Sheet forming three-dimensional structure

The three-dimensional structural architecture of plant defensins is exemplified by the structure of Rs-AFP, ° which comprises an N-terminal /3-strand followed by an ct-helix and two /3-strands (/3a/3/3 configuration). The /3-strands form a triple-stranded antiparallel /3-sheet. The three-dimensional structure is stabilized by three disulfide bonds. In general, in plant defensins two disulfide bonds form between the ct-helix and the central /3-strand. A third disulfide bond stabilizes the structure by linking the /3-strand after the helix to the coiled part after the ct-helix. This motif is called the cysteine-stabilized a/3-motif (CSa/3)" and also occurs in toxins isolated from insects, spiders, and scorpions.The fourth disulfide bond links the C-terminal end of the peptide with the N-terminal /3-strand. Two plant defensins, PhDl and PhD2, feature a fifth disulfide bond and have been proposed to be the prototypes of a new subclass within plant defensins." As a result of these structural features the global structure of plant defensins is notably different from o //3-thionins, which is one of the reasons for the different nomenclature. The structures of plant defensins Rs-AFP ° and NaDf are shown in Figure 6, where they are compared to the thionin /3-purothionin and the structurally more related drosomycin and charybdotoxin. ... 

In most silicate minerals a large number of silicate tetrahedra are linked together to form chains, sheets, or three-dimensional structures. We can connect... 

Polypeptide chains are folded into one or several discrete units, domains, which are the fundamental functional and three-dimensional structural units. The cores of domains are built up from combinations of small motifs of secondary structure, such as a-loop-a, P-loop-p, or p-a-p motifs. Domains are classified into three main structural groups a structures, where the core is built up exclusively from a helices p structures, which comprise antiparallel p sheets and a/p structures, where combinations of p-a-P motifs form a predominantly parallel p sheet surrounded by a helices. 

Figure 39-13. A schematic representation of the three-dimensional structure of Cro protein and its binding to DNA by its helix-turn-helix motif. The Cro monomer consists of three antiparallel p sheets (P1-P3) and three a-helices (a,-a3).The helix-turn-helix motif is formed because the aj and U2 helices are held at about 90 degrees to each other by a turn offour amino acids. The helix of Cro is the DNA recognition surface (shaded). Two monomers associate through the antiparallel P3 sheets to form a dimer that has a twofold axis of symmetry (right). A Cro dimer binds to DNA through its helices, each of which contacts about 5 bp on the same surface of the major groove. The distance between comparable points on the two DNA a-helices is 34 A, which is the distance required for one complete turn of the double helix. (Courtesy of B Mathews.)...

It is the sequence and types of amino acids and the way that they are folded that provides protein molecules with specific structure, activity, and function. Ionic charge, hydrogen bonding capability, and hydrophobicity are the major determinants for the resultant three-dimensional structure of protein molecules. The a-chain is twisted, folded, and formed into globular structures, a-helicies, and P-sheets based upon the side-chain amino acid sequence and weak intramolecular interactions such as hydrogen bonding between different parts of the peptide... 

X-ray diffraction analysis reveals the three-dimensional structure of both IL-l molecules to be quite similar. Both are globular proteins, composed of six strands of antiparallel P pleated sheet forming a barrel that is closed at one end by a further series of P sheets. 

An important element in the three-dimensional structure of a protein is the secondary structure. The secondary structure results from the formation of hydrogen bonds between the—N—H groups and the carbonyl (C O) groups of the peptide bonds —N—H 0=C. There are two basic ways to do this. We can form a helix or we can form a sheet. The great American chemist Linus Pauling won the Nobel Prize in Chemistry in 1954 for the elucidation of these structures. 

The three-dimensional structure of the catalytic core domain of HIV-1 integrase is centered on a mixed five stranded (3 sheet flanked by several helices forming... 

The polypeptide backbone does not assume a random three-dimensional structure, but instead generally forms regular arrangements of amino acids that are located near to each other in the linear sequence. These arrangements are termed the secondary structure of the polypeptide. The a-helix, 3-sheet, and 3-bend are examples of secondary structures frequently encountered in proteins. [Note The collagen helix, another example of secondary structure, is discussed on p. 43.]... 

Class II aminoacyl-tRNA synthetases contain a different set of three "signature sequences," two of which form an ATP-binding catalytic domain. The active site structure is built on an antiparallel (3 sheet and is surrounded by two helices (Fig. 29-9). Each class contains subgroups with inserted loops that form other domains. In the following tabulation the reference numbers refer to three-dimensional structural studies. 

---