Proteins: Structure and Functions

Protein Structure

• Proteins are large macromolecules made of carbon, hydrogen, oxygen, and nitrogen, and sometimes sulfur.
• Adult human bodies contain 12-18% protein by weight.
• Proteins have four levels of structural organization.

Protein Functions

• Proteins form the structural framework of tissues like bone and connective tissue.
• They act as enzymes, catalyzing biochemical reactions (e.g., salivary amylase, sucrase, and ATPase).
• Proteins serve as hormones and neurotransmitters. Some examples are the hormone insulin and the neurotransmitter substance P.
• Proteins provide support and protection aiding in the body's response to foreign substances and pathogens such as antibodies.
• Some proteins, such as myosin and actin, are involved in muscle contraction.
• Proteins act as carriers carrying vital substances such as hemoglobin which carries oxygen.

Amino Acids

• Amino acids are the monomers (building blocks) of proteins.
• There are 20 common amino acids.
• The liver synthesizes 10 of these.
• 10 amino acids are considered essential as they must be consumed through diet.

Amino Acid Structure

• Each amino acid has an amino group (NH2), a carboxyl group (COOH), a hydrogen atom (H), and a variable side chain (R group) attached to a central carbon atom.

Levels of Protein Structure

• Primary Structure: The linear sequence of amino acids connected by peptide bonds.
• Secondary Structure: The regular sub-structures (e.g., α-helices and β-sheets) of the polypeptide chain stabilized by hydrogen bonds.
• Tertiary Structure: The three-dimensional shape of the polypeptide chain resulting from the folding of the secondary structure.
• Quaternary Structure: The arrangement of multiple polypeptide chains to form a functional protein.

Polypeptide Chains

• Dipeptide: Two amino acids
• Tripeptide: Three amino acids
• Peptide: 4-9 amino acids
• Polypeptide: 10 or more amino acids

Classification of Amino Acids

• Amino acids are classified based on their interaction with water.
• Neutral nonpolar: Hydrophobic (interacts poorly with water) – No charge
• Neutral polar: Hydrophilic (interacts with water) – No charge
• Acidic: Charged negatively
• Basic: Charged positively

Differences in the Function of Proteins

• Various protein shapes result in different functions.
• The kinds and number of amino acids in the chain dictate the function.

Protein Metabolism

• Metabolism is the sum of all chemical processes.
• Catabolism breaks down complex substances into simpler building blocks.
• Anabolism is the building of complex substances from simpler ones. Digestion is an example of catabolism breaking down proteins in food into constituent amino acids which then are used in anabolism (building up) the body's protein structure.