Proteins

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Proteins

• Chapter 3
• A. P. Biology
• Mr. Knowles
• Liberty Senior High School

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Proteins are Most Common
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Functions of Proteins

• Enzymes- Metabolism
• Structural- Collagen and Keratin
• Cell Recognition- proteins on cellular surface.
• Regulation of Gene Expression- Gene Repressors or
  Enhancers.
• Defense- Antibodies.

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• An overview of protein functions

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Two Types of Proteins

• Fibrous Proteins- rope-like, structural proteins
  form shape of cells and tissues. Ex.
  Collagen-the most abundant protein of
  vertebrates.
• Globular Proteins- have specific shapes for their
  functions. Ex. Enzymes and antibodies.

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1. Proteins can be Structural
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2. Proteins can be Globular
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• X-ray crystallography
• Is used to determine a proteins
  three-dimensional structure.

Figure 5.24
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Papain
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Proteins

• Most diverse organic compound.
• Composed of amino acids- each with an amino group
  (NH2) and a carboxylic acid group (COOH).
• Different chemical group(s) attached to central
  C- R group .

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Amino Acid Polymers

• Amino acids
• Are linked by peptide bonds

OH
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• 20 different amino acids make up proteins

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Amino Acids

• Are the monomers of proteins.
• Only 20 naturally occurring amino acids.
• The R group gives each of the amino acids its
  unique property.
• All 20 amino acids can be grouped into 5 basic
  groups.

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5 Groups of Amino Acids (Fig. 3.15)

• Nonpolar- have R groups that contain CH2 and CH3.
• Polar Uncharged- R groups that have O or only H.
• Ionizable- have R groups that are acids and
  bases.
• Aromatic- R groups that have organic rings.

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5 Groups of Amino Acids (Fig. 3.15)

• 5. Special-function- amino acids that are only
  used for very specific functions methionine
  begins protein synthesis, proline causes kinks in
  the protein polymer, cysteine links chains
  together.

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The 20 Common Amino Acids (Fig. 3.15) Click
below for another view!
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Proteins

• Are polymers of amino acids.
• Joined by peptide bonds.
• Di- Tri- and Polypeptides.

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Globular Proteins

• Are long amino acids chains folded into complex
  shapes.
• All of the internal amino acids are nonpolar.
• Water excludes nonpolar amino acids hydrophobic
  interactions.

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Globular Proteins Have Four Levels of Structure

• Primary- the specific sequence of amino acids in
  the polypeptide chain.
• R groups have no role in the backbone, so any
  sequence of amino acids is possible.
• Therefore, 100 amino acids may be rearranged in
  20100 different possible sequences.

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• Primary Structure
• Is the unique sequence of amino acids in a
  polypeptide.

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Globular Protein Structure

• 2. Secondary- folding or coiling of the chain
  into a pattern due to weak H bonds between amino
  acids.
• H bonds form between the main chain of amino
  acids.
• Two Kinds of Secondary Structure

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Secondary Structures

• Alpha Helix- H bonds between one amino acid and
  another further down the chain. Pulls the chain
  into a coil.
• Beta Sheet- H bonds occur across two separate
  chains. If chains are parallel, they may form a
  sheet-like structure.

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• Secondary Structure
• Is the folding or coiling of the polypeptide into
  a repeating configuration.
• Includes the ? helix and the ? pleated sheet.

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Alpha Helix- The First Type of Secondary Protein
Structure
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Beta Sheet- Another Type of Protein Secondary
Structure
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Show me the levels of protein structure.
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Secondary Structures

• Some patterns of alpha helices and/or beta sheets
  are very common in protein structures.
• When secondary structures are organized into
  specific structures within proteins-motifs. Ex.
  ?-Barrel or a-turn-a motifs

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?-barrel Motif in a Cell Membrane Protein
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Globular Protein Structure

• 3. Tertiary Structure- folding and positioning
  of nonpolar R groups into the interior of the
  protein (hydrophobic interactions).
• Held together by weak van der Waals forces.
• Precise fitting of R groups within the interior.
  A change may destabilize a proteins shape.

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• Tertiary Structure
• Is the overall three-dimensional shape of a
  polypeptide.
• Results from interactions between amino acids and
  R groups.

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Globular Protein Structure

• Quaternary Structure- two or more polypepetide
  chains associate to form a protein.
• Each chain is called a subunit.
• Subunits are not necessarily the same.
• Ex. Hemoglobin 2 a-chain subunits 2 ß-chain
  subunits.

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• Quaternary Structure
• Is the overall protein structure that results
  from the aggregation of two or more polypeptide
  subunits.

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• The four levels of protein structure

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Quaternary Structure of Hemoglobin
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• Hemoglobin structure and sickle-cell disease

Fibers of abnormalhemoglobin deform cell into
sickle shape.
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Is Protein Folding Important?
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Normal Prion Scrapie Prion
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Reverse Transcriptase of HIV
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Cobra Toxin
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Shape of the Protein

• Tertiary and Quaternary structures provide shape.
• These structures are maintained by H bonds and
  other weak forces between R groups of amino acids.

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Protein Folding
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Conditions that Affect Protein Shape

• Can disrupt H bonds by
• High Temperature
• pH Changes (Acidic or Basic)
• Ion Concentration (Salt)
• Disrupting the 2, 3, 4 structure is called
  denaturation.

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• Denaturation
• Is when a protein unravels and loses its native
  conformation.

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• Enzymes
• Are a type of protein that acts as a catalyst,
  speeding up chemical reactions.

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Enzymes are Proteins

• Organic catalysts - increase the rate of chemical
  reactions in cells.
• Hold reactant molecules close together for
  reaction to occur- uses an active site.
• The active site is used to bind the reactant
  molecules-substrate.

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Lock-and-Key Model
Show me the model, Luke!
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Write your predictions!
Pineapple Papain (Enzyme)
Gelatin Substrate