Amino Acids and Proteins Overview

Amino Acids/Proteins

• Proteins/polypeptides are chains formed by the condensation/combination of 20 different α-amino acids.
• Polypeptides can be di-, tri-, etc., up to 10 amino acids (aa).
• Proteins are longer than 10 aa units; molecular weight (MW) >10,000.
• An amino acid contains a carboxyl group (-COOH) and an amino group (-NH₂).
• All amino acids in proteins have the -NH₂ group attached to the α carbon of the -COOH group (as well as the H and R).
• Naturally occurring α-amino acids, except glycine (R=H), are chiral and the L-stereoisomer.
• There are 20 α-amino acids in naturally occurring proteins.
• By convention, the -NH₂ group is placed to the left.
• Each amino acid has a common name often ending in "-ine."
• There are approximately 150 other physiologically important amino acids, such as GABA.

Classification of Amino Acids

• Amino acids can be categorized as nonpolar (hydrophobic) or polar (hydrophilic).

Nonpolar R Groups (Hydrophobic)

• Examples include glycine, alanine, proline, valine, tryptophan, isoleucine, methionine, leucine, and phenylalanine.

Polar R Groups (Hydrophilic)

• Examples include serine, asparagine, threonine, cysteine, glutamic acid, glutamine, tyrosine, histidine, lysine, arginine.

Amino Acids - General Properties

• Amino acids contain both an acidic functional group (-COOH) and a basic one (-NH₂), NH or N.
• Reactions involving amino acids are highly pH-dependent.

Peptide Bonds

• Peptide bonds form by condensation reactions, where the acid group (COOH) and amine group (-NH₂) of two different amino acids combine and release a water molecule.
• When peptide bonds are broken, this is called hydrolysis. The hydroxyl group (OH) and hydrogen (H) of a water molecule are added to the peptide bond.

pH-Dependent Properties

• Zwitterionic structures contain both N-H⁺ and COO⁻.
• At low pH, the COO⁻ group is protonated.
• At high pH, the N-H⁺ group loses a hydrogen.
• Isoelectric pH differs for each amino acid due to structural differences.

Essential, Nonessential, and Conditional Amino Acids

• Essential amino acids must be consumed in the diet.
• Nonessential amino acids can be synthesized in the body.
• Some amino acids are conditionally essential. They cannot be synthesized due to illness, lack of necessary precursors.

Dipeptides

• Dipeptides are composed of two amino acids.
• Examples include GG, AA, GA, and AG.

Tripeptides

• Tripeptides are composed of three amino acids.
• Examples include AAA, PPP, GGG, and variations.

Levels of Protein Structure

• Primary structure: The sequence of amino acids in the protein chain and the location of disulfide bridges.
• Secondary structure: The conformation or shape of the protein backbone.
• Tertiary structure: The 3D structure of the entire polypeptide.
• Quaternary structure: Occurs in proteins with more than one polypeptide chain describing the arrangement of the chains.

Specific Protein Examples

• Insulin: A protein hormone with a specific amino acid sequence that regulates blood sugar levels.
• Hemoglobin: A protein that transports oxygen in the blood, made up of multiple polypeptide chains.

Myoglobin structure

Myoglobin is a protein that stores oxygen in muscles. It has a single polypeptide chain.

Protein Classification by Structure

• Simple proteins consist of only amino acids.
  • Fibrous proteins are insoluble and mostly structural (e.g., hair, horn).
  • Globular proteins are soluble and often reactive (e.g., enzymes).
• Conjugated proteins have non-amino acid components (e.g., Lipoproteins, glycoproteins, metalloproteins).

Common Protein Functions

• Enzymes: Biological catalysts
• Contractile: Muscle movement
• Hormones: Signaling molecules (insulin, growth hormone)
• Neurotransmitters: Nerve signals
• Storage: Nutrient storage.
• Transport: Transporting substances (hemoglobin, transferrin)
• Protective: Antibodies
• Toxins: Poisonous substances (snake venom)

Vegetarian Diets

• The main challenge for vegetarians is obtaining enough high-quality proteins with the correct balance of essential amino acids.

Protein Content

• Different foods have varying protein content.

Malnutrition

• Malnutrition occurs when insufficient complete protein, especially essential amino acids, leads to poor bodily function.
• Symptoms include extreme emaciation, bloated abdomen, and ultimately death.

Glycolipids

• Glycolipids are derivatives of ceramides with carbohydrate directly attached to the ceramide.
• They lack a phosphocholine group that is present in sphingomyelin.

Glycoproteins

• Glycoproteins are proteins that have oligosaccharide chains (glycans) covalently attached to their peptide side-chains.
• The attaching process is called glycosylation and happens during co-translational modification and post-translational modification.

Roles of Glycolipids and Glycoproteins

• Glycolipids and glycoproteins serve many functions in the body, such as being cell surface markers, receptors for viruses and toxins, participating in cell interactions, blood group antigens, and undergoing changes during cell transformation, and more.

Denaturation

• Denaturation is any physical or chemical process that changes a protein's structure and makes it unable to perform its normal function.
• Denaturation can be reversible or irreversible depending on the protein and the extent of the change.