caseins). The contribution of these components to the buffering of milk was discussed in detail by Singh, McCarthy and Lucey (1997). 

In addition to participating in acid-base catalysis, some amino acid side chains may enter into covalent bond formation with substrate molecules, a phenomenon that is often referred to as covalent catalysis.174 When basic groups participate this may be called nucleophilic catalysis. Covalent catalysis occurs frequently with enzymes catalyzing nucleophilic displacement reactions and examples will be considered in Chapter 12. They include the formation of an acyl-enzyme intermediate by chymotrypsin (Fig. 12-11). Several of the coenzymes discussed in Chapters 14 and 15 also participate in covalent catalysis. These coenzymes combine with substrates to form reactive intermediate compounds whose structures allow them to be converted rapidly to the final products. 

Each of the monomeric proteins c-jun and c-fos, as well as other members of the leucine zipper family, has an N-terminal DNA-binding domain rich in positively charged basic amino acid side chains, an activation domain that can interact with other proteins in the initiation complex, and the leucine-rich dimerization domain.363 The parallel coiled-coil structure (Fig. 2-21) allows for formation of either homodimers or heterodimers. However, cFos alone does not bind to DNA significantly and the cjun/cFos heterodimer binds much more tightly than does cjun alone.364 The yeast transcriptional activator protein GCN4 binds to the same 5 -TGACTCA sequence as does the mammalian AP-1 and also has a leucine zipper structure.360 364 365... 

Salt bridges between positively charged basic amino acid side chains of histones and the negatively charged DNA phosphates play a major role in stabilizing the DNA-histone complex. Indeed, treatment of chromatin with concentrated NaCl (1-2 m), which is known to disrupt electrostatic bonds, causes a complete dissociation of DNA and histone in the nucleohistone complex. 

According to the lock-and-key model, an enzyme is pictured as a large, irregularly shaped molecule with a cleft, or crevice, in its middle. Inside the crevice is an active site, a small region with the shape and chemical composition necessary to bind the substrate and catalyze the appropriate reaction. In other words, the active site acts like a lock into which only a specific key can fit (Figure 24.10). An enzyme s active site is lined by various acidic, basic, and neutral amino acid side chains, all properly positioned for maximum interaction with the substrate. 

The chemical modification studies have thus not yet led to a much more conclusive picture of the active site than that outlined in Fig. 12, and the identification of amino acid side chains involved in catalysis or substrate binding may have to await the completion of the crystal structure determination. The reporter properties of the Co(II) enzyme clearly show, however, that an open coordination position is of decisive functional importance, that the metal ion is intimately associated with the basic group participating in the reaction, and that the metal ion is probably also involved in the binding of one of the substrates, HCO3. 

C is correct The solubilities of amino acids differ based upon the R group. Phenylalanine has a benzene R group and is the least polar amino acid listed. The carboxylic acid and amines on the other R groups increase solubility. You may have also memorized the four groups of amino acid side chains as either nonpolar, polar, acidic, or basic. Acidic, basic, and polar amino adds have greater water sohibility than nonpolar amino acids. 

Basic-stable protecting groups for sugar hydroxyl and amino acids side chain functional groups... 

Proteins are large polypeptides with molecular weights ranging from a few thousand into the millions. They contain 16% N on the average. Because some 20 amino acids are present in most proteins and there is no limitation to the size and amount of each amino acid a protein may contain, the number and types of proteins found in nature are almost limitless. Each protein has certain physical and chemical properties that are uniquely suited to its role in the living organism. The properties a protein may exhibit are ultimately a result of its amino acid content. Amino acid side chains determine whether the protein is water soluble, whether it is acidic or basic, and the shape it assumes. It is thus extremely important to study the amino acid content and sequences in proteins, as well as to ascertain their shapes and sizes. To exhibit a specific biologic property, a protein must not only contain the correct amino acid sequence but it must also have the appropriate size and shape. 

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