Figure 217

Factors that contribute to the folding of polypeptide chains and thus to their conformation are (1) hydrogen bonds between side chains or with surrounding water molecules, (2) ionic bonds between polar or ionized side chains, (3) van der Waals forces between nonpolar side chains, and (4) covalent bonds between side chains.

of a polypeptide chain run approximately parallel to each other, forming a relatively straight, extended region known as a beta sheet (Figure 2-19). However, for several reasons, a given region of a polypeptide chain may not assume either a helical or beta sheet conformation. For example, the sizes of the side chains and the ionic bonds between oppositely charged side chains can interfere with the repetitive hydrogen bonding required to produce these shapes. These irregular regions are known as loop conformations and occur in regions linking the more regular helical and beta sheet patterns (Figure 2-19).

Covalent bonds between certain side chains can also distort the regular folding patterns. For example, the side chain of the amino acid cysteine contains a sulfhydryl group (R—SH), which can react with a sulfhydryl group in another cysteine side chain to produce a disulfide bond (R—S—S—R), which links the two amino acid side chains together (Figure 2-20). Disulfide bonds form covalent bonds between portions of a polypeptide chain, in contrast to the weaker hydrogen and ionic bonds, which are more easily broken. Table 2-6 provides a summary of the types of bonding forces that contribute to the conformation of polypeptide chains. These same bonds are also involved in other intermolecular interactions, which will be described in later chapters.

A number of proteins are composed of more than one polypeptide chain and are known as multimeric proteins (many parts). The same factors that influence the conformation of a single polypeptide also determine the interactions between the polypeptides in a multimeric protein. Thus, the chains can be held together by interactions between various ionized, polar, and nonpolar side chains, as well as by disulfide co-valent bonds between the chains.

The polypeptide chains in a multimeric protein may be identical or different. For example, hemoglobin, the protein that transports oxygen in the blood, is a multimeric protein with four polypeptide chains, two of one kind and two of another (Figure 2-21).

The primary structures (amino acid sequences) of a large number of proteins are known, but

Hydrogen bond

Hydrogen bond

C II

C II

Alpha helix

Alpha helix

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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