Protein

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PROTEIN
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PROTEINS
Comprise over 50% of dry mass of most cells
Very diverse in structure and function
Consist of one or more polypeptides
Polypeptides are chains
of amino acids
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PROTEINS
Proteins are polymers of amino acids
20 different amino
acids
Different “R” groups
Some hydrophilic
Some hydrophobic
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PROTEINS
Amino acids are linked via condensation reactions
Amino acid of one reacts with
the acid group of another
Form covalent
“peptide bonds”
All proteins are made by
assembling the same 20
amino acids in different
orders to different lengths
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PROTEINS
Humans can make tens of thousands of distinctly different proteins
All use the same 20 amino acids
Different orders
Different lengths
How many distinctly different tripeptides can be assembled from these 20 amino acids?
…tetrapeptides?
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ESSENTIAL & NONESSENTIAL
All organisms require all 20 amino acids
Humans can manufacture only ten of these amino acids from precursor molecules*
“Non-essential amino acids”
Humans cannot manufacture the other ten amino acids
“Essential amino acids”
Must be acquired from diet
The bacterium Escherichia coli can manufacture all twenty amino acids
All amino acids are nonessential for E. coli

* Humans can produce 12 amino acids from precursor molecules, but the precursor molecules for the production of two of these 12 amino acids are themselves essential amino acids (met  cys, phe  tyr)
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ROLES OF PROTEINS
Key roles of proteins
Structural
Movement
Transport
Chemical messengers
Receptors
Storage
Defensive
Enzymes
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ROLES OF PROTEINS
Hormones are one group of proteins
Chemical messengers involved in numerous processes
e.g., Insulin, glucagon, growth hormone, etc.
Remember, not all hormones are proteins
Steroid hormones are lipids
Other hormones are proteins
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ROLES OF PROTEINS
Enzymes are biological catalysts
One important class of proteins
Speed up the rate of chemical reactions
Perhaps 10,000 times faster
Not consumed in this reactions
Highly specific
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PROTEIN CONFORMATION
Each protein folds up into a particular three-dimensional shape
Protein function is related to its structure
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PROTEIN CONFORMATION
Each protein folds up into a particular three-dimensional shape
Four levels of protein organization are recognized
Primary (1o) structure
Secondary (2o) structure
Tertiary (3o) structure
Quaternary (4o) structure
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PROTEIN CONFORMATION
Primary structure
Linear sequence of amino acids
Genetically determined
Ultimately determines higher
structural levels

The primary structure of the
enzyme lysozyme is depicted
129 amino acids long
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PROTEIN CONFORMATION
Some regions of the polypeptide exhibit secondary structure
Formed by H-bonding at regular intervals along the polypeptide backbone
Two common types of
secondary structure
Alpha-helices
Beta-pleated sheets
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PROTEIN CONFORMATION
Tertiary structure involve interactions among
R-groups
Four main factors
Hydrogen bonds
Ionic attractions
Hydrophobic interactions
Covalent bonds
“Disulfide bridges”
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PROTEIN CONFORMATION
Quaternary structure results from interactions between multiple polypeptide chains
Not all proteins possess multiple polypeptide chains
Not all proteins possess quaternary structure
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PROTEIN CONFORMATION
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PROTEIN CONFORMATION
The three-dimensional conformation of a protein is critical to its function
Form determines function
Altered conformation can compromise function
Altered structure at any level can alter the final conformation
Altered chemical bonds
can alter conformation
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MUTATION & CONFORMATION
The primary structure of a polypeptide is genetically determined
Primary structure ultimately determines the final shape of a protein
Mutations can alter the DNA
sequence of a gene
Can alter the primary structure
of the polypeptide
Can result in an altered three-
dimensional shape
Basis of many genetic disorders
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PROTEIN DENATURATION
Proteins can be denatured by a variety of factors
Temperature
pH
etc.
Denaturation alters
the conformation of the protein
Often irreversible
It is difficult to “un-hard-boil” an egg
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ROLES OF PROTEINS
Structural proteins
e.g., Collagen, elastin, keratin, etc.
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ROLES OF PROTEINS
Proteins facilitating movement
e.g., Actin, myosin, tubulin, etc.
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ROLES OF PROTEINS
Transport proteins
e.g., Hemoglobin, lac permease, etc.
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ROLES OF PROTEINS
Chemical messengers
e.g., Hormones and neurotransmitters
Receptors
Respond to chemical messengers
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ROLES OF PROTEINS
Storage
e.g., Casein, ovalbumin, etc.
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ROLES OF PROTEINS
Defensive
e.g., Antibodies, fibrin, etc.
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ROLES OF PROTEINS
Enzymes
Highly specific catalysts of chemical reactions
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REFERENCES
Campbell, Neil A. and Reese, Jane B. Biology, 7th edition. Pearson Education, Inc. 2005.
Campbell, Neil A., Reese, Jane B., Taylor, Martha R., and Simon, Eric J. Biology, Concepts and Connections, 5th edition. Pearson Education, Inc. 2006.
Nester, Eugene W., Anderson, Denise G., Roberts, C. Evans Jr., and Nester, Martha T. Microbiology, A Human Perspective, 5th edition. McGraw-Hill Companies, Inc. 2007.
Limson, Janice. 2002. http://www.scienceinafrica.co.za/2002/june/lactose.htm