Figure 214

Linkage of amino acids by peptide bonds to form a polypeptide.

Note that when two amino acids are linked together, one end of the resulting molecule has a free amino group, and the other has a free carboxyl group. Additional amino acids can be linked by peptide bonds to these free ends. A sequence of amino acids linked by peptide bonds is known as a polypeptide. The pep-tide bonds form the backbone of the polypeptide, and the side chain of each amino acid sticks out from the side of the chain. If the number of amino acids in a polypeptide is 50 or less, the molecule is known as a peptide; if the sequence is more than 50 amino acid units, it is known as a protein. The number 50 is arbitrary but has become the convention for distinguishing between large and small polypeptides.

One or more monosaccharides can be covalently attached to the side chains of specific amino acids (serine and threonine) to form a class of proteins known as glycoproteins.

Primary Protein Structure Two variables determine the primary structure of a polypeptide: (1) the number of amino acids in the chain, and (2) the specific type of amino acid at each position along the chain (Figure 2-15). Each position along the chain can be occupied by any one of the 20 different amino acids. Let us consider the number of different peptides that can be formed that have a sequence of three amino acids. Any one of the 20 different amino acids may occupy the first position in the sequence, any one of the 20 the second position, and any one of the 20 the third position, for a total of 20 X 20 X 20 = 203 = 8000 possible sequences of three amino acids. If the peptide is 6 amino acids in length, 206 = 64,000,000 possible combinations can be formed. Peptides that are only 6 amino acids long are still very small compared to proteins, which may have sequences of 1000 or more amino acids. Thus, with 20 different amino acids, an almost

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART ONE Basic Cell Functions

Image Polypeptide Image

FIGURE 2-15

The position of each type of amino acid in a polypeptide chain and the total number of amino acids in the chain distinguish one polypeptide from another. The polypeptide illustrated contains 223 amino acids with different amino acids represented by different-colored circles. The bonds between various regions of the chain (red to red) represent covalent disulfide bonds between cysteine side chains.

FIGURE 2-15

The position of each type of amino acid in a polypeptide chain and the total number of amino acids in the chain distinguish one polypeptide from another. The polypeptide illustrated contains 223 amino acids with different amino acids represented by different-colored circles. The bonds between various regions of the chain (red to red) represent covalent disulfide bonds between cysteine side chains.

unlimited variety of polypeptides can be formed by altering both the amino acid sequence and the total number of amino acids in the chain.

Protein Conformation A polypeptide is analogous to a string of beads, each bead representing one amino acid (Figure 2-15). Moreover, since amino acids can ro tate around their peptide bonds, a polypeptide chain is flexible and can be bent into a number of shapes, just as a string of beads can be twisted into many configurations. The three-dimensional shape of a molecule is known as its conformation (Figure 2-16). The conformations of peptides and proteins play a major role in their functioning, as we shall see in Chapter 4.

Four factors determine the conformation of a polypeptide chain once the amino acid sequence has been formed: (1) hydrogen bonds between portions of the chain or with surrounding water molecules; (2) ionic bonds between polar and ionized regions along the chain; (3) van der Waals forces, which are very weak forces of attraction between nonpolar (hy-drophobic) regions in close proximity to each other; and (4) covalent bonds linking the side chains of two amino acids (Figure 2-17).

An example of the attractions between various regions along a polypeptide chain is the hydrogen bond that can occur between the hydrogen linked to the nitrogen atom in one peptide bond and the double-bonded oxygen in another peptide bond (Figure 2-18). Since peptide bonds occur at regular intervals along a polypeptide chain, the hydrogen bonds between them tend to force the chain into a coiled conformation known as an alpha helix. Hydrogen bonds can also form between peptide bonds when extended regions

FIGURE 2-16

Conformation (shape) of the protein molecule myoglobin. Each dot corresponds to the location of a single amino acid.

Adopted from Albert L. Lehninger.

FIGURE 2-16

Conformation (shape) of the protein molecule myoglobin. Each dot corresponds to the location of a single amino acid.

Adopted from Albert L. Lehninger.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Chemical Composition of the Body CHAPTER TWO

Chemical Composition of the Body CHAPTER TWO

Polypeptide chain

Polypeptide chain

(1) Hydrogen bond

(2) Ionic bond

(3) van der Waals forces

(4) Covalent (disulfide) bond

(1) Hydrogen bond

(2) Ionic bond

(3) van der Waals forces

(4) Covalent (disulfide) bond

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.

Get My Free Ebook


Post a comment