Amino acid polypeptide chain formed

The results obtained with the oligomeric forms of D.gigas ferredoxin and reconstituted material, implicate that the same amino-acid polypeptide chain can accomodate both [3 Fe—xS] and [4 Fe—4 S] centers. Fdll represents a very pure and homogeneous preparation in respect to the type of centers present, being an example of a protein containing only [3 Fe—xS] cores. The oligomeric Fdl is isolated in a form that contains both molecules with one [4 Fe-4 S] center and a small number (up to 25 %) of molecules with one (3 Fe-xS] center. This situation is different from that of the AvFe—Sill protein where in each molecule one center of the novel structure is associated with one [4 Fe-4 S] center of HiPIP type . 

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). 

Figure 1.2 Proteins are built up by amino acids that are linked by peptide bonds to form a polypeptide chain, (a) Schematic diagram of an amino acid. Illustrating the nomenclature used in this book. A central carbon atom (Ca) is attached to an amino group (NH2), a carboxyl group (COOH), a hydrogen atom (H), and a side chain (R). (b) In a polypeptide chain the carboxyl group of amino acid n has formed a peptide bond, C-N, to the amino group of amino acid + 1. One water molecule is eliminated in this process. The repeating units, which are called residues, are divided into main-chain atoms and side chains. The main-chain part, which is identical in all residues, contains a central Ca atom attached to an NH group, a C =0 group, and an H atom. The side chain R, which is different for different residues, is bound to the Ca atom.

Imately 65 X 55 X 50 It Is composed of four polypeptide chains each resembling quite closely the myoglobin chain The three dimensional structure of the subunits Is held together by weak noncovalent bonds The polar amino acid side chains are In contact with the solvent, and the nonpolar residues are located In the Interior of the molecule or In regions which form the contacts between chains The heme group Is located In a pocket In each chain residues In contact with heme are Invariable ( e are the same In different mammalian hemoglobins) and the bonds between heme and chain are hydrophobic Interactions Contacts between like chains (a-a are... 

The nature of the amino acid residues is of prime importance in the development and maintenance of protein structure. Polypeptide chains composed of simple aliphatic amino acids tend to form helices more readily than do those involving many different amino acids. Sections of a polypeptide chain which are mainly non-polar and hydrophobic tend to be buried in the interior of the molecule away from the interface with water, whereas the polar amino acid residues usually lie on the exterior of a globular protein. The folded polypeptide chain is further stabilized by the presence of disulphide bonds, which are produced by the oxidation of two cysteine residues. Such covalent bonds are extremely important in maintaining protein structure, both internally in the globular proteins and externally in the bonding between adjacent chains in the fibrous proteins. 

Alpha helix one form of secondary structure in proteins in which the polypeptide chain forms a helix having 3.6 amino acid residues per turn. 

The unique three-dimensional structure of each polypeptide is determined by its amino acid sequence. Interactions between the amino acid side chains guide the folding of the polypeptide to form a compact structure. Four types of interactions cooperate in stabilizing the tertiary structures of globular proteins. 

Amino acids and the structure of the polypeptide chain. Polypeptides are composed of L-amino acids covalently linked together in a sequential manner to form linear chains, (a) The generalized structure of the amino acid. The zwitterion form, in which the amino group and the carboxyl group are ionized, is strongly favored. (b) Structures of some of the R groups found for different amino acids, (c) Two amino acids become covalently linked by a peptide bond, and water is lost, (d) Repeated peptide bond formation generates a polypeptide chain, which is the major component of all proteins. 

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