Amino Acids: Structure, Classification, and Biological Role – Foundations of Bioorganic Chemistry

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AMINO ACIDS: STRUCTURE, CLASSIFICATION, AND BIOLOGICAL ROLE

Introduction to Bioorganic Chemistry

Bioorganic chemistry is a foundational discipline that explores the chemical processes and molecular structures underlying biological systems. It bridges organic chemistry and biochemistry, focusing on the structure, function, and interactions of biomolecules essential for life.

Bioorganic chemistry includes structural, metabolic, and clinical aspects.
It is crucial for understanding medical practice and the molecular basis of life.
Key biomolecules: amino acids, proteins, nucleic acids, carbohydrates, and lipids.

Biomolecules: Types and Functional Groups

Biomolecules are organic molecules present in living organisms, classified by size and complexity:

Macromolecules: Large, complex molecules (e.g., proteins, nucleic acids, polysaccharides).
Micromolecules: Small molecules (e.g., amino acids, monosaccharides, fatty acids).
Complex molecules: Assemblies of macromolecules (e.g., ribosomes, membranes).

Functional groups are specific groups of atoms within molecules that determine their chemical properties and reactivity:

Hydroxyl (-OH): Alcohols
Amino (-NH2): Amines
Carboxyl (-COOH): Carboxylic acids
Amide (-CONH2): Amides
Sulfhydryl (-SH): Thiols
Carbonyl (C=O): Ketones, aldehydes

Bioelements: Classification and Biological Roles

Bioelements are chemical elements essential for life, classified as:

Major (macroelements): C, H, O, N, P, S, Ca, K, Na, Mg, Cl
Minor (microelements): Fe, Cu, Zn, Mn, Co, Mo, I, F, Se, Cr, V, Ni

These elements are involved in the structure and function of biomolecules and physiological processes.

Element	Role
Oxygen (O)	Component of water and organic molecules; essential for respiration
Carbon (C)	Backbone of all organic molecules
Hydrogen (H)	Component of water and organic molecules; involved in acid-base balance
Nitrogen (N)	Component of amino acids, proteins, nucleic acids
Phosphorus (P)	Component of nucleic acids, ATP, phospholipids
Sulfur (S)	Component of some amino acids (cysteine, methionine), vitamins
Calcium (Ca)	Bone and teeth structure, muscle contraction, blood clotting
Potassium (K)	Main intracellular cation, nerve impulse transmission
Sodium (Na)	Main extracellular cation, fluid balance, nerve impulse transmission
Magnesium (Mg)	Enzyme cofactor, ATP stabilization, bone structure

Additional info: The table above summarizes the main macroelements and their biological roles as described in the notes.

Types of Chemical Bonds in Biomolecules

Biomolecules are held together by various types of chemical bonds, which determine their structure and function:

Covalent bonds: Strong bonds formed by sharing electron pairs between atoms (e.g., C–C, C–N, C–O).
Polar covalent bonds: Electrons are shared unequally, leading to partial charges (e.g., O–H, N–H).
Non-covalent interactions: Weaker forces important for molecular recognition and structure:
- Hydrogen bonds: Attraction between a hydrogen atom and an electronegative atom (O, N).
- Van der Waals forces: Weak attractions due to transient dipoles.
- Ionic interactions: Attraction between oppositely charged ions (e.g., Na+ and Cl–).

Properties of Living Matter

Living matter is characterized by:

High degree of structural organization and complexity.
Functional specificity of each component.
Ability to transform and utilize energy.
Capacity for self-replication and transmission of genetic information.

AMINO ACIDS: STRUCTURE AND CLASSIFICATION

General Structure of Amino Acids

Amino acids are organic compounds containing both an amino group (-NH2) and a carboxyl group (-COOH) attached to a central (α) carbon atom. The α-carbon also bears a hydrogen atom and a variable side chain (R group) that determines the identity and properties of each amino acid.

General formula:
At physiological pH, amino acids exist as zwitterions (internal salts with both positive and negative charges).

