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Amino Acids Proteins, and Enzymes

Types of Proteins

Amino Acids

The Peptide Bond

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Types of Proteins

Type Examples• Structural tendons, cartilage, hair, nails• Contractile muscles• Transport hemoglobin• Storage milk• Hormonal insulin, growth hormone• Enzyme catalyzes reactions in cells• Protection immune response

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Amino Acids

• Building blocks of proteins• Carboxylic acid group• Amino group• Side group R gives unique characteristics

R side chain I

H2H—C —COOH I

H

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Examples of Amino Acids

H I

H2N—C —COOH I

H glycine

CH3 I

H2N—C —COOH I

H alanine

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Types of Amino Acids

Nonpolar R = H, CH3, alkyl groups, aromatic

OPolar ll

R = –CH2OH, –CH2SH, –CH2C–NH2,

(polar groups with –O-, -SH, -N-)

Polar/Acidic

R = –CH2COOH, or -COOH

Polar/ Basic

R = –CH2CH2NH2

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Learning Check AA1

Identify each as (1) polar or (2) nonpolar

A. NH2–CH2–COOH (Glycine)

CH3

| CH–OH

|

B. NH2–CH–COOH (Serine)

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Solution AA1

Identify each as (1) polar or (2) nonpolar

A.(2) NH2–CH2–COOH (Glycine)

CH3

| CH–OH |

B. (1) NH2–CH–COOH (Serine)

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Essential Amino Acids

• 10 amino acids not synthesized by the body

• arg, his, ile, leu, lys, met, phe, thr, trp, val

• Must obtain from the diet

• All in diary products

• 1 or more missing in grains

and vegetables

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Amino Acids as Acids and Bases

• Ionization of the –NH2 and the –COOH group

• Zwitterion has both a + and – charge• Zwitterion is neutral overall

+

NH2–CH2–COOH H3N–CH2–COO–

glycine Zwitterion of glycine

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pH and ionization

H+ OH–

+ +

H3N–CH2–COOH H3N–CH2–COO– H2N–CH2–

COO–

Positive ion zwitterion Negative ion

Low pH neutral pH High pH

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Learning Check AA2

CH3 CH3

+

H3N–CH–COOH H2N–CH2–COO–

(1) (2)

Select from the above structures

A. Alanine in base.

B. Alanine in acid.

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Solution AA2

CH3 CH3

+

H3N–CH–COOH H2N–CH2–COO–

(1) (2)

Select from the above structures

A. (2) Alanine in base.

B. (1) Alanine in acid.

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The Peptide Bond

Amide bond formed by the –COOH of an amino acid and the –NH2 of the next amino acid

O CH3

+ | | + |

NH3–CH2–COH + H3N–CH–COO–

O CH3

+ | | |

NH3–CH2–C – N–CH–COO–

| peptide bond H

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Peptides

• Amino acids linked by amide (peptide) bonds

Gly Lys Phe Arg Ser

H2N- -COOH

end Peptide bonds end

Glycyllysylphenylalanylarginylserine

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Learning Check AA3

What are the possible tripeptides formed from one each of leucine, glycine, and alanine?

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Solution AA3

Tripeptides possible from one each of leucine, glycine, and alanine

Leu-Gly-Ala

Leu-Ala-Gly

Ala-Leu-Gly

Ala-Gly-Leu

Gly-Ala-Leu

Gly-Leu-Ala

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Learning Check AA4

Write the three-letter abbreviations for the following tetrapeptide:

H3N CH

CH3

C

O

N

H

CH C

O

N

H

CH C

O

N

H

CH C O-

OCH

CH CH3

CH3

CH2

SH

CH2

CH2

S

CH3

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Solution AA4

Ala-Leu-Cys-Met

H3N CH

CH3

C

O

N

H

CH C

O

N

H

CH C

O

N

H

CH C O-

OCH

CH CH3

CH3

CH2

SH

CH2

CH2

S

CH3