Structure of Proteins

Proteins consist of long chains of subunits called amino acids. As the name implies, each amino acid contains an amino group (NH2) on one end of the molecule and a carboxyl group (COOH) on another end. There are about twenty different amino acids, each with a distinct structure and chemical properties, that are used to build proteins. The differences between the amino acids are due to differences in their functional groups. "R" is the abbreviation for functional group in the general formula for an amino acid (fig. 2.24). The R symbol actually stands for the word residue, but it can be thought of as indicating the "rest of the molecule."

When amino acids are joined together by dehydration synthesis, the hydrogen from the amino end of one amino acid combines with the hydroxyl group of the carboxyl end of another amino acid. As a covalent bond is formed between the two amino acids, water is produced (fig. 2.25). The bond between adjacent amino acids is called a peptide bond, and the compound formed is called a peptide. Two amino acids bound together is called a dipeptide; three, a tripeptide. When numerous amino acids are joined in this way, a chain of amino acids, or a polypeptide, is produced.

The lengths of polypeptide chains vary widely. A hormone called thyrotropin-releasing hormone, for example, is only three amino acids long, whereas myosin, a muscle protein, contains about 4,500 amino acids. When the length of a polypeptide

Fox: Human Physiology, Eighth Edition

2. Chemical Composition of the Body

Text

© The McGraw-Hill Companies, 2003

Chemical Composition of the Body

Functional group R

Amino group

Carboxyl group

Amino group

Carboxyl group

Figure The Double Helical Structure

Basic

Polar amino acids

Sulfur-containing

Acidic OOH

CH2 H2

Basic

H2N C NH

Arginine

Polar amino acids

Sulfur-containing

CH2 O

Acidic OOH

CH2 H2

Cysteine

Aspartic acid

■ Figure 2.24 Representative amino acids. The figure depicts different types of functional (R) groups. Each amino acid differs from other amino acids in the number and arrangement of its functional groups.

chain becomes very long (containing more than about 100 amino acids), the molecule is called a protein.

The structure of a protein can be described at four different levels. At the first level, the sequence of amino acids in the protein is described; this is called the primary structure of the protein. Each type of protein has a different primary structure. All of the billions of copies of a given type of protein in a person have the same structure, however, because the structure of a given protein is coded by the person's genes. The primary structure of a protein is illustrated in figure 2.26a.

Weak hydrogen bonds may form between the hydrogen atom of an amino group and an oxygen atom from a different amino acid nearby. These weak bonds cause the polypeptide chain to assume a particular shape, known as the secondary structure of the protein (fig. 2.26b,c). This can be the shape of an alpha (a) helix, or alternatively, the shape of what is called a beta (P) pleated sheet.

Most polypeptide chains bend and fold upon themselves to produce complex three-dimensional shapes called the tertiary structure of the protein (fig. 2.26d). Each type of protein has its own characteristic tertiary structure. This is because the folding and bending of the polypeptide chain is produced by chemical interactions between particular amino acids located in different regions of the chain.

Most of the tertiary structure of proteins is formed and stabilized by weak chemical bonds (such as hydrogen bonds) between the functional groups of widely spaced amino acids. Since most of the tertiary structure is stabilized by weak bonds, this structure can easily be disrupted by high temperature or by changes in pH. Irreversible changes in the tertiary structure of proteins that occur by these means are referred to as denaturation of the proteins. The tertiary structure of some

3H,0

■ Figure 2.25 The formation of peptide bonds by dehydration synthesis reactions. Water molecules are split off as the peptide bonds (highlighted in green) are produced between the amino acids.

40 Chapter Two

40 Chapter Two

Tertiary Hemoglobin

(d) Tertiary structure (e) Quaternary structure

(hemoglobin)

(d) Tertiary structure (e) Quaternary structure

(hemoglobin)

■ Figure 2.26 The structure of proteins. (a) The primary structure refers to the sequence of amino acids in the polypeptide chain. The secondary structure refers to the conformation of the chain created by hydrogen bonding between amino acids; this can be either an alpha helix (b) or a beta pleated sheet (c). The tertiary structure (d) is the three-dimensional structure of the protein. The formation of a protein by the bonding together of two or more polypeptide chains is the quaternary structure (e) of the protein.

proteins, however, is made more stable by strong covalent bonds between sulfur atoms (called disulfide bonds and abbreviated S—S) in the functional group of an amino acid known as cysteine (fig. 2.27).