Classification of Amino Acids

Amino acids can be classified based on:

Structure of the side chain (R group):
- Nonpolar (hydrophobic): e.g., alanine, valine, leucine
- Polar uncharged: e.g., serine, threonine, asparagine
- Acidic (negatively charged): e.g., aspartic acid, glutamic acid
- Basic (positively charged): e.g., lysine, arginine, histidine
- Aromatic: e.g., phenylalanine, tyrosine, tryptophan
Essentiality:
- Essential amino acids: Cannot be synthesized by the body; must be obtained from the diet (e.g., valine, leucine, isoleucine, lysine, methionine, phenylalanine, threonine, tryptophan, histidine).
- Non-essential amino acids: Can be synthesized by the body.

Functions of Amino Acids

Building blocks of proteins (proteinogenic amino acids).
Precursors for hormones, neurotransmitters, and other biomolecules.
Involved in metabolic pathways and energy production.
Some amino acids have specialized roles (e.g., glutamate as a neurotransmitter).

Stereochemistry of Amino Acids

All amino acids (except glycine) have a chiral α-carbon, leading to two possible stereoisomers: L- and D-forms. Only L-amino acids are incorporated into proteins in living organisms.

Enantiomers: Non-superimposable mirror images.
L-form: The biologically active form in proteins.

Acid-Base Properties of Amino Acids

Amino acids can act as both acids and bases (amphoteric), due to the presence of both amino and carboxyl groups. They have characteristic dissociation constants (pKa) and an isoelectric point (pI), at which the molecule has no net charge.

At low pH: Amino acids are fully protonated (positively charged).
At high pH: Amino acids are deprotonated (negatively charged).
At pI: Zwitterionic form predominates (net charge = 0).

Equation for isoelectric point (for amino acids without ionizable side chains):

PROTEINS: STRUCTURE AND PEPTIDE BONDS

Peptide Bond Formation

Proteins are polymers of amino acids linked by peptide bonds. A peptide bond is a covalent bond formed between the carboxyl group of one amino acid and the amino group of another, releasing a molecule of water.

Peptide bond:
Peptides: Short chains of amino acids (dipeptides, tripeptides, oligopeptides).
Polypeptides: Longer chains (usually >50 amino acids).
Proteins: One or more polypeptide chains folded into a functional structure.

Primary Structure of Proteins

The primary structure is the linear sequence of amino acids in a polypeptide chain, determined by the genetic code. The order of amino acids dictates the protein's higher-order structure and function.

Written from the N-terminus (amino end) to the C-terminus (carboxyl end).
Determined by the sequence of codons in DNA.

Methods for Determining Primary Structure

Hydrolysis: Breaking peptide bonds to identify constituent amino acids.
Enzymatic cleavage: Using proteases (e.g., trypsin, chymotrypsin) to generate fragments.
Edman degradation: Sequentially removes N-terminal amino acids for identification.
Mass spectrometry: Modern technique for sequencing peptides and proteins.

Biological Importance of Proteins

Structural components (e.g., collagen, keratin).
Enzymes catalyzing biochemical reactions.
Transport and storage (e.g., hemoglobin, albumin).
Hormones and signaling molecules (e.g., insulin).
Immune response (e.g., antibodies).

Summary Table: Types of Amino Acids by Side Chain Properties

Type	Examples	Properties
Nonpolar (hydrophobic)	Alanine, Valine, Leucine	Hydrophobic, found in protein interiors
Polar uncharged	Serine, Threonine, Asparagine	Hydrophilic, can form hydrogen bonds
Acidic	Aspartic acid, Glutamic acid	Negatively charged at physiological pH
Basic	Lysine, Arginine, Histidine	Positively charged at physiological pH
Aromatic	Phenylalanine, Tyrosine, Tryptophan	Contain aromatic rings, absorb UV light

Additional info: The notes also reference the importance of amino acids in clinical and metabolic contexts, including their role in disease, nutrition, and as precursors for pharmacologically active substances.