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  • 774 Chapter 24

    The Chemistry of Life

    CHAPTER 24

    What Youll LearnYou will learn the functionsof the four major classes ofbiological molecules: pro-teins, carbohydrates, lipids,and nucleic acids.

    You will identify the build-ing blocks that form themajor biological molecules.

    You will compare andcontrast the metabolicprocesses of cellular respira-tion, photosynthesis, andfermentation.

    Why Its ImportantThe large biological moleculesin your body are essential tothe organization and opera-tion of its millions of cells.The structure of these mole-cules is directly related totheir function, and how theyfunction affects your healthand survival.

    The silk that makes up thisspiders web is, gram for gram,stronger than steel, yet it islightweight and stretchable.Spider silk is made of protein,a biological molecule.

    Visit the Chemistry Web site atchemistrymc.com to find linksabout the chemistry of life.

    http://chemistrymc.com

  • 24.1 Proteins 775

    DISCOVERY LAB

    Materials

    400-mL beakerhot plate10-mL graduated cylinderboiling chip10% glucose solutiontest tubetongsBenedicts solutionstirring rodother food solutions such as10% starch or honey

    Testing for Simple Sugars

    Your body constantly uses energy. Many different food sources cansupply that energy, which is stored in the bonds of moleculescalled simple sugars. What foods contain simple sugars?

    Safety Precautions

    Procedure

    1. Fill a 400-mL beaker one-third full of water. Place this water bathon a hot plate and begin to heat it to boiling.

    2. Place 5.0 mL 10% glucose solution in a test tube.

    3. Add 3.0 mL Benedicts solution to the test tube. Mix the two solu-tions using a stirring rod. Add a boiling chip to the test tube.

    4. Using tongs, place the test tube in the boiling water bath and heatfor five minutes.

    5. Record a color change from blue to yellow or orange as a positivetest for a simple sugar.

    6. Repeat the procedure using food samples such as a 10% starchsolution, a 10% gelatin (protein) suspension, or a few drops ofhoney suspended in water.

    Analysis

    Was a color change observed? Which foods tested positive for thepresence of a simple sugar?

    Objectives Describe the structures of

    amino acids and proteins.

    Explain the roles of proteinsin cells.

    Vocabularyproteinamino acidpeptide bondpeptidedenaturationenzymesubstrateactive site

    Section 24.1 Proteins

    An amazing variety of chemical reactions take place in living organisms. Atthe forefront of coordinating the numerous and intricate reactions of life arethe large molecules called proteins, whose name comes from the Greek rootword protos, meaning first.

    Protein StructureYou have learned that polymers are large molecules made of many repeatingbuilding blocks called monomers. Proteins are organic polymers made ofamino acids linked together in a specific way. But proteins are not just large,randomly arranged chains of amino acids. To function properly, each proteinmust be folded into a specific three-dimensional structure. The spider silkshown on the opposite page would not be the incredibly strong yet light-weight protein that it is if it were not constructed in its specific way. You willlearn in this section how proteins are made from their amino-acid buildingblocks and how different types of proteins function.

  • Amino acids As you saw in Chapter 23, many different functional groups arefound in organic compounds. Amino acids, as their name implies, are organicmolecules that have both an amino group and an acidic carboxyl group. Thegeneral structure of an amino acid is shown below.

    Each amino acid has a central carbon atom around which are arranged fourgroups: an amino group ( NH2), a carboxyl group ( COOH), a hydrogenatom, and a variable side chain, R. The side chains range from a single hydro-gen atom to a complex double-ring structure. Examine the different side chainsof the amino acids shown in Figure 24-1. Identify the nonpolar alkanes, polarhydroxyl groups, acidic and basic groups such as carboxyl and amino groups,aromatic rings, and sulfur-containing groups. This wide range of side chainsgives the different amino acids a large variety of chemical and physical prop-erties and is an important reason why proteins can carry out so many differentfunctions.

    Twenty different amino acids are commonly found in the proteins of liv-ing things. The name of each amino acid and its three-letter abbreviation arelisted in Table 24-1. What is the abbreviation for glycine?

    The peptide bond The amino and carboxyl groups provide convenientbonding sites for linking amino acids together. Since an amino acid is bothan amine and a carboxylic acid, two amino acids can combine to form an

    Figure 24-1

    A large variety of side chains canbe found on amino acids. Someof them are shown on these rep-resentative amino acids and arehighlighted in green.

