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Transcript of α-helix - SRI LANKA'S EDUCATIONAL HUB - the quaternary structure of the protein ... Carbohydrates...

  • 1072 CHAPteR 24 the Chemistry of Life: Organic and Biological Chemistry

    Globular proteins fold into a compact, roughly spherical shape. Globular proteins are generally soluble in water and are mobile within cells. They have nonstructural functions, such as combating the invasion of foreign objects, transporting and storing oxygen (hemoglobin and myoglobin), and acting as catalysts. The fibrous proteins form a second class of proteins. In these substances the long coils align more or less in parallel to form long, water-insoluble fibers. Fibrous proteins provide structural integrity and strength to many kinds of tissue and are the main components of muscle, tendons, and hair. The largest known proteins, in excess of 27,000 amino acids long, are muscle proteins.

    The tertiary structure of a protein is maintained by many different interactions. Certain foldings of the protein chain lead to lower energy (more stable) arrangements than do other folding patterns. For example, a globular protein dissolved in aque-ous solution folds in such a way that the nonpolar hydrocarbon portions are tucked within the molecule, away from the polar water molecules. Most of the more polar acidic and basic side chains, however, project into the solution, where they can inter-act with water molecules through iondipole, dipoledipole, or hydrogen-bonding interactions.

    R

    R

    R

    R

    R

    R

    R

    O

    O

    O

    O

    O

    O

    OH

    H

    H

    H

    H

    H

    NN

    NNN

    NNCC

    CC

    CCCC

    CCCC

    C

    C

    Primary structure

    Secondary structureTertiary structure

    Quaternary structure

    -helix

    -sheet

    R group represents side chain

    Figure 24.19 The four levels of structure of proteins.

  • seCtION 24.8 Carbohydrates 1073

    Some proteins are assemblies of more than one polypeptide chain. Each chain has its own tertiary structure, and two or more of these tertiary subunits may aggregate into a larger functional macromolecule. The way the tertiary subunits are arranged is called the quaternary structure of the protein (Figure 24.19). For example, hemoglo-bin, the oxygen-carrying protein of red blood cells, consists of four tertiary subunits. Each subunit contains a component called a heme with an iron atom that binds oxygen as depicted in Figure 23.15. The quaternary structure is maintained by the same types of interactions that maintain the tertiary structure.

    One of the most fascinating current hypotheses in biochemistry is that misfolded proteins can cause infectious disease. These infectious misfolded proteins are called prions. The best example of a prion is the one thought to be responsible for mad cow disease, which can be transmitted to humans.

    24.8 | CarbohydratesCarbohydrates are an important class of naturally occurring substances found in both plant and animal matter. The name carbohydrate (hydrate of carbon) comes from the empirical formulas for most substances in this class, which can be written as Cx1H2O2y. For example, glucose, the most abundant carbohydrate, has the molecular formula C6H12O6, or C61H2O26. Carbohydrates are not really hydrates of carbon; rather, they are polyhydroxy aldehydes and ketones. Glucose, for example, is a six-carbon alde-hyde sugar, whereas fructose, the sugar that occurs widely in fruit, is a six-carbon ketone sugar ( Figure 24.20).

    The glucose molecule, having both alcohol and aldehyde functional groups and a reasonably long and flexible backbone, can form a six-member-ring structure, as shown in Figure 24.21. In fact, in an aqueous solution only a small percentage of the glucose molecules are in the open-chain form. Although the ring is often drawn as if it were planar, the molecules are actually nonplanar because of the tetrahedral bond angles around the C and O atoms of the ring.

    Figure 24.21 shows that the ring structure of glucose can have two relative ori-entations. In the a form the OH group on C1 and the CH2OH group on C5 point in opposite directions, and in the b form they point in the same direction. Although the difference between the a and b forms might seem small, it has enormous bio-logical consequences, including the vast difference in properties between starch and cellulose.

    Figure 24.20 Linear structure of the carbohydrates glucose and fructose.

    C OHH

    C HHO

    C OHH

    C OHH

    C OHH

    HGlucose

    OH

    H

    C HHO

    C OHH

    C OHH

    C OHH

    H

    CH

    Fructose

    O

    CH

    C O

    1

    2

    3

    4

    5

    6

    1

    2

    3

    4

    5

    6

    Aldehyde

    Ketone

    Figure 24.21 Cyclic glucose has an A form and a B form.

