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20.4 Formation of Peptides20.5 Protein Structure: Primary and Secondary Levels

Chapter 20 Amino Acids and Proteins

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The Peptide Bond A peptide bond is an amide bond between the carboxyl group

of one amino acid and the amino group of the next amino acid. O CH3 O

+ || + | ||H3N—CH2—C—OH + H3N—CH—C—O–

O H CH3 O + || | | ||

H3N—CH2—C—N—CH—C—O–

peptide bond

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A Dipeptide A peptide is named from the free amine (NH3

+) using -yl endings for the names of the amino acids.

The last amino acid with the free carboxyl group (COO-) uses its amino acid name.

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A polypeptide containing 50 or more amino acids is called a protein.

The primary structure of a protein is the sequence of amino acids in the peptide chain.

Ala-Leu-Cys-Met

Primary Structure

CH CH3

CH3

H3N CH C

O

N

H

CH C

O

N

H

CH C

O

N

H

CH C O-

OCH CH2

CH2

S

CH3

CH2

SH

CH3

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Primary Structures

The nonapeptides oxytocin and vasopressin have similar primary structures.

Only the amino acids at positions 3 and 8 differ.

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InsulinInsulin: Was the first

protein to have its primary structure determined.

Of humans has a primary structure that is similar to the insulin of pigs and cows.

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The secondary structures of proteins indicate the arrangement of the polypeptide chains in space.

The alpha helix is a three-dimensional arrangement of the polypeptide chain that gives a corkscrew shape like a coiled telephone cord.

Secondary Structure: Alpha Helix

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Alpha Helix The coiled shape

of the alpha helix is held in place by hydrogen bonds between the amide groups and the carbonyl groups of the amino acids along the chain.

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Secondary Structure: Pleated Sheet

The pleated sheet: Holds proteins in a

parallel arrangement with hydrogen bonds.

Has R groups that extend above and below the sheet.

Is typical of fibrous proteins such as silk.

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A triple helix: Consists of three alpha helix chains. Contains large amounts glycine, proline,

hydroxy proline and hydroxylysine that contain –OH groups for hydrogen bonding.

Is found in collagen, connective tissue, skin, tendons, and cartilage.

Secondary Structure: Triple Helix

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Essential amino acids: Are the ten amino

acids that are not synthesized by the body.

Must be obtained from the diet.

Essential Amino Acids

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Essential amino acids are: Found in milk and eggs

(complete proteins). Not all found in grains

and vegetables (incomplete proteins). Obtained by combining

two or more vegetables that provide complementary proteins.

Essential Amino Acids