amino acids and peptides

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Amino acids and peptides The “Lego” of proteins www.freelivedoctor.com

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Page 1: Amino Acids And Peptides

Amino acids and peptides

The “Lego” of proteins

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Page 2: Amino Acids And Peptides

Amino Acids

• Same general structure

• Called alpha amino acids

• L- isomer is physiologically active

• Side chain or R group determines other properties

• Acid-base properties

H3N+

R

O

O

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Page 3: Amino Acids And Peptides

Amino Acids, General

• pK of carboxyl group around 2.3– Deprotonated at physiological pH

• R – COO-

• pK of amino group around 9.5 – Protonated at physiological pH

• H3N+-R

• Some R-groups are acidic or basic

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Page 4: Amino Acids And Peptides

Nonpolar amino acids

NH3+CH

O

CH3

O

NH3+CH2

O

O

NH3+

CH2

CH

O

CH3 CH3

ONH3+

CH CH2

O

CH3

CH3

O

N+

O

O

HH

glycine(G) Alanine (A)

NH3+

CH3

O

O

CH3

Valine (V)

Leucine (L)

Isoleucine (I)

Proline (P)

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Page 5: Amino Acids And Peptides

Aromatic Amino Acids

H3N+O

O

H3N+

O

N

O

H3N+

O

OH

O

phenylalanine (F) tyrosine (Y)

pKr = 10.07

tryptophan (W)

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Page 6: Amino Acids And Peptides

Polar Amino Acids, alcohols

H3N+

OH

O

O H3N+

CH3 OH

O

O

Serine (S) Threonine (T)

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Page 7: Amino Acids And Peptides

Polar Amino Acids(sulfur containing)

H3N+O

SH

O

N3N+O

SCH3

O

Cysteine (C)

Methionine (M)

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Page 8: Amino Acids And Peptides

Polar amino acids (amides)

H3N+ O

O

NH2

O

H3N+O

ONH2

O

Asparagine (N)

Glutamine (Q)

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Page 9: Amino Acids And Peptides

Charged Amino Acids (Acidic)

H3N+ O

O

O

O

H3N+O

OO

O

Aspartate (D)

Glutamate (E)pKr = 3.86

pKr = 4.25

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Page 10: Amino Acids And Peptides

Charged Amino Acids (Basic)

H3N+O

NH

NH2+NH2

O

H3N+O

N

NH

OH3N+

O

NH3+

O

Arginie (R)

pKr = 12.48

Histidine (H)

pKr = 6.0

Lysine (K)

pKr = 10.53

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Page 11: Amino Acids And Peptides

Titration of Amino acids

• Titration of glycine

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Page 12: Amino Acids And Peptides

Titration of Glutamate

H3N+O

OO

O

Glutamate (E)

pKr = 4.25

pK = 2.19

pK = 9.67

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Page 13: Amino Acids And Peptides

Purification of amino acids

• Chromatography– Various types– Ion exchange

• Uses net charge or can change with pH– Separate K, D and A

– HPLC

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Page 14: Amino Acids And Peptides

Electrophoresis

• Using electricity to move particles through a gelatinous matrix

• Isoelectric point

• IEF

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Page 15: Amino Acids And Peptides

Peptides

• Peptide bond is amide linkage between amino acids• No free rotation about C-N bond due to partial double

bond character of bond– resonance

NH

O

R2

H3N+

O

R1

O

N

O

R2

H3N+

O

R1

O

-

peptide bond

+

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Page 16: Amino Acids And Peptides

Tetrapeptide

• Note planes of peptide bonds

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Page 17: Amino Acids And Peptides

Peptides

• Peptides are vectorial– Have N and C termini

• Sequences read H3N+---COO-

P-I-G

NH

OH2N+

O

NH

O

O

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Page 18: Amino Acids And Peptides

Draw a tetrapeptide of L-A-R-D at physiological pH

• Answer

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Page 19: Amino Acids And Peptides

Biologically Interesting Peptides

• Aspartame– L-aspartyl-Lphenylalanine methyl ester

NH

OH3N+

O

OO

OCH3

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Page 20: Amino Acids And Peptides

Other small interesting peptides

• Enkephalins Y-G-G-F-LY-G-G-F-M

• OxytocinC-Y-I-Q-N-C-P-L-G-NH2

• Has C—C disulfide

• VasopressinC-Y-F-Q-N-C-P-R-G-NH2

• Also disulfide

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Page 21: Amino Acids And Peptides

Chemical Synthesis of Peptides

• Why?

• Done solid phase– Problem with in solution

• Hard to make pure desired sequence

– Purify by filtration

• Side chains blocked

• Made backwards ( C---N)

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Page 22: Amino Acids And Peptides

Scheme for peptide synthesis

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Page 23: Amino Acids And Peptides

Tetrapeptide answer

NH

OH3N+

O

NH N

H

O

O

O

O

O

NH

NH2H2N+

L A R D

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