Topic 4: Amino Acids. Proteins - Detailed Solutions

Problem 1: Ionization of Amino Acids

📋 Task:

Write the equations of ionization in an alkaline and acidic mediums for the following amino acids: a. Cysteine, b. Lysine, c. Glutamic acid

a. Cysteine

Cysteine Structure

In Acidic Medium (pH < 2):

H₃N⁺-CH(CH₂SH)-COOH
Fully protonated form: Both amino group and carboxyl group are protonated
Net charge: +1

In Neutral Medium (pH ≈ 5-6, pI):

H₃N⁺-CH(CH₂SH)-COO⁻
Zwitterion form: Amino group protonated, carboxyl group deprotonated
Net charge: 0

In Alkaline Medium (pH > 10):

H₂N-CH(CH₂SH)-COO⁻
Fully deprotonated form: Both groups deprotonated
Net charge: -1

pKa values: pKa₁(COOH) ≈ 1.96, pKa₂(NH₃⁺) ≈ 8.18, pKa(SH) ≈ 10.28
Isoelectric point (pI): ≈ 5.07

b. Lysine

Lysine Structure

In Acidic Medium (pH < 2):

H₃N⁺-CH((CH₂)₄NH₃⁺)-COOH
Fully protonated form: α-amino, ε-amino, and carboxyl all protonated
Net charge: +2

In Neutral Medium (pH ≈ 9-10, pI):

H₃N⁺-CH((CH₂)₄NH₃⁺)-COO⁻
Zwitterion form: Both amino groups protonated, carboxyl deprotonated
Net charge: +1

In Alkaline Medium (pH > 11):

H₂N-CH((CH₂)₄NH₂)-COO⁻
Fully deprotonated form: All groups deprotonated
Net charge: -1

pKa values: pKa₁(COOH) ≈ 2.18, pKa₂(NH₃⁺) ≈ 8.95, pKa(ε-NH₃⁺) ≈ 10.53
Isoelectric point (pI): ≈ 9.74 (basic amino acid)

c. Glutamic Acid

Glutamic Acid Structure

In Acidic Medium (pH < 2):

H₃N⁺-CH(CH₂CH₂COOH)-COOH
Fully protonated form: Both carboxyl groups and amino group protonated
Net charge: +1

In Neutral Medium (pH ≈ 3-4, pI):

H₃N⁺-CH(CH₂CH₂COO⁻)-COO⁻
Zwitterion form: Both carboxyl groups deprotonated, amino protonated
Net charge: -1

In Alkaline Medium (pH > 10):

H₂N-CH(CH₂CH₂COO⁻)-COO⁻
Fully deprotonated form: All groups deprotonated
Net charge: -2

pKa values: pKa₁(COOH) ≈ 2.19, pKa₂(NH₃⁺) ≈ 9.67, pKa(side chain COOH) ≈ 4.25
Isoelectric point (pI): ≈ 3.22 (acidic amino acid)

Problem 2: Carnosine Formation

📋 Task:

Dipeptide carnosine is naturally produced by the body in the liver. Carnosine works as an antioxidant, reduces reactive oxygen and alpha-beta-unsaturated aldehydes in the body. Write down the reaction of carnosine formation from alanine and histidine. Write down the three-letter abbreviated name for this dipeptide. Identify the total charge of the dipeptide at pH=1 and pH=13

Carnosine Formation Reaction

Reaction:
H₂N-CH(CH₃)-COOH + H₂N-CH(CH₂C₃H₃N₂)-COOH →
H₂N-CH(CH₃)-CO-NH-CH(CH₂C₃H₃N₂)-COOH + H₂O

Alanine + Histidine → Carnosine + Water

Alanine

Histidine

Carnosine

At pH = 1 (Strongly Acidic):

N-terminal amino group: NH₃⁺ (protonated) → +1
C-terminal carboxyl: COOH (protonated) → 0
Histidine imidazole ring: protonated → +1
Total charge: +2

At pH = 13 (Strongly Alkaline):

N-terminal amino group: NH₂ (deprotonated) → 0
C-terminal carboxyl: COO⁻ (deprotonated) → -1
Histidine imidazole ring: deprotonated → 0
Total charge: -1

Three-letter abbreviation: Ala-His or β-Ala-His
IUPAC Name: N-(2-amino-1-oxopropyl)histidine
Charge at pH=1: +2
Charge at pH=13: -1

Biological Importance:

Antioxidant: Scavenges reactive oxygen species
pH buffer: In muscle cells (pH ≈ 6.83)
Neurotransmitter: Acts in the brain
Anti-glycation: Prevents protein glycation
Anti-aging: Protects against cellular aging

