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Nutritional needs

CHAPTER 3: AMINO ACIDS, PEPTIDES, AND PROTEINS

Pg. 71-89

Nutrional needs

Nonpolar, aliphatic R group

SemiEssential histidine

Non-Essential cysteine homocysteine tyrosine glycine glutathione serine aspartic acid

glutamine glutamic acid arginine alanine proline hydroxyproline

Essential: cannot make trytophan valine leucine isoleucine methionine phenylalanine threonine lysine

Polar, uncharged R groups

Reversible formation of disulfide bond

Fig. 3-7

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Aromatic R groups

Positively charged R groups

H+

ERROR pKr = 6.0

Negatively charged R groups

Table 3-1: Amino acids

110

Box 3-1 = Lambert-Beer Law

Io Absorbance = log ---- = cl I Absorbance (A) Optical Density (O.D.) Concentration

Why detect proteins at 280?

2

Fig. 3-6: Absorption of UV light by aromatic amino acids

Optical Density at 280 nm OD280nm

Column chromatography

Color compound http://www.youtube.com/watch? v=EytuRMS1154&feature=related

Most proteins have no color

Need UV detector

High Performance Liquid Chromatography

http://www.youtube.com/watch? v=kz_egMtdnL4&feature=related

Fig. 3-10: Titration curves

Removal of H+ Gradual pH Glycine:

Fig. 3-10: Titration curves

Removal of H+ Gradual pH Glycine:

Deprotonation

COOH NH3+

Deprotonation

COOH NH3+

of two groups:

of two groups:

Two

Diprotic

distinct stages = 2 pKs

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Fig. 2-17

NOTE Acetate = Carboxylate Protonated<pKa 0 Deprotonated>pKa -1

TITRATION OF ACETIC ACID (CH3COOH) WITH AQUEOUS BASE (OH-)

http://www.youtube.com/watch?v=vvRRsw6SUOs

Fig. 2-17

NOTE Ammonium= amido Protonated<pKa +1 Deprotonated>pKa 0

Fig. 3-10: Titration curves

Table 3-1 pK1 COOH = 2.34 pK2 NH3+ = 9.60

Fig. 3-10: Titration curves

Fig. 3-10: Titration curves

pI = isoelectric point Net charge = 0 pI = 1/2 (pK1+pK2) = 1/2 (2.34+9.60) = 5.97

pK1 COOH = 2.34 pK2 NH3+ = 9.60 pH < pK1 COOH and NH3+ pK1 < pH < pK2 COO- and NH3+ pH > pK2 COO- and NH2

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Fig. 3-10: Titration curves

Below pI, + aa Above pI, - aa Predict the electric charge of aa Two buffering regions: 2.3 and 9.6

Zwitterions

Fig. 3-9

Table 3-1: Amino acids with ionizable R groups

LIFE IS NOT THAT SIMPLE!

Example: Histine

His pK1 = 1.82 pK2 = 9.17 pKr = 6.0 pI = 7.59 pK1? pK2? pKr? + COOH H3N-CH H+ CH2 C N

+H

Draw a titration curve

What H+ is being removed at:

C H

CH N+ H H

His pK1 = 1.82 pK2 = 9.17 pKr = 6.0 pI = 7.59

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Why is the pI = 7.59?

Why is the pI = 7.59?

His pK1 = 1.82 pK2 = 9.17 pKr = 6.0 pI = 7.59

1.82 + 9.17 + 6.0 pI = --------------------- = 5.7 3

His pK1 = 1.82 pK2 = 9.17 pKr = 6.0 pI = 7.59

9.17 + 6.0 pI = ------------- = 7.59 2

Peptides and proteins

Polymers of amino acids

Amino acids & peptide bond

http://www.5min.com/Video/Proteins-AminoAcids-and-the-Peptide-Bond-150614984

Fig. 3-13: Peptide bond

Fig. 3-14: Polypeptide

Polymer 1 2 3 4 5

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Fig. 3-14: Polypeptide

Fig. 3-14: Polypeptide

Amino-terminal end = N-terminus Carboxyl-terminal end = C-terminus

Peptide bond

Hydrolysis

of a peptide bond is exothermic but the reaction has a high activation energy Stable

N-terminus

C-terminus

Fig. 3-15: Ionization of peptide

One free amino group and one free carboxyl group Ionization of free amino and carboxyl groups R-groups are also ionized in peptide

Determine the charge of a peptide: Table 3-1

Charge at pH 7.0 NH3+ pK = 9.69 NH3+ or NH2

COO- pK = 4.25

COO- or COOH

NH3+ pK = 10.53 COO- pK = 2.18 Total charge =

NH3+ or NH2 COO- or COOH

Fig. 3-8: Nonstandard amino acids

Review: Nomenclature & Facts

Multisubunit proteins/Oligomers: Protein with 2 or more polypeptide chains Protomer: Chains are identical Chains may be covalently linked = disulfide bond Approximate the average weight of protein: 110 Dalton x number of amino acids Never have equal amounts of the 20 amino acids in a protein

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Levels of protein structure

Conjugated proteins

Contain chemical groups other than amino acid Prosthetic group: Table 3-4

Lipoprotein:

lipids sugar Metalloprotein: metal

Glycoprotein:

Next few weeks

Next lecture

Homework

Calculate the pH at which the amino group on the side chain of lysine is 20 % deprotonated.

Homework

1: 2:

Absolute configuration Titration curve 3: Henderson-Hasselbalch on amino acids 4: Ionization state of amino acid 7: Effect of pH on peptides 8: General facts 11: Net charges of peptides 13: Histones 14: Solubility of peptides

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