    -Glucose

    Open formCH2OH

    C C

    C

    C

    HOH

    H

    H

    OH

    H

    HO

    H

    OH

    O

    C

    -Glucose

    CH2OH

    C C

    C

    C

    HOH

    H

    H OH

    HHOH

    OH

    O

    C

    CH2OH

    C C

    C

    C

    HOH

    H

    H O

    HHOH

    OH

    O H

    C23

    5

    6

    6

    5

    4

    3 2

    1

    6

    5

    4

    3 2

    1

    4 1

  • 1074 CHAPteR 24 the Chemistry of Life: Organic and Biological Chemistry

    Fructose can cyclize to form either five- or six-member rings. The five-member ring forms when the C5 OH group reacts with the C2 carbonyl group:

    C

    C

    HO

    H

    C

    HO

    H

    6

    5

    4 2

    1

    3

    CO

    CH2OH

    C

    H

    CH2OH

    C

    H

    CH2OH

    OH

    H

    OH

    C

    H

    OH C

    OH

    6

    5

    4 2

    13 CH2OH

    O

    The six-member ring results from the reaction between the C6 OH group and the C2 carbonyl group.

    How many chiral carbon atoms are there in the open-chain form of glucose (Figure 24.20)?

    sOLuTiONAnalyze We are given the structure of glucose and asked to determine the number of chiral carbons in the molecule.Plan A chiral carbon has four different groups attached (Section 24.5). We need to identify those carbon atoms in glucose.Solve Carbons 2, 3, 4, and 5 each have four different groups attached to them:

    1

    2

    CH

    C

    O

    H OH

    HHO C

    4C

    3

    H OH

    OH5H C

    6CH OH

    H

    1

    2

    CH

    C

    O

    H OH

    HHO C

    4C

    3

    H OH

    OH5H C

    6CH OH

    H

    1

    2

    CH

    C

    O

    H OH

    HHO C

    4C

    3

    H OH

    OH5H C

    6CH OH

    H

    1

    2

    CH

    C

    O

    H OH

    HHO C

    4C

    3

    H OH

    OH5H C

    6CH OH

    H

    Thus, there are four chiral carbon atoms in the glucose molecule.

    Practice exercise 1How many chiral carbon atoms are there in the open-chain form of fructose (Figure 24.20)? (a) 0, (b) 1, (c) 2, (d) 3, (e) 4

    Practice exercise 2Name the functional groups present in the beta form of glucose.

    saMPLe exerCise 24.8 identifying Functional Groups and Chiral

    Centers in Carbohydrates

    DisaccharidesBoth glucose and fructose are examples of monosaccharides, simple sugars that cannot be broken into smaller molecules by hydrolysis with aqueous acids. Two monosaccha-ride units can be linked together by a condensation reaction to form a disaccharide. The structures of two common disaccharides, sucrose (table sugar) and lactose (milk sugar), are shown in Figure 24.22.

  • seCtION 24.8 Carbohydrates 1075

    The word sugar makes us think of sweetness. All sugars are sweet, but they differ in the degree of sweetness we perceive when we taste them. Sucrose is about six times sweeter than lactose, slightly sweeter than glucose, but only about half as sweet as fruc-tose. Disaccharides can be reacted with water (hydrolyzed) in the presence of an acid catalyst to form monosaccharides. When sucrose is hydrolyzed, the mixture of glucose and fructose that forms, called invert sugar,* is sweeter to the taste than the original sucrose. The sweet syrup present in canned fruits and candies is largely invert sugar formed from hydrolysis of added sucrose.

    PolysaccharidesPolysaccharides are made up of many monosaccharide units joined together. The most important polysaccharides are starch, glycogen, and cellulose, all three of which are formed from repeating glucose units.

    Starch is not a pure substance. The term refers to a group of polysaccharides found in plants. Starches serve as a major method of food storage in plant seeds and tubers. Corn, potatoes, wheat, and rice all contain substantial amounts of starch. These plant products serve as major sources of needed food energy for humans. Enzymes in the digestive system catalyze the hydrolysis of starch to glucose.

    Some starch molecules are unbranched chains, whereas others are branched. Figure 24.23(a) illustrates an unbranched starch structure. Notice, in particular, that

    Figure 24.22 Two disaccharides.

    Glucose unit Sucrose LactoseFructose unit

    HOCH2

    C

    O

    O

    Galactose unit

    C

    H H

    O

    Glucose unitHOCH2

    OH

    HOH

    O

    CH2OH

    CH2OH

    C

    C

    C

    HOH

    C

    HO

    H H

    H

    C

    H

    C

    H

    HOH

    OH

    O

    C

    H

    C

    H

    C C OH

    H

    C

    CH2OH

    C

    C

    C

    HOH

    H

    H

    C

    HO

    H

    OH

    O

    C

    H

    C

    HO

    Figure 24.23 Structures of (a) starch and (b) cellulose.

    CH2OH

    C C

    C

    C

    OHOHO

    OH

    O

    C

    CH2OH

    C C

    C

    C

    OHO

    OH

    O

    C

    CH2OH

    C C

    C