Problem 3: Glutathione Formation

📋 Task:

Glutathione is a tripeptide involved in redox reactions occurring in the cells of a living organism, performs the function of protecting cysteine-containing proteins. Write down the reaction of glutathione formation from glutamic acid, cysteine and glycine. Write down the three-letter abbreviated name for this tripeptide. Identify the total charge of the tripeptide at pH=1 and pH=13

Glutathione Formation Reaction

Reaction:
Glu + Cys + Gly → Glutathione + 2H₂O

Structure:
γ-Glu-Cys-Gly (γ-peptide bond between Glu and Cys)

Glutathione

At pH = 1 (Strongly Acidic):

N-terminal amino (Glu): NH₃⁺ → +1
C-terminal carboxyl (Gly): COOH → 0
Glu side chain COOH: protonated → 0
Cys side chain SH: protonated → 0
Total charge: +1

At pH = 13 (Strongly Alkaline):

N-terminal amino (Glu): NH₂ → 0
C-terminal carboxyl (Gly): COO⁻ → -1
Glu side chain COO⁻: deprotonated → -1
Cys side chain S⁻: deprotonated → -1
Total charge: -3

Three-letter abbreviation: Glu-Cys-Gly or GCG
IUPAC Name: γ-Glutamylcysteinylglycine
Charge at pH=1: +1
Charge at pH=13: -3
Special feature: γ-peptide bond (unusual linkage)

Biological Importance:

Master antioxidant: Most important cellular antioxidant
Redox reactions: Protects against oxidative stress
Detoxification: Conjugates with toxins
Immune function: Supports immune system
Protein protection: Protects cysteine-containing proteins

Problem 4: Alanylglutamine Dipeptide

📋 Task:

Alanylglutamine dipeptide is used in medicine to increase the body's resistance to various negative influences. Write down the reaction of alanylglutamine formation from alanine and glutamine. Write down the three-letter abbreviated name for this dipeptide. Identify the total charge of the dipeptide at pH=1 and pH=13

Alanylglutamine Formation

Reaction:
Ala + Gln → Alanylglutamine + H₂O

Structure:
H₂N-CH(CH₃)-CO-NH-CH(CH₂CH₂CONH₂)-COOH

Alanylglutamine

At pH = 1:

N-terminal NH₃⁺ → +1
C-terminal COOH → 0
Total charge: +1

At pH = 13:

N-terminal NH₂ → 0
C-terminal COO⁻ → -1
Total charge: -1

Three-letter abbreviation: Ala-Gln or AQ
Charge at pH=1: +1
Charge at pH=13: -1

Medical Applications:

Immunity: Increases resistance to infections
Parenteral nutrition: Stable glutamine source
Gut health: Supports intestinal mucosa
Post-surgery: Aids recovery

Problem 5: Glycylleucine Dipeptide

📋 Task:

Glycylleucine dipeptide has a bitter taste, it increases appetite and acts in a living organism as a digestive stimulant. Write down the reaction of glycylleucine formation from glycine and leucine. Write down the three-letter abbreviated name for this dipeptide. Identify the total charge of the dipeptide at pH=1 and pH=13

Glycylleucine Formation

Reaction:
Gly + Leu → Glycylleucine + H₂O

Structure:
H₂N-CH₂-CO-NH-CH(CH₂CH(CH₃)₂)-COOH

Glycylleucine

At pH = 1:

N-terminal NH₃⁺ → +1
C-terminal COOH → 0
Total charge: +1

At pH = 13:

N-terminal NH₂ → 0
C-terminal COO⁻ → -1
Total charge: -1

Three-letter abbreviation: Gly-Leu or GL
Charge at pH=1: +1
Charge at pH=13: -1

Biological Properties:

Taste: Bitter taste receptor activator
Appetite stimulant: Increases food intake
Digestive stimulant: Enhances digestion
Protein degradation marker: Released during proteolysis

Problem 6: Arginylglycine Dipeptide

📋 Task:

Arginylglycine dipeptide has a salty taste, it improves digestion and appetite, enhances intestinal motility, relieves bloating and gas formation. Write down the reaction of arginylglycine formation from arginine and glycine. Write down the three-letter abbreviated name for this dipeptide. Identify the total charge of the dipeptide at pH=1 and pH=13

Arginylglycine Formation

Reaction:
Arg + Gly → Arginylglycine + H₂O

Structure:
H₂N-C(NH)-NH-(CH₂)₃-CH(NH₂)-CO-NH-CH₂-COOH

Arginylglycine

At pH = 1:

