Proteins Concept 5.4: Proteins have many structures, resulting in a wide range of functions - PowerPoint Presentation

Slide1

ProteinsSlide2

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]Slide3
Slide4

Animation: Structural Proteins

Animation: Storage Proteins

Animation: Transport Proteins

Animation: Receptor Proteins

Animation: Contractile Proteins

Animation: Defensive Proteins

Animation: EnzymesSlide5

Animation: Hormonal Proteins

Animation: Sensory Proteins

Animation: Gene Regulatory ProteinsSlide6

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 lifeSlide7

LE 5-16

Substrate

(sucrose)

Enzyme

(sucrose)

Fructose

GlucoseSlide8

Polypeptides

Polypeptides are polymers of amino acids

A protein consists of one or more polypeptidesSlide9

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 proteinsSlide10

LE 5-UN78

Amino

group

Carboxyl

group

a

carbonSlide11

LE 5-17a

Isoleucine (Ile)

Methionine (Met)

Phenylalanine (Phe)

Tryptophan (Trp)

Proline (Pro)

Leucine (Leu)

Valine (Val)

Alanine (Ala)

Nonpolar

Glycine (Gly)Slide12

LE 5-17b

Asparagine (Asn)

Glutamine (Gln)

Threonine (Thr)

Polar

Serine (Ser)

Cysteine (Cys)

Tyrosine (Tyr)Slide13

LE 5-17c

Electrically

charged

Aspartic acid (Asp)

Acidic

Basic

Glutamic acid (Glu)

Lysine (Lys)

Arginine (Arg)

Histidine (His)Slide14

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 thousandEach polypeptide has a unique linear sequence of amino acidsSlide15

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 automatedSlide16

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 conformationSlide17

LE 5-19

A ribbon model

Groove

Groove

A space-filling modelSlide18

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 IntroductionSlide19

LE 5-20

Amino acid

subunits

b

pleated sheet

+

H

3

N

Amino end



helixSlide20

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 StructureSlide21

LE 5-20a

Amino acid

subunits

Carboxyl end

Amino endSlide22

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 StructureSlide23

LE 5-20b

Amino acid

subunits

b

pleated sheet



helixSlide24

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 StructureSlide25

LE 5-20d

Hydrophobic

interactions and

van der Waals

interactions

Polypeptide

backbone

Disulfide bridge

Ionic bond

Hydrogen

bondSlide26

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 StructureSlide27

LE 5-20e

b

Chains

a

Chains

Hemoglobin

Iron

Heme

Collagen

Polypeptide chain

Polypeptide

chainSlide28

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 hemoglobinSlide29

LE 5-21a

Red blood

cell shape

Normal cells are

full of individual

hemoglobin

molecules, each

carrying oxygen.

10 µm

10 µm

Red blood

cell shape

Fibers of abnormal

hemoglobin deform

cell into sickle

shape.Slide30

LE 5-21b

Primary

structure

Secondary

and tertiary

structures

1

2

3

Normal hemoglobin

Val

His

Leu

4

Thr

5

Pro

6

Glu

Glu

7

Primary

structure

Secondary

and tertiary

structures

1

2

3

Sickle-cell hemoglobin

Val

His

Leu

4

Thr

5

Pro

6

Val

Glu

7

Quaternary

structure

Normal

hemoglobin

(top view)

a









a

a

a

Function

Molecules do

not associate

with one

another; each

carries oxygen.

Quaternary

structure

Sickle-cell

hemoglobin

Function

Molecules

interact with

one another to

crystallize into

a fiber; capacity

to carry oxygen

is greatly reduced.

Exposed

hydrophobic

region

b

subunit

b

subunitSlide31

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 inactiveSlide32

LE 5-22

Denaturation

Renaturation

Denatured protein

Normal proteinSlide33

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 proteinsSlide34

LE 5-23a

Chaperonin

(fully assembled)

Hollow

cylinder

CapSlide35

LE 5-23b

Polypeptide

Correctly

folded

protein

An unfolded poly-

peptide enters the

cylinder from one

end.

Steps of Chaperonin

Action:

The cap comes

off, and the

properly folded

protein is released.

The cap attaches, causing

the cylinder to change

shape in such a way that

it creates a hydrophilic

environment for the

folding of the polypeptide.Slide36

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

Another method is nuclear magnetic resonance (NMR) spectroscopy, which does not require protein crystallizationSlide37

LE 5-24a

Photographic film

Diffracted X-rays

X-ray

source

X-ray

beam

X-ray

diffraction pattern

CrystalSlide38

LE 5-24b

Nucleic acid

3D computer model

X-ray diffraction pattern

Protein