Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Proteins have many structures, resulting in a wide range of functions Proteins

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Proteins have many structures, resulting in a wide range of functions

• Proteins account for more than 50% of the dry mass of most cells

• The monomers (building units) of proteins are amino acids.

• Cells use 20 amino acids to make thousands of proteins


Amino Acid Monomers

• Amino acids are organic molecules with carboxyl and amino groups

Aminogroup

Carboxylgroup

carbon


Peptidebond

Amino end(N-terminus)

Peptidebond

Side chains

Backbone

Carboxyl end(C-terminus)

(a)

(b)

Amino acids are linked by peptide bonds formed by dehydration reactions


Polypeptides

• Polypeptides are polymers of amino acids

• A protein consists of one or more polypeptides

Lysozyme: an antibacterial enzyme (protein) found in human tears.It is made of one polypeptide.

Hemoglobin protein is made of four polypeptides


Fig. 5-22

Primarystructure

Secondaryand tertiarystructures

Quaternarystructure

Normalhemoglobin(top view)

Primarystructure

Secondaryand tertiarystructures

Quaternarystructure

Function Function

subunit

Molecules donot associatewith oneanother; eachcarries oxygen.

Red bloodcell shape

Normal red bloodcells are full ofindividualhemoglobinmoledules, eachcarrying oxygen.

10 µm

Normal hemoglobin

1 2 3 4 5 6 7

Val His Leu Thr Pro Glu Glu

Red bloodcell shape

subunit

Exposedhydrophobicregion

Sickle-cellhemoglobin

Moleculesinteract withone another andcrystallize intoa fiber; capacityto carry oxygenis greatly reduced.

Fibers of abnormalhemoglobin deformred blood cell intosickle shape.

10 µm

Sickle-cell hemoglobin

GluProThrLeuHisVal Val

1 2 3 4 5 6 7



Four Levels of Protein Structure

• Primary structure

• Secondary structure

• Tertiary structure

• Quaternary structure


• Primary structure, the sequence of amino acids in a protein, is like the order of letters in a long word


Figure 3.7 The Four Levels of Protein Structure (Part 1)


• The coils and folds of secondary structure result from hydrogen bonds between atoms of the polypeptide backbone (NOT the amino acid side chain or R-groups).

• Typical secondary structures are a coil called an alpha helix and a folded structure called a beta pleated sheet.

Secondary Structure




Fig. 5-21d

Abdominal glands of thespider secrete silk fibers

made of a structural proteincontaining -pleated sheets.

The radiating strands, madeof dry silk fibers, maintain

the shape of the web.

The spiral strands (capturestrands) are elastic, stretching

in response to wind, rain,and the touch of insects.


• Tertiary structure is determined by interactions between R groups, rather than interactions between backbone constituents

• These interactions between R groups include:

– hydrogen bonds,

– ionic bonds,

– hydrophobic interactions,

– van der Waals interactions

– Strong covalent bonds called disulfide bridges may reinforce the protein’s conformation.

Tertiary Structure




• Quaternary structure results when two or more polypeptide chains form one macromolecule

• Collagen is a fibrous protein consisting of three polypeptides coiled like a rope

• Hemoglobin is a globular protein consisting of four polypeptides: two alpha and two beta chains

Quaternary structure


Concept 5.5: Nucleic acids store and transmit hereditary information

• The amino acid sequence of a polypeptide is programmed by a unit of inheritance called a gene

• Genes are made of DNA, a nucleic acid


The Roles of Nucleic Acids

• There are two types of nucleic acids:

– Deoxyribonucleic acid (DNA)

– Ribonucleic acid (RNA)

• DNA replicates in order for the cells to divide

• DNA directs the synthesis of messenger RNA (mRNA) and, through mRNA, controls protein synthesis

• Protein synthesis occurs in ribosomes


Concept 3.1 Nucleic Acids Are Informational Macromolecules

• DNA’s information is encoded in the sequence of bases. DNA has two functions:

– Replication

– Information is copied to RNA and used to specify amino acid sequences in proteins.

LE 5-25

NUCLEUS

DNA

CYTOPLASM

mRNA

mRNA

Ribosome

Aminoacids

Synthesis ofmRNA in the nucleus

Movement ofmRNA into cytoplasmvia nuclear pore

Synthesis of protein

Polypeptide

DNA directs the synthesis of messenger RNA (mRNA) and, through mRNA, controls protein synthesis


The Structure of Nucleic Acids

• Nucleic acids are polymers called polynucleotides

• Each polynucleotide is made of monomers called nucleotides

• Each nucleotide consists of:

– a nitrogenous base

– a pentose sugar

– and a phosphate group

LE 5-26a5 end

3 end

Nucleoside

Nitrogenousbase

Phosphategroup

Nucleotide

Polynucleotide, ornucleic acid

Pentosesugar


Nitrogenous bases

• There are two families of nitrogenous bases:

– Pyrimidines have a single six-membered ring

– Purines have a six-membered ring fused to a five-membered ring.

Nitrogenous bases

Pyrimidines

Purines

CytosineC

Thymine (in DNA)T

Uracil (in RNA)U

AdenineA

GuanineG


Pentose sugar

• In DNA, the sugar is deoxyribose

• In RNA, the sugar is ribose.

Pentose sugars

Deoxyribose (in DNA) Ribose (in RNA)


The DNA Double Helix

• A DNA molecule has two polynucleotides spiraling around an imaginary axis, forming a double helix

• The nitrogenous bases in DNA form hydrogen bonds in a complementary fashion: A always pairs with T, and G always pairs with C


Figure 3.4 DNA



• The two strands are antiparallel (running in opposite directions), and the double helix is right-handed.


RNA

• RNA is made of one polynucleotide (one strand of nucleotides)

• The nucleotide of RNA is made of:

– A nitrogenous bases, Adenine (A), Uracil (U), Guanine (G), or Cytosine (C).

– A ribose sugar.

– A phosphate group.


• DNA replication and transcription depend on base pairing:

• 5′-TCAGCA-3′

• 3′-AGTCGT-5′

•

• transcribes to RNA with the

• sequence 5′-UCAGCA-3′.



Differences between DNA and RNA

DNA RNA

Composed of two strands of polynucleotides twisted together helically to form a double helix

Composed of one strand of polynucleotides.

Contains the 5-carbon sugar Deoxyribose

Contains the 5-carbon sugar Ribose


Differences between DNA and RNA

DNA RNA

Contains the nitrogenous bases Adenine (A), Thymine (T), Guanine (G), and Cytosine (C)

Adenine pairs with Thymine and Guanine pairs with Cytosine.

Contains the nitrogenous bases Adenine (A), Uracil (U), Guanine (G), and Cytosine (C)

Larger molecule. Shorter than DNA

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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Proteins have many structures, resulting in a wide range of functions Proteins