Proteins amide linkages

The amide linkage between monomer units in a protein is called a peptide bond. Peptides and polypeptides, which often exhibit biological activity (see Antibiotics, peptides Neuroregulators), are smaller than proteins. Although the differentiation between polypeptide and protein is somewhat arbitrary, the usual distinction is drawn around 100 monomer units. Proteins are also characterized by higher levels of stmcture resulting from internal interactions. 

The chloroisocyanurates can be used in the bleaching of cotton, synthetics, and their blends they do, however, attack proteinaceous fibers, such as silk or wool, presumably via active chlorine reaction with the peptide (amide) linkage. However, the chloroisocyanurates can be used as shrink-proofing agents in wool finishing (131), (see Textiles Wool). The same action of chlorine upon proteins contributes to the effectiveness of chloroisocyanurates in automatic dishwashers. 

Chemically, proteins are unbranched polymers of amino acids linked head to tail, from carboxyl group to amino group, through formation of covalent peptide bonds, a type of amide linkage (Figure 5.1). 

Larger polymers are known as proteins. Aside from the amide linkages, the polymer chain is very flexible, giving rise to the possibility of an enormous number of different conformers. It is nothing short of remarkable, therefore, that proteins rapidly fold into a single conformation. Very strong forces must be at work. 

Amide hydrolysis is common in biological chemistry. Just as the hydrolysis of esters is the initial step in the digestion of dietary fats, the hydrolysis of amides is the initial step in the digestion of dietary proteins. The reaction is catalyzed by protease enzymes and occurs by a mechanism almost identical to that we just saw for fat hydrolysis. That is, an initial nucleophilic acyl substitution of an alcohol group in the enzyme on an amide linkage in the protein gives an acyl enzyme intermediate that then undergoes hydrolysis. 

Proteins are polyamides formed by the polymerization, through amide linkages, of a-amino acids. Three of the 25-30 important natural a-amino acids are shown in Figure 18-13. Each acid has an amine group, —NHj, attached to the a-carbon, the carbon atom immediately adjacent to the carboxylic acid group. 

The protein molecule may involve hundreds of such amino acid molecules connected through the amide linkages. A portion of this chain might be represented as shown in Figure 18-14. 

C13-0002. Amino acids are the molecular building blocks of proteins. Any two amino acids can condense in two ways each creates an amide linkage. Draw the stmetures of the two condensation products that link these two amino acids ... 

Polymers that contain amide linkage groups are called polyamides. Proteins, which are biological polyamides, are described in Section 13-1. Here we focus on two commercially important polyamides Nylon 66 and Kevlar. 

Proteins form in a sequence of condensation reactions in which the amine end of one amino acid combines with the carboxyl end of another, eliminating a water molecule to create an amide linkage. The amide group that connects two amino acids is called a peptide linkage, and the resulting molecule is known as a peptide. When two amino acids are linked, the product is a dipeptide. A dipeptide formed from alanine and glycine is shown in Figure 13-33. 

Sulfosuccinimidyl-2-(7-azido-4-methylcoumarin-3-acetamide)ethyl-l,3 -dithiopropionate (SAED) is a photoreactive heterobifunctional crosslinking agent that also contains a fluorescent group (Thermo Fisher). The sulfo-NHS ester end of the reagent reacts with primary amines in proteins and other molecules to form stable amide linkages. The photoreactive end is an AMCA... 

Three main forms of amine-reactive AMCA probes are commonly available. One of them is simply the free acid form of AMCA, which can be used to couple to amine-containing molecules using the carbodiimide reaction (Chapter 3, Section 1.1). The other two are active-ester derivatives of AMCA—the water-insoluble NHS ester and the water-soluble sulfo-NHS ester forms—both of which spontaneously react with amines to create stable amide linkages. All of them react under mild conditions with primary amines in proteins and other molecules to form highly fluorescent derivatives. 

AMCA-NHS, succinimidyl-7-amino-4-methylcoumarin-3-acetic acid, is an amine-reactive derivative of AMCA containing an NHS ester on its carboxylate group (Thermo Fisher). The result is reactivity directed toward amine-containing molecules, forming amide linkages with the AMCA fluorophore (Figure 9.23). Proteins labeled with AMCA show little-to-no effect on the isoelectric point of the molecule. 

Amine-reactive biotinylation reagents contain reactive groups off biotin s valeric acid side chain that are able to form covalent bonds with primary amines in proteins and other molecules. Two basic types are commonly available N-hydroxysuccinimide (NHS) esters and carboxylates. NHS esters spontaneously react with amines to form amide linkages (Chapter 2, Section 1.4),... 

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