Chapter-9 Biomolecules

Chemical Constituents of Living Cells

Living systems are composed of various chemical compounds known as biomolecules. These organic and inorganic molecules interact to maintain life processes, growth, development, and reproduction within a cell.

Biomolecules

Biomolecules are carbon-based organic molecules synthesized by living organisms. They include micromolecules (such as amino acids, sugars, and nucleotides) and macromolecules (such as proteins, polysaccharides, and nucleic acids).

• Micromolecules: Low molecular weight compounds found in the acid-soluble pool.
• Macromolecules: High molecular weight polymers found in the acid-insoluble fraction.

Proteins

Proteins are complex, heteropolymer macromolecules made up of chains of amino acids linked together by peptide bonds. They are essential for almost all cellular activities.

Structure of Proteins

Protein structure is organized into four distinct levels of complexity:

• Primary Structure: The linear sequence of amino acids joined by peptide bonds. Exam-Oriented Note: The first amino acid has a free N-terminus and the last has a free C-terminus.
• Secondary Structure: The folding or coiling of the polypeptide chain into regular patterns such as the alpha-helix or beta-pleated sheet, maintained by hydrogen bonds.
• Tertiary Structure: The overall three-dimensional folding of a single polypeptide chain, stabilized by ionic bonds, hydrogen bonds, disulfide linkages, and hydrophobic interactions. This is necessary for biological activity.
• Quaternary Structure: The arrangement of two or more polypeptide chains (subunits) relative to each other, as seen in hemoglobin.

Functions of Proteins

Proteins perform diverse roles in living cells:

• Enzymatic Catalysis: Accelerating biochemical reactions (e.g., pepsin, amylase).
• Transport: Carrying substances across membranes or in blood (e.g., hemoglobin transports oxygen).
• Structure: Providing support and shape (e.g., collagen, keratin).
• Defense: Protecting against pathogens (e.g., antibodies/immunoglobulins).
• Regulation: Acting as hormones or receptors (e.g., insulin).

Carbohydrates

Carbohydrates are polyhydroxy aldehydes or ketones, or substances that yield such compounds on hydrolysis. They serve as primary energy sources and structural components.

Structure and Classification

Carbohydrates are classified based on their complexity:

• Monosaccharides: Simple sugars that cannot be hydrolyzed further (e.g., glucose, fructose, ribose). General formula is Cn(H2O)n.
• Oligosaccharides: Short chains of monosaccharides linked by glycosidic bonds, yielding 2 to 10 sugar units upon hydrolysis (e.g., sucrose, lactose, maltose).
• Polysaccharides: Long, complex polymers of monosaccharide units (e.g., starch, glycogen, cellulose).

Functions of Carbohydrates

• Energy Storage: Starch in plants and glycogen in animals.
• Structural Support: Cellulose in plant cell walls and chitin in fungal cell walls and arthropod exoskeletons.

Nucleic Acids

Nucleic acids are macromolecules that store and transmit genetic information from one generation to the next. The two main types are DNA (Deoxyribonucleic Acid) and RNA (Ribonucleic Acid).

Structure of Nucleic Acids

Nucleic acids are polymers of nucleotides. Each nucleotide consists of three components:

• A pentose sugar (Ribose in RNA, Deoxyribose in DNA).
• A nitrogenous base (Purines: Adenine, Guanine; Pyrimidines: Cytosine, Thymine in DNA, Uracil in RNA).
• A phosphate group.

A nucleoside consists only of a nitrogenous base attached to a pentose sugar, whereas a nucleotide includes the phosphate group as well.

Functions of Nucleic Acids

• DNA: Stores the genetic blueprint of living organisms and directs protein synthesis.
• RNA: Translates genetic information from DNA into functional proteins through transcription and translation.

Enzymes

Enzymes are biological catalysts that speed up the rate of biochemical reactions without being consumed or permanently altered in the process. Almost all enzymes are proteins (with few exceptions like ribozymes).

Types of Enzymes

• Simple Enzymes: Consist entirely of protein.
• Conjugated Enzymes (Holoenzymes): Consist of a protein part (apoenzyme) and a non-protein part (cofactor).

Cofactors can be further divided into:

• Co-enzymes: Organic compounds loosely bound to the apoenzyme (e.g., vitamins like NAD, FAD).
• Prosthetic Groups: Organic compounds tightly bound to the apoenzyme (e.g., heme group in peroxidase).
• Metal Ions: Inorganic ions that form coordination bonds (e.g., Zn2+ for carboxypeptidase).

Properties of Enzymes

• Highly specific for their substrates.
• Required in small amounts because they are regenerated after each reaction.
• Sensitive to changes in temperature and pH.
• Lower the activation energy of a reaction.

Enzyme Action

Enzymes catalyze reactions through the formation of an enzyme-substrate complex:

1. The substrate binds to the active site of the enzyme, forming an enzyme-substrate complex.
2. The enzyme strains or aligns chemical bonds, converting the substrate into product.
3. The products are released, and the free enzyme is ready to bind with another substrate molecule.

Lock-and-Key Model vs Induced-Fit Model: The lock-and-key model states the active site is rigid, while the induced-fit model suggests the active site alters its shape slightly to fit the substrate perfectly.

Classification and Nomenclature of Enzymes

The International Union of Biochemistry (IUB) classifies enzymes into six functional classes:

Exam-Oriented Note: Enzyme names usually end in the suffix "-ase" (e.g., sucrase, lactase) and indicate the substrate they act upon or the type of reaction they catalyze.