Tertiary -Sheet Structures

These spectra are similar to that of native RNase A at 95°C (not shown). The far-UV spectrum at 95°C indicates a retention of substantial p-sheet secondary structure, but a significant loss of the a-helix conformation as indicated by the decrease of intensity at 222 nm.48 The near-UV spectrum at 95°C indicates a complete collapse of tertiary structure as is seen in molten globule proteins.49 Trace 3 is the sample from trace 2 after cooling the protein to 23°C. Both spectra reveal little recovery of either secondary or tertiary protein structure. 

Type i and ii membrane proteins only contain one transmembrane helix of this type, while type ill proteins contain several. Rarely, type i and ii polypeptides can aggregate to form a type iV transmembrane protein. Several groups of integral membrane proteins—e.g., the porins (see p. 212)—penetrate the membrane with antiparallel (3-sheet structures. Due to its shape, this tertiary structure is known as a P-barrel. ... 

The template used for generating P-sheet structures described in this section is based on the structure of gramicidin S (1, Scheme 1). Gramicidin S is a head-to-tail cyclic decapeptide discovered over 50 years ago and has the sequence c[-Val-Om-Leu-D-Phe-Pro-]2. 13 The tertiary structure of gramicidin S has since been elucidated and found to exist in a P-sheet/p-turn conformation. 14,15 As shown in Scheme 1, two antiparallel P-strands containing the Val-Om-Leu sequence are held in place by two type II P-tums defined by the D-Phe-Pro sequence. Val and Leu residues occupy H-bonded sites while Orn residues are located in non-H-bonded sites. 

The biological properties of HA in aqueous solution is controlled by reversible tertiary structures, as defined by NMR spectroscopy. Evidence suggests a ft-pleated sheet-like array stabilized by H- and hydrophobic bonds. Easy transitions between secondary and tertiary structures occur that are convenient mechanisms for switching between functions. The 20 kDa or 50-disaccharide unit is around the size at which such stable tertiary structures are expected to form.199,200 Polymers greater than 20 kDa provide the preferred substrate for Hyal-2. The enzyme cleaves at a much slower rate once the HA substrate loses tertiary structure. The hyaladherins may also provide additional substrate specificity.201 The array of hyaladherins that bind to tertiary HA structures may differ from those that bind to HA chains with exclusively secondary structure. The substrate specificity of Hyal-2 may depend on a combination of differences in bound hyaladherins and on secondary versus tertiary structure. 

Hydrophobic interactions are formed when two or more hydrophobic groups (for example, side chains of valine, leucine, phenylalanine, and so on) in an aqueous environment find themselves sufficiently close to exclude water molecules from their vicinity. These interactions are primarily a result of entropy effects and are believed to be of major importance in the maintenance of the tertiary structures of proteins. Scheraga and coworkers have also proposed that hydrophobic interactions may be involved in the stabilization of the a helix and the pleated sheet structures. 

The tertiary structure of a protein is its complete three-dimensional conformation. Think of the secondary structure as a spatial pattern in a local region of the molecule. Parts of the protein may have the a-helical structure, while other parts may have the pleated-sheet structure, and still other parts may be random coils. The tertiary structure includes all the secondary structure and all the kinks and folds in between. The tertiary structure of a typical globular protein is represented in Figure 24-17. 

A prerequisite for the catalytic function of an enzyme is its native tertiary structure which is determined by the number and sequence of amino acids (primary structure) forming the molecule. Favoured by hydrogen bonds, parts of the polypeptide chain exist in an a-helical or a (3-sheet structure (secondary structure). Most enzymes are globular proteins, the tertiary structure of which may be fixed by disulfide bonds between cysteine residues. A famous example is lysozyme (Fig. 20), consisting of 129 amino acids. A defined three-dimensional structure is... 

In this section we describe the patterns found in supersecondary motifs and in complete tertiary structures. We deal first with helical structures and then describe P-sheet structures in Section 15.4.2. 

See also Secondary Structures (Specific examples). Prediction of Secondary and Tertiary Protein Structure, oi-Helix, / -Sheet... 

Figure 4.23 Schematic representation of the tertiary structure of bovine -lactoglobulin. showing the binding of retinol arrows indicate antiparallel -sheet structures (from Papiz et...

The strongest advantage of the FTIR approach as a structure-specific probe is that a complete spectrum is available for each time point of measurement. Methods to study protein folding by stopped-flow FTIR have been reviewed.In this way, several spectral windows are accessible simultaneously for the observation of the formation of different secondary structure elements and also events that can be attributed to changes in tertiary structure. One specific advantage of the infrared technique is the ability to monitor directly the kinetics of processes involving jS-sheet structures, which is exceptionally difficult to do with other techniques. 

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