Proteins and Nucleic Acids: Structure and Function

Proteins: The Most Abundant and Versatile Macromolecules

Introduction to Proteins

Proteins are the most diverse and abundant biological macromolecules, essential for nearly every dynamic function in living organisms. Each protein has a unique structure that determines its specific function, with tens of thousands of different proteins found in the human body alone.

• Proteins are polymers composed of 20 different amino acids.

• Each protein's structure is directly related to its function in the cell.

• Proteins perform a wide variety of roles, including catalysis, defense, transport, signaling, movement, and structural support.

Amino Acids: Building Blocks of Proteins

Core Structure of Amino Acids

All amino acids share a common core structure, but differ in their side chains (R groups), which confer unique properties.

• Central carbon atom (α-carbon)

• Amino group (–NH2)

• Carboxyl group (–COOH)

• Hydrogen atom

• Variable side chain (R group)

Ionization States: Amino acids can exist in non-ionized or ionized forms depending on the pH of the environment.

• Non-ionized: Both amino and carboxyl groups are uncharged.

• Ionized: Amino group is protonated (–NH3+), carboxyl group is deprotonated (–COO–).

Properties of Side Chains (R Groups)

The side chain determines the chemical nature and reactivity of each amino acid.

• Nonpolar side chains: Consist mainly of carbon and hydrogen; hydrophobic and do not participate in hydrogen bonding.

• Polar and charged side chains: Contain functional groups that can form hydrogen bonds or ionic interactions; hydrophilic and often participate in chemical reactions.

Example: Glycine has a single hydrogen as its side chain (nonpolar), while aspartic acid has a carboxyl group (negatively charged, polar).

Levels of Protein Structure

Primary Structure

The primary structure of a protein is its unique sequence of amino acids, linked by peptide bonds.

• There are 20 types of amino acids available for protein synthesis.

• Protein length can range from just two amino acids to tens of thousands.

• The sequence is determined by genetic information and dictates higher levels of structure.

Peptide Bond Formation: Amino acids are joined by condensation reactions, forming covalent peptide bonds.

Secondary Structure

Secondary structure refers to local folding patterns within a polypeptide, stabilized by hydrogen bonds.

• Hydrogen bonds form between the carbonyl oxygen of one amino acid and the amino hydrogen of another.

• Two main types:

  • α-helix (alpha-helix): A right-handed coil stabilized by hydrogen bonds.

  • β-pleated sheet (beta-pleated sheet): Sheet-like structures formed by hydrogen bonding between parallel or antiparallel strands.

Tertiary Structure

The tertiary structure is the overall three-dimensional shape of a single polypeptide chain, resulting from interactions among side chains (R groups).

• Stabilized by hydrogen bonds, ionic bonds, hydrophobic interactions, van der Waals forces, and disulfide bridges.

• Determines the protein's functional conformation.

Quaternary Structure

Some proteins consist of two or more polypeptide subunits, which assemble into a functional complex.

• Quaternary structure describes the arrangement and interaction of these subunits.

• Examples: Hemoglobin (four subunits), collagen (three subunits).

Protein Folding and Stability

Protein folding is often spontaneous, driven by hydrogen bonds and van der Waals interactions. Proper folding is essential for function, and is sometimes assisted by molecular chaperones.

• Denaturation: Loss of three-dimensional structure (and function) due to heat, pH changes, or chemicals.

• Some proteins can renature, while others must be synthesized anew.

Protein Function and Diversity

Major Functions of Proteins

Proteins perform a wide array of functions in living cells, each determined by their unique structure.

Type	Function	Example
Enzymatic	Catalyze biochemical reactions	Digestive enzymes
Defensive	Protection against disease	Antibodies
Storage	Store amino acids	Casein in milk
Transport	Transport substances	Hemoglobin
Hormonal	Coordinate organismal activities	Insulin
Receptor	Response to chemical stimuli	Neurotransmitter receptors
Contractile and Motor	Movement	Actin, myosin
Structural	Support	Keratin, collagen

Relationship Between Structure and Function

• The specific shape of a protein determines its function.

• Even a single change in the amino acid sequence (primary structure) can alter protein function (e.g., sickle cell anemia).

Nucleic Acids: Information Storage and Transfer

Structure of Nucleic Acids

Nucleic acids are polymers of nucleotide monomers, responsible for storing and transmitting genetic information.

• Two main types: Deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA).

• Each nucleotide consists of a phosphate group, a five-carbon sugar (deoxyribose or ribose), and a nitrogenous base.

Nucleotide Polymerization

• Nucleotides are joined by phosphodiester bonds, forming a sugar-phosphate backbone.

• Polymerization is a condensation reaction, releasing water.

• Energy for polymerization comes from nucleoside triphosphates (e.g., ATP).

DNA Structure

• DNA consists of two antiparallel strands forming a double helix.

• Strands are held together by hydrogen bonds between complementary bases: Adenine (A) pairs with Thymine (T), and Guanine (G) pairs with Cytosine (C).

• Strands are complementary and run in opposite directions (5' to 3' and 3' to 5').

RNA Structure and Function

• RNA is usually single-stranded but can form secondary structures (e.g., hairpins) through intramolecular base pairing.

• RNA contains ribose sugar and uses uracil (U) instead of thymine.

• Some RNA molecules have catalytic activity (ribozymes).

Comparison of DNA and RNA

Feature	DNA	RNA
Strands	Double-stranded	Single-stranded (usually)
Sugar	Deoxyribose	Ribose
Bases	A, T, C, G	A, U, C, G
Function	Genetic information storage	Information transfer, catalysis

Key Equations

• Peptide bond formation (condensation reaction):

$\text{Amino acid}_1 + \text{Amino acid}_2 \rightarrow \text{Dipeptide} + H_2O$

• Phosphodiester bond formation (nucleotide polymerization):

$\text{Nucleotide}_1 + \text{Nucleotide}_2 \rightarrow \text{Dinucleotide} + H_2O$

Additional info: Some details about protein folding, denaturation, and nucleic acid catalysis were expanded for academic completeness.