Download File - Mrs. LeCompte

3. PROTEINS
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Polypeptide Chains = polymers of amino acids that are arranged in a specific linear
sequence and are linked by peptide bonds
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Protein = one or more polypeptide chains that make up one macromolecule

Make up 50% or more of cellular dry weight

Functions include:
o Metabolism: ENZYMES speed up chemical reactions in cells
o Structural support
o Transport: carriers in cell membranes and large-scale, like hemoglobin
o Defense: Antibodies
o Regulation: Hormones and other intercellular messengers
o Motion: Contractile proteins (muscles) and intracellular transport proteins
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Made of only 20 amino acid monomers
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Amino Acids = monomer building blocks of proteins
o Made of an alpha carbon, which is covalently bonded to:

H atom
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amino group
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carboxyl group
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Variable R group (side chain)
o Grouped by properties of side chains

Nonpolar side groups (hydrophobic)

Polar side groups (hydrophilic)


May be Uncharged or charged polar
Polypeptides are held together by Peptide Bonds = covalent bond formed by a
dehydration reaction that links the carboxyl group of one amino acid with the amino
group of another
o Gives it a repeating backbone of N-C-C-N-C-C-…

Protein function depends on its 3-D shape = Conformation

Native Conformation = normal shape
o Enables a protein to recognize and bind to another molecule
4 Levels of Protein Structure
1. Primary Structure = unique sequence of amino acids in a protein

Determined by genes

Can be sequenced in the lab
2. Secondary Structure = regular, repeated coiling and folding of a protein’s polypeptide
backbone

Stabilized by H-bonds between peptide linkages
o Carbonyl of one a.a. and amino group of another a.a.

Fibrous Proteins (e.g. keratin or silk) are composed of this level

Two types:
a) Alpha Helix = helical coil stabilized by H-bonding between every 4th
peptide bond
b) Beta-Pleated Sheet = sheet of antiparallel chains folded into accordion
pleats

Held together by either intrachain or interchain H-bonds
3. Tertiary Structure = the overall 3-D shape of a single polypeptide chain

Shape is held together by bonding between and among side chains (R- groups) and by
interactions between R groups and the aqueous environment

Weak Interactions
o H-bonds between polar side chains
o Ionic bonding between charged side chains
o Hydrophobic side chains orient themselves so that they are minimally exposed
to water in the protein’s interior
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Covalent Linkage
o Disulfide Bridges form between 2 cysteine amino acid monomers brought
together by the folding of the protein.


This is a strong bond!
Globular Proteins (ex. many enzymes) stop at this level
4. Quaternary Structure = a 3-D structure that results from the interactions between and
among two or more polypeptide chains that work together to act as one overall protein

Ex. Hemoglobin (4 globular polypeptides)
Protein Conformation

Conformation is influenced by the PHYSICAL and CHEMICAL environmental
conditions

If the environment changes too much, the protein may become denatured and lose its
native conformation

Can happen if:
o The protein is placed in an organic (nonpolar) solvent
o Chemical agents disrupt H-bonds, ionic bonds, or difulfide bridges

Ex. Change in pH
o The protein is subjected to excess heat

The protein may or may not return to the native conformation when the environment
returns to normal, depending on:
o The degree of denaturation
o The need for chaperone proteins = temporarily scaffold proteins as they fold in
the cell

Believed that they don’t fold it really, but help correct it if it misfolds

May play a role in Alzheimer’s or Cystic Fibrosis?

Prion = a misfolded protein that causes other proteins of the same
type to fold incorrectly as well
o TSE diseases, like Mad Cow Disease, are due to these