Proteins

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Proteins

• Account for over 50 of dry weight of cells
• Used for structural support
• (see page 72) storage
• transport
• signaling
• movement
• defense
• metabolism regulation (enzymes)
• Are the most structurally sophisticated molecules
  known
• Are polymers constructed from 20 different amino
  acids

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Table 5.1 An Overview of Protein Functions
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Hierarchy of structure

• Amino acids building blocks
• (are 20 different ones)
• Polypeptides polymers of amino acids
• Protein one or more polypeptides folded and
  coiled into specific conformations.

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General structure of amino acid
H

• All differ in the R-group
• The physical and chemical properties of the
  R-group determine the characteristics of the
  amino acid.

COOH
NH2
R
R
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Nonpolar, Polar, and Electrically Charged
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Amino Acids join

• Carboxyl group of one is adjacent to amino group
  of another, dehydration synthesis occurs, forms a
  covalent bond
• PEPTIDE BOND
• When repeated over and over, get a polypeptide
• On one end is an N-terminus (amino end)
• On other is a C-terminus (carboxyl end)

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Figure 5.16 Making a polypeptide chain
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Proteins function depends on its conformation

• Functional proteins consist of one or more
  polypeptides twisted, folded, and coiled into a
  unique shape
• Amino acid sequence determines shape
• 2 big categories 1. globular
• 2. Fibrous
• Function of a protein depends on its ability to
  recognize and bind to some other molecule.

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Lysozyme
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Four levels of protein structure

• 1. Primary structure unique sequence of amino
  acids (long chain)
• 2. Secondary structure segments of polypeptide
  chain that repeatedly coil or fold in patterns
  that contribute to overall configuration
• are the result of hydrogen bonds at
  regular intervals along the polypeptide
  backbone

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Figure 5.18 The primary structure of a protein
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Secondary structure Alpha helix and Beta
pleated sheet

• Helix delicate coil held together by H-bonding
  between every fourth amino acid
• Ex. Lysozyme, keratin
• Beta pleated sheet two or more regions of the
  polypeptide chain lie parallel to one another.
  H-bonds form here, and keep the structure
  together.
• Ex. Lysozyme, silk

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Figure 5.20 The secondary structure of a protein
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• 3. Tertiary structure superimposed on secondary
  structure irregular contortions from
  interactions between side chains
• nonpolar side chains end up in clusters at the
  core of a protein caused by the action of water
  molecules which exclude nonpolar substances
• (hydrophobic interaction)
• -van der Waals interactions, H-bonds, and ionic
  bonds all add together to stabilize tertiary
  structure
• may also have disulfide bridges form (when amino
  acids with 2 sulfhydryl groups are brought
  together)

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Figure 5.22 Examples of interactions
contributing to the tertiary structure of a
protein
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• Quaternary Structure the overall protein
  structure that results from the aggregation of
  the polypeptide subunits
• Ex. collagen structural
• hemoglobin globular

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Figure 5.23 The quaternary structure of proteins
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Figure 5.24 Review the four levels of protein
structure
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What determines Protein configuration

• Polypeptide chain of given amino acid sequence
  can spontaneously arrange into
• 3-D shape
• Configuration also depends on physical and
  chemical conditions of proteins environment
• (if pH, salt , temp, etc. are altered,
  protein may unravel and lose native conformation
  
• denaturation

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Figure 5.25 Denaturation and renaturation of a
protein
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Protein-Folding problem

• HOW proteins fold is not always clear may be
  several intermediate states on the way to stable
  conformation
• Are a few ways to track, though
• chaperonins protein molecules that assist
  the proper folding of other proteins.
• Ex. From E. coli keeps polypeptide
  segrated from problems in environment
• computer simulations Blue Gene, a
  supercomputer able to generate the 3-D
  structure of any protein starting from its aa
  sequence (medical uses)

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Figure 5.26 A chaperonin in action
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Chemical Tests for Protein ID

• For a liquid
• Biuret Reagent Test
• solutions containing a protein will turn
  lavender/purple when combined with blue Biuret
  Reagent
• For a solid
• Xanthoproteic Test
• solids (that are light in color) will turn
  yellow when exposed to nitric acid

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Overconsumption of fat and protein