AP Biology

Proteins

2008-2009 AP Biology

ProteinsDiverse group

Made of Amino Acids

AP Biology

Proteins Most structurally & functionally diverse group Function: involved in almost everything

Enzymes (pepsin, DNA polymerase) Structural Support (keratin, collagen) Transport of substances (hemoglobin, aquaporin)

Hormones (insulin & other hormones) Defense (antibodies) Contraction and Movement (actin & myosin) Storage (bean seed proteins)

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Proteins Structure

monomer = Amino Acid 20 different amino acids

polymer = Chains of AA protein can be one or more polypeptide

chains folded & bonded together large & complex molecules complex 3-D shape

Rubisco

hemoglobin

growthhormones

H2O

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Amino acids Structure

central carbon amino group carboxyl group (acid) R group (side chain)

variable group different for each amino acid confers unique chemical

properties to each amino acid like 20 different letters of an

alphabet can make many words (proteins)

—N—H

HC—OH

||O

R

|—C—

|

H

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Effect of different R groups:Nonpolar amino acids

Why are these nonpolar & hydrophobic?Why are these nonpolar & hydrophobic?

nonpolar & hydrophobic

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Effect of different R groups:Polar amino acids

polar or charged & hydrophilic

Why are these polar & hydrophillic?Why are these polar & hydrophillic?

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Ionizing in cellular waters H+ donorsH+ donors

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Ionizing in cellular waters H+ acceptorsH+ acceptors

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Sulfur containing amino acids Form Disulfide bonds

covalent cross links betweens sulfhydryls stabilizes 3-D structure

H-S – S-HH-S – S-H

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Fig. 7.12 (TEArt)

Constant regionVariable regionDisulfide bond

s

chain chain

T Receptor B Receptor

Light chain Light chain chain

Heavychains

Plasmamembrane

MHC-II

chain

MHC-I

-2microglobulin

SS

sss

ss

ss

s ssss

ssss

ss

ssss

ssss

ss

ss

s

ss

ss

ss

ss

ss

ss

ss

sss

sss

ss

ss

ss

chain

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

AP Biology

Building proteins Peptide Bond

covalent bond between NH2 (amine) of one amino acid & COOH (carboxyl) of another

C–N bond

peptidebond

dehydration synthesisH2O

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Building proteins Polypeptide chains have direction

N-terminus = NH2 end C-terminus = COOH end repeated sequence (N-C-C) is the

polypeptide backbone can only grow in one direction

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Protein structure & function

hemoglobin

Function depends on structure 3-D structure

twisted, folded, coiled into unique shape

collagen

pepsin

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Primary (1°) structure Chain of Amino Acids

amino acid sequence determined by gene (DNA)

slight change in amino acid sequence can affect protein’s structure & its function even just one amino acid change

can make all the difference!

lysozyme: enzyme in tears & mucus that kills bacteria

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Sickle cell anemia

I’mhydrophilic!

But I’mhydrophobic!

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Secondary (2°) structure

folding along short sections of polypeptide interactions between

adjacent amino acids Hydrogen Bonds

weak bonds between R groups

forms sections of 3-D structure Alpha Helix B Pleated Sheet

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Secondary (2°) structure

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Tertiary (3°) structure interactions between distant amino acids

Hydrophobic Interactionscytoplasm is

water-basednonpolar amino

acids cluster away from water

H-Bonds Disulfide Bonds

covalent bonds between sulfurs in sulfhydryls (S–H)

anchors 3-D shape

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Quaternary (4°) structure Consists of the interactions of two or more

polypeptide chains only then does polypeptide become

functional protein

collagen = skin & tendons hemoglobin

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Protein structure (review)

amino acid sequence

peptide bonds

1°

determinedby DNA R groups

H bonds

R groupshydrophobic interactions

disulfide bridges(H & ionic bonds)

3°multiple

polypeptideshydrophobic interactions

4°

2°

2008-2009 AP Biology

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.

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Protein Folding in the Cell

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

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

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

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

AP Biology

Fig. 5-24

Hollowcylinder

Cap

Chaperonin(fully assembled)

Polypeptide

Steps of ChaperoninAction:

An unfolded poly-peptide enters thecylinder from one end.

1

2 3The cap attaches, causing thecylinder to change shape insuch a way that it creates ahydrophilic environment forthe folding of the polypeptide.

The cap comesoff, and the properlyfolded protein isreleased.

Correctlyfoldedprotein

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Protein denaturation Unfolding a protein

alter 3-D shape some proteins can return to their functional

shape after denaturation, many cannot

2008-2009 AP Biology

Let’s build some

Proteins!