Proteins

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]


Animation: Structural Proteins

Animation: Storage Proteins

Animation: Transport Proteins

Animation: Receptor Proteins

Animation: Contractile Proteins

Animation: Defensive Proteins

Animation: Enzymes


Animation: Hormonal Proteins

Animation: Sensory Proteins

Animation: Gene Regulatory Proteins


• 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

LE 5-16

Substrate(sucrose)

Enzyme(sucrose)

Fructose

Glucose


Polypeptides

• Polypeptides are polymers of amino acids

• A protein consists of one or more polypeptides


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

LE 5-UN78

Aminogroup

Carboxylgroup

a carbon

LE 5-17a

Isoleucine (Ile)

Methionine (Met) Phenylalanine (Phe) Tryptophan (Trp) Proline (Pro)

Leucine (Leu)Valine (Val)Alanine (Ala)

Nonpolar

Glycine (Gly)

LE 5-17b

Asparagine (Asn) Glutamine (Gln)Threonine (Thr)

Polar

Serine (Ser) Cysteine (Cys) Tyrosine (Tyr)

LE 5-17c

Electricallycharged

Aspartic acid (Asp)

Acidic Basic

Glutamic acid (Glu) Lysine (Lys) Arginine (Arg) Histidine (His)


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


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


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

LE 5-19

A ribbon model

Groove

Groove

A space-filling model


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

LE 5-20

Amino acidsubunits

b pleated sheet+H3N

Amino end

a helix


• 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 Structure

LE 5-20a

Amino acidsubunits

Carboxyl end

Amino end


• 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 Structure

LE 5-20b

Amino acidsubunits

b pleated sheet

a helix


• 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 Structure

LE 5-20d

Hydrophobicinteractions andvan der Waalsinteractions

Polypeptidebackbone

Disulfide bridge

Ionic bond

Hydrogenbond


• 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 Structure

LE 5-20e

b Chains

a ChainsHemoglobin

IronHeme

CollagenPolypeptide chain

Polypeptidechain


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 5-21a

Red bloodcell shape

Normal cells arefull of individualhemoglobinmolecules, eachcarrying oxygen.

10 µm 10 µm

Red bloodcell shape

Fibers of abnormalhemoglobin deformcell into sickleshape.

LE 5-21b

Primarystructure

Secondaryand tertiarystructures

1 2 3

Normal hemoglobinVal His Leu

4Thr

5Pro

6Glu Glu

7Primarystructure

Secondaryand tertiarystructures

1 2 3

Sickle-cell hemoglobinVal His Leu

4Thr

5Pro

6Val Glu

7

Quaternarystructure

Normalhemoglobin(top view)

a

b

b

b

b

a

a

a

Function Molecules donot associatewith oneanother; eachcarries oxygen.

Quaternarystructure

Sickle-cellhemoglobin

Function Molecules interact withone another tocrystallize intoa fiber; capacityto carry oxygenis greatly reduced.

Exposedhydrophobicregionb subunit b subunit


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

LE 5-22

Denaturation

Renaturation

Denatured proteinNormal protein


The Protein-Folding Problem

• It is hard to predict a protein’s conformation from its primary structure

• Most proteins probably go through several states on their way to a stable conformation

• Chaperonins are protein molecules that assist the proper folding of other proteins

LE 5-23a

Chaperonin(fully assembled)

Hollowcylinder

Cap

LE 5-23b

PolypeptideCorrectlyfoldedprotein

An unfolded poly-peptide enters thecylinder from oneend.

Steps of ChaperoninAction:

The cap comesoff, and theproperly foldedprotein is released.

The cap attaches, causingthe cylinder to changeshape in such a way that it creates a hydrophilicenvironment for thefolding of the polypeptide.


• Scientists use X-ray crystallography to determine a protein’s conformation

• Another method is nuclear magnetic resonance (NMR) spectroscopy, which does not require protein crystallization

LE 5-24a

Photographic film

Diffracted X-raysX-raysource X-ray

beam

X-raydiffraction pattern

Crystal

LE 5-24b

Nucleic acid

3D computer modelX-ray diffraction pattern

Protein

Proteins

Documents

Transcript of Proteins