Proteins condensation polymers

Much of protein engineering concerns attempts to explore the relationship between protein stmcture and function. Proteins are polymers of amino acids (qv), which have general stmcture +H3N—CHR—COO , where R, the amino acid side chain, determines the unique identity and hence the stmcture and reactivity of the amino acid (Fig. 1, Table 1). Formation of a polypeptide or protein from the constituent amino acids involves the condensation of the amino-nitrogen of one residue to the carboxylate-carbon of another residue to form an amide, also called peptide, bond and water. The linear order in which amino acids are linked in the protein is called the primary stmcture of the protein or, more commonly, the amino acid sequence. Only 20 amino acid stmctures are used commonly in the cellular biosynthesis of proteins (qv). 

The name Nylon was given by the Du Pont company of America to their first synthetic condensation polymer formed by the reaction of difuncfional acids with difuncfional amines, ft had been made as part of the fundamental programme of W. H. Carothers to investigate the whole topic of polymerisation. The term has gradually been extended to other related polymers. These materials are strictly polyamides, but this term includes that otherwise distinct class of natural macromolecules, the proteins. The term nylon is retained for its usefulness in distinguishing synthetic polyamides from the broader class of such polymers. 

Representative condensation polymers are listed in Table I. The list is by no means exhaustive, but it serves to indicate the variety of condensation reactions which may be employed in the synthesis of polymers. Cellulose and proteins, although their syntheses have not been accomplished by condensation polymerization in the laboratory, nevertheless are included within the definition of condensation polymers on the ground that they can be degraded, hydrolytically, to monomers differing from the structural units by the addition of the elements of a molecule of water. This is denoted by the direction of the arrows in the table, indicating depolymerization. 

The essential distinction between the approaches used to formulate and evaluate proteins, compared with conventional low molecular weight drugs, lies in the need to maintain several levels of protein structure and the unique chemical and physical properties that these higher-order structures convey. Proteins are condensation polymers of amino acids, joined by peptide bonds. The levels of protein architecture are typically described in terms of the four orders of structure [23,24] depicted in Fig. 2. The primary structure refers to the sequence of amino acids and the location of any disulfide bonds. Secondary structure is derived from the steric relations of amino acid residues that are close to one another. The alpha-helix and beta-pleated sheet are examples of periodic secondary structure. Tertiary... 

Proteins are nature s polyamide condensation polymers. A protein is formed by polymerization of o-artiino acids, with the amino group on the carbon atom next to the carboxylic acid. Biologists call the bond formed a peptide rather than an amide. In the food chain these amino acids are continuously hydrolyzed and polymerized back into polymers, which the host can use in its tissues. These polymerization and depolymerization reactions in biological systems are all controlled by enzyme catalysts that produce extreme selectivity to the desired proteins. 

Polymers are large molecules (macromolecules) that consist of one or two small molecules (monomers) joined to each other in long, often highly branched, chains in a process called polymerization. Both natural and synthetic polymers exist. Some examples of natural polymers are starch, cellulose, chitin (the material of which shells are made), nucleic acids, and proteins. Synthetic polymers, the subject of this chapter, include polyethylene, polypropylene, polystyrene, polyesters, polycarbonates, and polyurethanes. In their raw, unprocessed form, synthetic polymers are sometimes referred to as resins. Polymers are formed in two general ways by addition or by condensation. 

Besides addition polymerization, the other general way to prepare polymers is known as condensation polymerization or step growth polymerization. Much of the pioneering work on condensation polymerization was conducted by Wallace Carothers while he was employed by DuPont. He recognized that many natural polymers are formed from monomers with two reactive functional groups. For example, proteins are polymers of amino acids, which contain both amine and carboxylic acid groups. The formation of amide bonds is used to connect one monomer to another. Carothers s attempts to imitate nature led to a whole industry based on condensation polymerization. 

Because they contain two functional groups, amino acids can react to produce condensation polymers by forming amide bonds. These polymers are called peptides, polypeptides, or proteins. Although there is no universally accepted distinction, the term protein is usually reserved for naturally occurring polymers that contain a relatively large number of amino acid units and have molecular masses in the range of a few thousand or larger. The term peptide is used for smaller polymers. 

