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Amino Acids and Proteins

Biology 12

E. McIntyre

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• What do gelatin desserts, hair, antibodies, spider webs, blood clots, egg whites, tofu, and fingernails all have in common? They are all made of protein.

Where do we find proteins?

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Proteins

• genetic information in DNA codes specifically for the production of proteins

• 50% of the dry mass of most cells

• As with carbohydrates and lipids, their three-dimensional structure is directly related to their function

• enzymes are proteins

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Proteins: Basic Characteristics

• Proteins are amino acid polymers• central carbon atom surrounded by

an amino group, a carboxyl group, a hydrogen atom, and a variable group of atoms called a side chain, usually symbolized by the letter ‘R’. Remember that carboxyl and amino groups are functional groups.

• There are 20 different R groups commonly found in living organisms and, so there are 20 different amino acids.

General structure of an amino acid

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• There are 20 amino acids.

• Note all are the same except for the side chains

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• Amino acids are amphiprotic (which means that they possess both acidic (carboxyl) and basic (amino) functional groups.)

• When dissolved in water, the carboxyl group donates an H ion to the amino group, causing the carboxyl group to become negatively charged and the amino group to possess an extra hydrogen and a net positive charge.

Amino acids: General Properties

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• Amino acids may be polar (hydrophilic), nonpolar (hydrophobic), or charged (acidic or basic)

• Acidic amino acids possess a carboxyl group on their side chains, and basic amino acids contain amino groups on their side chains.

• These characteristics are determined by an amino acid’s side chain.

…Amino acids: General Properties

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Examples of Amino Acids

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Types of Amino Acids

Nonpolar R = H, CH3

OPolar ll

R = –CH2OH, –CH2SH, –CH2C–NH2, (polar groups with –O-, -SH, -N-)

Acidic

R = –CH2COOH, or -COOHBasic

R = –CH2CH2NH2

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Learning Check AA1

Identify each as polar or nonpolar

A.

B.

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Solution AA1

Identify each as polar or nonpolar

A

B

polar

Non-polar

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Essential Amino Acids

• 10 amino acids not synthesized by the body• Arginine (arg) histadine (his) isoleucine (ile)

leucine (leu) threonine (thr) lysine (lys), methionine (met) phenylalanine (phe) trytophan (trp)valine (val)

• Must obtain from the diet• All in diary products • 1 or more missing in grains and vegetablesWant to remember the essential amino acids?

Any Help In Learning These Little Molecules Proves Truly Valuable

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Amino Acids as Acids and Bases

• Ionization of the –NH2 (amino) and the –COOH (carboxyl) group

• An amphiprotic molecule has both a + and – charge

• An amphiprotic molecule is neutral overall

Zwitterion of alanine

In water, an amino acid loses a H+ ion from the carboxyl group and

donates it to the amino group.

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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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Stop & Think

• Remember that carboxyl and amino groups are functional groups.

• Think: What are functional groups?

• How are polymers of amino acids made?

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Solution to Stop & Think

• Carboxyl groups and amino groups react together to link amino acids.

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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-AlaLeu-Ala-GlyAla-Leu-GlyAla-Gly-LeuGly-Ala-LeuGly-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

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Types of Proteins

Type Examples• Structuraltendons, cartilage, hair, nails• Contractile muscles• Transporthemoglobin• Storage milk• Hormonal insulin, growth hormone• Enzyme catalyzes reactions in cells• Protectionimmune response

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Protein Structure