copyright © 2005 pearson education, inc. publishing as benjamin cummings concept 5.4: proteins have...
TRANSCRIPT
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 5.4: Proteins have many structures, resulting in a wide range of functions
• Proteins account for more than 50% of the dry mass of most cells
• Protein functions include structural support, storage, transport, cellular communications, movement, and defense against foreign substances
[Animations are listed on slides that follow the figure]
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Enzymes are a type of protein that acts as a catalyst, speeding up chemical reactions
• Enzymes can perform their functions repeatedly, functioning as workhorses that carry out the processes of life
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Polypeptides
• Polypeptides are polymers of amino acids
• A protein consists of one or more polypeptides
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Amino Acid Monomers
• Amino acids are organic molecules with carboxyl and amino groups
• Amino acids differ in their properties due to differing side chains, called R groups
• Cells use 20 amino acids to make thousands of proteins
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Amino Acid Polymers
• Amino acids are linked by peptide bonds
• A polypeptide is a polymer of amino acids
• Polypeptides range in length from a few monomers to more than a thousand
• Each polypeptide has a unique linear sequence of amino acids
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Determining the Amino Acid Sequence of a Polypeptide
• The amino acid sequences of polypeptides were first determined by chemical methods
• Most of the steps involved in sequencing a polypeptide are now automated
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Protein Conformation and Function
• A functional protein consists of one or more polypeptides twisted, folded, and coiled into a unique shape
• The sequence of amino acids determines a protein’s three-dimensional conformation
• A protein’s conformation determines its function
• Ribbon models and space-filling models can depict a protein’s conformation
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Four Levels of Protein Structure
• The primary structure of a protein is its unique sequence of amino acids
• Secondary structure, found in most proteins, consists of coils and folds in the polypeptide chain
• Tertiary structure is determined by interactions among various side chains (R groups)
• Quaternary structure results when a protein consists of multiple polypeptide chains
Animation: Protein Structure Introduction
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Primary structure, the sequence of amino acids in a protein, is like the order of letters in a long word
• Primary structure is determined by inherited genetic information
Animation: Primary Protein StructureAnimation: Primary Protein Structure
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The coils and folds of secondary structure result from hydrogen bonds between repeating constituents of the polypeptide backbone
• Typical secondary structures are a coil called an alpha helix and a folded structure called a beta pleated sheet
Animation: Secondary Protein StructureAnimation: Secondary Protein Structure
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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, and van der Waals interactions
• Strong covalent bonds called disulfide bridges may reinforce the protein’s conformation
Animation: Tertiary Protein StructureAnimation: Tertiary Protein Structure
LE 5-20dLE 5-20d
Hydrophobicinteractions andvan der Waalsinteractions
Polypeptidebackbone
Disulfide bridge
Ionic bond
Hydrogenbond
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
Animation: Quaternary Protein StructureAnimation: Quaternary Protein Structure
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Sickle-Cell Disease: A Simple Change in Primary Structure
• A slight change in primary structure can affect a protein’s conformation and ability to function
• Sickle-cell disease, an inherited blood disorder, results from a single amino acid substitution in the protein hemoglobin
LE 5-21aLE 5-21a
Red bloodcell shape
Normal cells arefull of individualhemoglobinmolecules, eachcarrying oxygen.
10 µm 10 µm
Red bloodcell shape
Fibers of abnormalhemoglobin deformcell into sickleshape.
LE 5-21bLE 5-21b
Primarystructure
Secondaryand tertiarystructures
1 2 3
Normal hemoglobin
Val His Leu
4Thr
5Pro
6Glu Glu
7Primarystructure
Secondaryand tertiarystructures
1 2 3
Sickle-cell hemoglobin
Val His Leu
4Thr
5Pro
6Val Glu
7
Quaternarystructure
Normalhemoglobin(top view)
Function Molecules donot associatewith oneanother; eachcarries oxygen.
Quaternarystructure
Sickle-cellhemoglobin
Function Molecules interact withone another tocrystallize intoa fiber; capacityto carry oxygenis greatly reduced.
Exposedhydrophobicregion subunit subunit
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
What Determines Protein Conformation?
• In addition to primary structure, physical and chemical conditions can affect conformation
• Alternations in pH, salt concentration, temperature, or other environmental factors can cause a protein to unravel
• This loss of a protein’s native conformation is called denaturation
• A denatured protein is biologically inactive