N-terminal NH₃⁺ → +1
Arg side chain (guanidino): NH₂⁺=C(NH₂)NH- → +1
C-terminal COOH → 0
Total charge: +2

At pH = 13:

N-terminal NH₂ → 0
Arg side chain: neutral → 0
C-terminal COO⁻ → -1
Total charge: -1

Three-letter abbreviation: Arg-Gly or RG
Charge at pH=1: +2
Charge at pH=13: -1

Biological Properties:

Taste: Salty taste
Digestion: Improves digestive function
Intestinal motility: Enhances peristalsis
Anti-bloating: Relieves gas formation
Appetite: Stimulates food intake

Problem 7: Alanylglutamine in Cosmetology

📋 Task:

Alanylglutamine dipeptide is used in cosmetology as an active ingredient in skin moisturizers. Write down the reaction of alanylglutamine formation from alanine and glutamine. Write down the three-letter abbreviated name for this dipeptide. Identify the total charge of the dipeptide at pH=1 and pH=13

Alanylglutamine for Skin Care

Same as Problem 4:
Three-letter abbreviation: Ala-Gln or AQ
Charge at pH=1: +1
Charge at pH=13: -1

Cosmetological Applications:

Skin moisturizer: Hydrates skin effectively
Anti-aging: Reduces wrinkles

Skin barrier:

Wound healing: Promotes tissue repair
Stable formulation: Better than free glutamine

Problem 8: Glutamylglycine Dipeptide

📋 Task:

The dipeptide glutamylglycine is used as an active ingredient in drops that reduce intraocular pressure. Write down the reaction of glutamylglycine formation from glutamic acid and glycine. Write down the three-letter abbreviated name for this dipeptide. Identify the total charge of the dipeptide at pH=1 and pH=13

Glutamylglycine Formation

Reaction:
Glu + Gly → Glutamylglycine + H₂O

Structure:
H₂N-CH(CH₂CH₂COOH)-CO-NH-CH₂-COOH

Glutamylglycine

At pH = 1:

N-terminal NH₃⁺ → +1
C-terminal COOH → 0
Glu side chain COOH → 0
Total charge: +1

At pH = 13:

N-terminal NH₂ → 0
C-terminal COO⁻ → -1
Glu side chain COO⁻ → -1
Total charge: -2

Three-letter abbreviation: Glu-Gly or EG
Charge at pH=1: +1
Charge at pH=13: -2

Medical Applications:

Intraocular pressure: Reduces eye pressure
Glaucoma treatment: Used in eye drops
Ophthalmology: Treats eye conditions

Problem 9: Aspartame Formation

📋 Task:

Aspartame is used in medicine as a sugar substitute for diabetics. Write 2 steps of aspartame molecule formation: a. Condensation of aspartic acid and phenylalanine, b. Esterification of aspartylphenylalanine dipeptide with methanol

Aspartame Synthesis

Step a: Condensation
Asp + Phe → Asp-Phe + H₂O

Step b: Esterification
Asp-Phe + CH₃OH → Aspartame + H₂O

Overall:
Asp + Phe + CH₃OH → Aspartame + 2H₂O

Aspartame

IUPAC Name: N-(L-α-Aspartyl)-L-phenylalanine 1-methyl ester
Three-letter code: Asp-Phe-OMe
Sweetness: 200 times sweeter than sucrose

Applications:

Sugar substitute: For diabetics
Low calorie: Zero-calorie sweetener
Food industry: Diet sodas, sugar-free products
PKU warning: Contains phenylalanine (phenylketonuria)

Problem 10: Noopept Formation

📋 Task:

Noopept is a nootropic drug that reduces anxiety and improves memory. Write 2 steps of noopept molecule formation: a. Condensation of phenylalanine, proline and glycine, b. Esterification of phenylalanylprolylglycine tripeptide with ethanol

Noopept Synthesis

Step a: Tripeptide Formation
Phe + Pro + Gly → Phe-Pro-Gly + 2H₂O

Step b: Esterification
Phe-Pro-Gly + C₂H₅OH → Noopept + 2H₂O

Overall:
Phe + Pro + Gly + C₂H₅OH → Noopept + 3H₂O

Noopept

Chemical Name: N-Phenylacetyl-L-prolylglycine ethyl ester
Molecular formula: C₁₇H₂₂N₂O₄
Three-letter code: Phe-Pro-Gly-OEt

Pharmacological Effects:

Nootropic: Enhances cognitive function
Memory improvement: Enhances learning and memory
Anxiolytic: Reduces anxiety
Neuroprotective: Protects brain cells
Antioxidant: Reduces oxidative stress in brain

🧪 Topic 4: Detailed Solutions

End of Topic 4 Solutions