Essential Biomolecules: Protein Structure, Enzyme Action, and Nucleic Acids

Biological Macromolecules: Proteins, Enzymes, and Nucleic Acids

Proteins: Structure and Function

Proteins are essential biological macromolecules (biomolecules) composed primarily of Carbon (C), Hydrogen (H), Oxygen (O), and Nitrogen (N). They are polymers formed by the linkage of subunits called amino acids (AAs).

Amino Acid Composition

Each amino acid consists of a central carbon atom bound to four groups:

• A carboxyl group (COOH)
• An amino group (NH₂)
• A hydrogen atom (H)
• A variable side chain or radical (R group)

There are 20 distinct amino acids that form proteins. These subunits are linked together by a peptide bond, which forms through a dehydration reaction between the amino group of one AA and the carboxyl group of another, releasing a molecule of water (H₂O).

Levels of Protein Structure

The three-dimensional structure of a protein determines its function. This structure is achieved as the polypeptide chain folds, a process determined by the sequence of amino acids. There are four levels of complexity:

Primary Structure (1°)

This is the unique linear sequence of amino acids in the polypeptide chain. This sequence dictates all subsequent structural levels.

Secondary Structure (2°)

The polypeptide chain folds into specific, repeating patterns stabilized by hydrogen bonds along the backbone. Two common types exist:

• Alpha-helix (α-helix): The chain folds in a helical pattern (e.g., hair keratin).
• Beta-pleated sheet (β-sheet): The chain folds in a zig-zag pattern.

Tertiary Structure (3°)

The secondary structures fold back upon themselves, resulting in complex three-dimensional shapes (fibrillar or globular structures). This structure is stabilized by various bonds, including hydrogen bonds between amino acid R groups.

Quaternary Structure (4°)

This level involves the association of two or more tertiary polypeptide subunits to form a large, functional molecule (e.g., hemoglobin).

Protein Denaturation

Extreme changes in the environment, such as an increase in temperature or a change in pH, lead to the loss of the protein's complex structure. This process, known as denaturation, results in the loss of the protein's function and properties.

Key Functions of Proteins

• Structural: Keratin
• Transport/Conveyor: Hemoglobin
• Regulatory: Insulin
• Contractile: Actin
• Defense: Antibodies
• Enzymatic: Enzymes

Enzymes: Biological Catalysts

Enzymes are specialized proteins that act as biological catalysts. They function by significantly increasing the rate of metabolic reactions without being consumed in the process.

Enzymes temporarily bind to the reactant molecules, called substrates, forming an Enzyme-Substrate (E-S) complex, thereby catalyzing their transformation into products.

Enzyme Action and Characteristics

• Specificity: Enzymes are highly specific, typically catalyzing only one particular chemical reaction involving a specific substrate.
• Efficiency: A single enzyme molecule can catalyze the transformation of many substrate molecules per minute, acting effectively in small quantities.
• Nomenclature: Enzymes are often named by adding the suffix -ase to the name of the substrate they act upon (e.g., the enzyme that acts on maltose is maltase).

Nucleic Acids: DNA and RNA

Nucleic acids are biomolecules formed by Carbon (C), Hydrogen (H), Oxygen (O), Phosphorus (P), and Nitrogen (N). They are polymers composed of repeating units called nucleotides.

Nucleotide Composition

A nucleotide is formed by the union of three components:

1. A pentose sugar (five-carbon sugar)
2. A phosphate group (derived from phosphoric acid)
3. A nitrogenous base

The combination of the pentose sugar and the nitrogenous base is called a nucleoside.

Nitrogenous Bases

The bases are categorized into two types:

• Pyrimidines: Cytosine (C), Thymine (T, found in DNA), and Uracil (U, found in RNA).
• Purines: Adenine (A) and Guanine (G).

There are two main types of nucleic acids, distinguished by their pentose sugar and specific bases:

• DNA (Deoxyribonucleic Acid): Contains deoxyribose sugar and the bases A, C, T, G.
• RNA (Ribonucleic Acid): Contains ribose sugar and the bases C, A, U, G.

DNA Structure and Role

DNA typically exists as a double helix structure, consisting of two chains of nucleotides coiled helically. These strands are antiparallel, and the nitrogenous bases are always directed toward the interior of the molecule. The structure is stable due to hydrogen bonds formed between complementary base pairs.

The primary function of DNA is to act as the carrier of hereditary information, which the cell uses to produce all necessary proteins.

RNA Structure and Types

RNA is typically formed by a single chain of nucleotides. The three main types of RNA are:

• Messenger RNA (mRNA)
• Transfer RNA (tRNA)
• Ribosomal RNA (rRNA)