Denatured proteins retain their primary structure (the pep-tide bonds are not broken) but have altered chemical properties. Cooking a pot roast, for example, alters the texture of the meat proteins—it doesn't result in an amino acid soup. Denaturation is most dramatically demonstrated by frying an egg. Egg albumin proteins are soluble in their native state, in which they form the clear, viscous fluid of a raw egg. When denatured by cooking, these proteins change shape, cross-bond with each other, and by this means form an insoluble white precipitate— the egg white.

Hemoglobin and insulin are composed of a number of polypeptide chains covalently bonded together. This is the quaternary structure of these molecules. Insulin, for exam ple, is composed of two polypeptide chains—one that is twenty-one amino acids long, the other that is thirty amino acids long. Hemoglobin (the protein in red blood cells that carries oxygen) is composed of four separate polypeptide chains (see fig. 2.26e). The composition of various body proteins is shown in table 2.4.

Many proteins in the body are normally found combined, or conjugated, with other types of molecules. Glycoproteins are proteins conjugated with carbohydrates. Examples of such molecules include certain hormones and some proteins found in the cell membrane. Lipoproteins are proteins conjugated with lipids. These are found in cell membranes and in the plasma (the fluid portion of the blood). Proteins may also be conjugated with pigment molecules. These include hemoglobin, which transports oxygen in red blood cells, and the cytochromes, which are needed for oxygen utilization and energy production within cells.

Chemical Composition of the Body

Table 2.4 Composition of Selected Proteins Found in the Body

Number of

Protein

Polypeptide Chains

Nonprotein Component

Function

Hemoglobin

4

Heme pigment

Carries oxygen in the blood

Myoglobin

1

Heme pigment

Stores oxygen in muscle

Insulin

2

None

Hormonal regulation of metabolism

Blood group proteins

1

Carbohydrate

Produces blood types

Lipoproteins

1

Lipids

Transports lipids in blood

Van der Waals forces

Van der Waals forces

Van Der Waals Bond Protein
Disulfide bond (covalent)

■ Figure 2.27 The bonds responsible for the tertiary structure of a protein. The tertiary structure of a protein is held in place by a variety of bonds. These include relatively weak bonds, such as hydrogen bonds, ionic bonds, and Van der Waals (hydrophobic) forces, as well as the strong covalent disulfide bonds.

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Responses

  • calvin
    What bonds are responsible for the tertiary level structure of proteins?
    6 years ago
  • Agostino
    What is has more than 100 amino acid groups?
    6 years ago
  • erkki
    Which bonds are created during the primary structure of a protein?
    6 years ago
  • belinda
    What is ionic bonding in protein structure?
    6 years ago
  • Cerdic
    What type of chemical bonds are responsible for the tertiary structure of a protein?
    6 years ago
  • CHRISTIAN SCHMITT
    What is the structure of van der waal bonds?
    6 years ago
  • ameera
    Which Protein structure is Formed by bonding between functional groups?
    6 years ago
  • Gabriele
    Is myosin a polypetide in human red blood cells?
    6 years ago
  • catherine
    How tertiary structure of proteins is formed?
    6 years ago
  • lonnie
    Why doesn't the denatured protein function?
    6 years ago
  • Columbus
    What is the difference between a peptide bond and a covalent bond?
    6 years ago
  • marjorie estrada
    How is hemoglobin formed?
    5 years ago
  • jemima bolger
    What end of the protein chain called that contains the amino group?
    5 years ago
  • Haylom Mewael
    Why doesn't a pot roast end up a amino acid soup?
    4 years ago
  • thorsten
    What type of chemical bond is responsible for proteins shape?
    4 years ago

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