Let s compare proteins to the polymers that were discussed in Chapter 24. One difference is that all the molecules of a particular protein are identical that is, they have the same molecular mass and contain the same number of amino acids connected in the same sequence. Recall that a typical condensation polymer consists of molecules containing many different numbers of monomers. More important, proteins are enormously more complex than simple condensation polymers because they are formed from a combination of 20 different monomer units. And these monomers are not randomly distributed in the protein. Rather, each molecule of a particular protein has an identical sequence of amino acid units. The exact sequence is of critical importance because it is the order of the side chains that determines the shape and function of that particular protein. 

Some types of natural condensation polymers play crucial roles in living systems. Proteins (Section 28-9) are polymeric chains of L-amino acids linked by peptide bonds. A peptide bond is formed by the elimination of a molecule of water between the amino group of one amino acid and the carboxylic acid group of another. 

Polymers are widely found in nature. The human body contains many natural polymers, such as proteins and nucleic acids. Cellulose, another natural polymer, is the main structural component of plants. Most natural polymers are condensation polymers, and in their formation from monomers water is a by-product. 

All proteins are condensation polymers of amino acids. An immense number of proteins exists in nature. Eor example, the human body is estimated to have 100,000 different proteins. What is amazing is that all of these proteins are derived from only twenty amino acids. In the condensation reaction whereby two amino acids become linked, one molecule of water forming from the carboxylic acid of one amino acid and the amine group of the other is eliminated. The result is a peptide bond hence, proteins are polypeptides containing from approximately fifty to thousands of amino acid residues. 

EXAMPLE 2.4 Proteins are polymers of polar and nonpolar amino acids the amino acid nnits in the polymer are called residues becanse when the peptide bond between an amino acid and a peptide is formed, water is removed in a condensation reaction, leaving a residne of the amino acid. Amino acid residnes that have polar side chains form hydrogen bonds with water so they are hydrophilic (Fig. 2-5). Nonpolar side chains of amino acid residnes do not form hydrogen bonds with water, so they do not dissolve readily in it they are said to be hydrophobic. Thns proteins tend to fold np so that their hydrophobic residnes are clnstered in an interior core, away from contact with the aqneons environment and the hydrophilic residnes tend to be arranged on the exterior interacting with water. Interactions also occnr with other proteins and other biomolecnles in general. 

The proteins also are condensation polymers. In this case, the fundamental molecular units are amino acids, which combine according to the reaction... 

Natural polymers are broadly classified as polysaccharides, proteins, polynucleotide, and natural rubber. This classification is based on the type of monomer building blocks for the polymer and type of covalent bonds for formation of the polymer. Polysaccharides that are carbohydrate polymers are formed by condensation of monomeric units called monosaccharides, proteins or peptide polymers are formed from amino acids and polynucleotide are synthesized from nucleotide condensation reaction forming glycosidic, peptide, and phosphodiester bonds, respectively. All natural polymers are condensation polymers. 

The carboxylic acids and amines link to form peptide bonds, also known as amide groups. Proteins are the condensation polymers made from amino acid monomers. Carbohydrates are also condensation polymers made from sugar monomers such as glucose and galactose. Condensation polymerization is occasionally used to form simple hydrocarbons. This method, however, is expensive and inefficient, so the addition polymer of ethene, i.e., polyethylene, is generally used. Condensation polymers, unlike addition polymers, may be biodegradable. The peptide or ester bonds between monomers can be hydrolyzed by acid catalysts or bacterial enzymes breaking the polymer chain into smaller pieces. The most commonly known condensation polymers are proteins and fabrics such as nylon, silk, or polyester. 

Proteins are polymers formed by condensation reactions of amino adds, which have the general structure... 

Is protein biosynthesis as shown in Figure 26.12 (page 1196) step growth or chain growth Is the protein that results an addition or a condensation polymer Why ... 

All proteins are condensation polymers of amino acids. A large number of... 

To summarize, then, proteins are polypeptides that are condensation polymers of amino acids. They vary greatly in size, and some proteins include non-amino acid groups. 

Enzymes are macromolecular protein catalysts, consisting of linear condensed polymers of a-amino acids joined in amide linkages. Each enzyme has a genetically determined and unique primary sequence, and it folds in three dimensions into a precise orientation (Lodish et al, 1999). 

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