Protein Secondary Structure:- β Turns Are Common in Proteins

In globular proteins, which have a compact folded structure, nearly one-third of the amino acid residues are in turns or loops where the polypeptide chain reverses direction (Fig. 4–8). These are the connecting elements that link successive runs of α helix orβconformation. Particularly common are β turns that connect the ends of two adjacent segments of an antiparallel β sheet. The structure is a 180-turn involving four amino acid residues, with the carbonyl oxygen of the first residue forming a hydrogen bond with the amino-group hydrogen of the fourth. The peptide groups of the central two residues do not participate in any interresidue hydrogen bonding. Gly and Pro residues often occur in β turns, the former because it is small and flexible, the latter because peptide bonds.

FIGURE 4–8 Structures of turns. (a) Type I and type II turns are most common; type I turns occur more than twice as frequently as type II. Type II β turns always have Gly as the third residue. Note the hydrogen bond between the peptide groups of the first and fourth residues of the bends. (Individual amino acid residues are framed by large blue circles.) (b) The trans and cis isomers of a peptide bond in volving the imino nitrogen of proline. Of the peptide bonds between amino acid residues other than Pro, over 99.95% are in the trans con figuration. For peptide bonds involving the imino nitrogen of proline, however, about 6% are in the cis configuration; many of these occur at β turns.

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مواضيع ذات صلة

Protein Tertiary and Quaternary Structures: -Fibrous Proteins Are Adapted for a Structural Function

Protein Secondary Structure:- Common Secondary Structures Have Characteristic Bond Angles and Amino Acid Content

Protein Secondary Structure:- Amino Acid Sequence Affects α Helix Stability

Overview of Protein Structure:- The Peptide Bond Is Rigid and Planar

Overview of Protein Structure:- A Protein’s Conformation Is Stabilized Largely by Weak Interactions

Overview of Protein Structure:- The Peptide Bond Is Rigid and Planar

Overview of Protein Structure:- A Protein’s Conformation Is Stabilized Largely by Weak Interactions

Protein Sequences and Evolution:- Protein Sequences Can Elucidate the History of Life on Earth

The Covalent Structure of Proteins:- Amino Acid Sequences Provide Important Biochemical Information

The Covalent Structure of Proteins:- Small Peptides and Proteins Can Be Chemically Synthesized

The Covalent Structure of Proteins:- Amino Acid Sequences Can Also Be Deduced by Other Methods

The Covalent Structure of Proteins:-Large Proteins Must Be Sequenced in Smaller Segments