    Amino acid plus yields

    Peptide bond

    plus

    H R1

    H OH

    N C C OH

    Amino acid

    H R2

    H OH

    N C C OH

    Dipeptide Water

    H HR1

    H OH

    N C C N C C OH

    OH

    R2

    0 H2O

    776 Chapter 24 The Chemistry of Life

    R

    Amino group Carboxyl group

    Variable side chain

    Hydrogen atom H O

    H2N C C OH

    Glycine Serine Cysteine Lysine

    Glutamic acid Glutamine Valine Phenylalanine

    H2N C C OH

    H

    H

    O

    H2N C C OH

    CH2

    OH

    H

    O

    H2N C C OH

    CH2

    SH

    H

    O

    H2N C C OH

    CH2

    CH2

    CH2 NH2

    CH2

    H

    O

    H2N C C OH

    CH3

    H

    O

    H2N C C OH

    CH2

    H2N C C OH

    H

    O

    O OH

    CH2

    CH2

    C

    H2N C C OH

    H

    O

    CH3CH2

    CH2

    CH

    O NH2C

    H

    O

    Figure 24-2

    The amino group of one aminoacid bonds to the carboxylgroup of another amino acid toform a dipeptide. The organicfunctional group formed is anamide linkage and is called apeptide bond.

  • amide, releasing water in the process. This reaction is a condensation reaction.As Figure 24-2 shows, the amino group of one amino acid reacts with thecarboxyl group of another amino acid to form an amide functional group.Where do the H and OH that form water come from?

    The amide bond that joins two amino acids is referred to by biochemistsas a peptide bond.

    A molecule that consists of two amino acids bound together by a peptidebond is called a dipeptide. Figure 24-3a shows the structure of a dipeptidethat is formed from the amino acids glycine (Gly) and phenylalanine (Phe).Figure 24-3b shows a different dipeptide, also formed by linking togetherglycine and phenylalanine. Is Gly-Phe the same compound as Phe-Gly? No,theyre different. Examine these two dipeptides to see that the order in whichamino acids are linked in a dipeptide is important.

    Each end of the two-amino-acid unit in a dipeptide still has a free groupone end has a free amino group and the other end has a free carboxyl group.Each of those groups can be linked to the opposite end of yet another aminoacid, forming more peptide bonds. A chain of two or more amino acids linkedtogether by peptide bonds is called a peptide. Living cells always build pep-tides by adding amino acids to the carboxyl end of a growing chain.

    Polypeptides As peptide chains increase in length, other ways of referringto them become necessary. A chain of ten or more amino acids joined by pep-tide bonds is referred to as a polypeptide. When a chain reaches a length ofabout 50 amino acids, its called a protein.

    Because there are only 20 different amino acids that form proteins, itmight seem reasonable to think that only a limited number of different pro-tein structures are possible. But a protein can have from 50 to a thousand ormore amino acids, arranged in any possible sequence. To calculate the num-ber of possible sequences these amino acids can have, you need to considerthat each position on the chain can have any of 20 possible amino acids. Fora peptide that contains n amino acids, there are 20n possible sequences of theamino acids. So a dipeptide, with only two amino acids, can have 202, or400, different possible amino acid sequences. Even the smallest protein con-taining only 50 amino acids has 2050, or more than 1 1065, possiblearrangements of amino acids! It is estimated that human cells make between80 000 and 100 000 different proteins. You can see that this is only a verysmall fraction of the total number of proteins possible.

    H

    Peptide bond

    O

    C N

    24.1 Proteins 777

    The 20 Amino Acids

    Amino acid Abbreviation

    Alanine AlaArginine ArgAsparagine AsnAspartic acid AspCysteine CysGlutamic acid GluGlutamine GlnGlycine GlyHistidine HisIsoleucine IleLeucine LeuLysine LysMethionine MetPhenylalanine PheProline ProSerine SerThreonine ThrTryptophan TrpTyrosine TyrValine Val

    Table 24-1

    Glycylphenylalanine (Gly-Phe)Gly Phe

    Phenylalanylglycine (Phe-Gly)Phe Gly

    H HH

    H OH

    N C C N C C OH

    OH

    CH2

    H H

    H OH

    N C C N C C OH

    OH

    H

    CH2

    Figure 24-3

    Glycine and phenylalaninecan be combined in this configuration.

    Glycine and phenylalaninecan also be combined in thisconfiguration. Why are thesetwo structures different substances?

    b

    a

    a b

  • Figure 24-4

    The folding of polypeptidechains into both helices (left)and sheets (right) involvesamino acids that are fairly closetogether in the chain being heldin position by hydrogen bonds.Other interactions among thevarious side chains are notshown here but play an impor-tant role in determining thethre

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