Biological Macromolecules: Structure and Classification of Carbohydrates and Proteins
Polymers are complex structures composed of repeating molecular units called a monomer.
Carbohydrates: Structure and Function
Functions of Carbohydrates
- Energy: Delivered or stored energy.
- Structural: Form structures (e.g., insect shells).
Classification by Monomer Count
- Monosaccharides: Single monomer unit.
- Disaccharides: Two monomer units.
- Trisaccharides: Three monomer units.
- Oligosaccharides: From 4 to 10 units.
- Polysaccharides: 11 to hundreds of thousands of units.
Classification by Monomer Carbon Atoms
- Trioses: 3 carbon atoms.
- Tetroses: 4 carbon atoms.
- Pentoses: 5 carbon atoms.
- Hexoses: 6 carbon atoms.
Glucose Cyclization
The Glucose Cyclic structure is formed when oxygen, which is negatively charged and nucleophilic, joins the carbon atom at the end of the chain, which is positively charged and electrophilic.
Alpha and Beta Glycosidic Linkages
The hydroxyl (OH) group at the end of the molecule can be located downward or upward relative to the ring plane, generating two different structures: the Alpha ($\alpha$) and Beta ($\beta$) anomers.
- Alpha ($\alpha$) Linkage: Results in a weaker bond. This structure typically corresponds to energy-storage carbohydrates (e.g., starch) because the binding energy is weaker, allowing for easier separation.
- Beta ($\beta$) Linkage: Results in a firmer structure (not easily separated). This corresponds to structural carbohydrates (e.g., cellulose).
Key Polysaccharides in Biology
- Starch: The most widespread energy reserve in plants. Starch is a mixture of two polysaccharides: amylose and amylopectin.
- Glycogen: The primary reserve polysaccharide of animal tissue. It is found in almost all cells, but its concentration is particularly high in hepatocytes (liver cells) and muscle cells.
Proteins: Structure, Function, and Classification
Two amino acids are joined by removing a water molecule, forming an amide bond called a peptide bond. When the molecular weight of the resulting polymer is less than 10,000 units, it is referred to as a polypeptide; if it exceeds that figure, it is generally called a protein.
Amino Acids: The Monomers of Protein
The monomers that constitute proteins are amino acids.
Essential Functions of Proteins
- Structural: Provides support (e.g., histones in DNA, elastin in the skin).
- Enzymatic/Catalytic: Acts as biological catalysts (enzymes).
- Transport: Carries substances (e.g., hemoglobin transports oxygen in the blood).
- Hormonal: Acts as chemical messengers (hormones).
- Defensive: Protects the body (e.g., antibodies).
- Contractile/Motor: Facilitates movement (e.g., actin and myosin).
- Homeostatic: Involved in maintaining osmotic balance.
- Storage: Functions as a reserve.
Classification of Proteins
Proteins are classified as simple or conjugated, depending on their constitution:
- Simple Proteins: Upon hydrolysis, they produce only alpha ($\alpha$)-amino acids.
- Conjugated Proteins: Upon hydrolysis, they produce amino acids along with other non-protein substances (prosthetic groups) such as monosaccharides, nucleic acids, or lipids.
Rotation in the Polypeptide Chain
Rotation that allows the polypeptide chain to form a spiral structure occurs around the alpha ($\alpha$) carbon atom. The specific side chain (R group) attached to this carbon determines the properties of the amino acid.
Properties of the Peptide Bond
The peptide bond exhibits several key properties:
- It has partial double bond character due to resonance stabilization.
- It has no freedom of rotation.
- It is shorter than typical C-N single bonds.
- The C=O and N-H groups are polar and participate actively in the formation of hydrogen bonds, which stabilize secondary structures.
The Zwitterion State
The polarity of amino acids is crucial for molecular interactions. A Zwitterion is a neutral molecule that carries both a positive and a negative electrical charge at different locations within the molecule (e.g., the amino group is protonated, and the carboxyl group is deprotonated).
- In acidic medium, groups are protonated (net positive charge).
- In basic medium, groups are deprotonated (net negative charge).