Proteins: Structure, Function, and Chemistry

Overview of Protein Functions

Proteins are essential biological macromolecules that perform a wide variety of functions in living organisms. Their diverse roles are determined by their unique structures and the chemical properties of their amino acid building blocks.

• Transport proteins: Move molecules across cell membranes (e.g., hemoglobin transports oxygen).

• Cellular functions: Most cellular processes depend on proteins.

• Amino acids: The building blocks of proteins; their order determines protein structure and function.

• Enzymes: Proteins that catalyze (speed up) chemical reactions (e.g., DNA polymerase, amylase).

• Antibodies: Proteins involved in immune defense.

• Structural proteins: Provide support and shape (e.g., collagen in connective tissue).

• Contractile proteins: Involved in movement (e.g., actin and myosin in muscles).

• Signaling proteins: Transmit signals within and between cells (e.g., hormones like insulin).

• Storage proteins: Store amino acids or ions (e.g., ferritin stores iron).

• Receptor proteins: Receive and transmit signals (e.g., neurotransmitter receptors).

Protein Structure and Amino Acids

Proteins are polymers made from amino acid monomers. The sequence and chemical properties of amino acids determine the protein's structure and function.

• Amino acid structure: Each amino acid has a central (alpha) carbon, an amino group (–NH2), a carboxyl group (–COOH), a hydrogen atom, and a variable R group (side chain).

• Functional groups: The amino group and carboxyl group are key functional groups in amino acids.

• Ionization: At physiological pH, the amino group is typically protonated (–NH3+) and the carboxyl group is deprotonated (–COO–).

• Alpha carbon: The central carbon to which all groups are attached.

General Structure of an Amino Acid

• Non-ionized form:

• Ionized form (at physiological pH):

Classification of Amino Acids

Amino acids are classified based on the properties of their side chains (R groups):

• Nonpolar (hydrophobic): Side chains are mostly hydrocarbons (e.g., alanine, leucine).

• Polar (hydrophilic): Side chains contain electronegative atoms (e.g., serine, threonine).

• Acidic: Side chains are negatively charged at physiological pH (e.g., aspartic acid).

• Basic: Side chains are positively charged at physiological pH (e.g., lysine).

Additional info: Some amino acids contain sulfur (e.g., cysteine, methionine) or aromatic rings (e.g., phenylalanine, tryptophan).

Protein Polarity and Solubility

• Interior of proteins: Usually hydrophobic (nonpolar side chains).

• Surface of proteins: Usually hydrophilic (polar or charged side chains), allowing interaction with water.

Isomerism in Amino Acids

• Optical isomers (enantiomers): Amino acids (except glycine) exist as L- and D- forms; only L-amino acids are used in proteins.

• Cis-trans isomerism: Occurs in double bonds (not common in standard amino acids, but relevant in peptide bonds and some side chains).

Peptide Bond Formation

Amino acids are linked by peptide bonds, formed through a condensation (dehydration synthesis) reaction:

• Peptide bond: Covalent bond between the carboxyl group of one amino acid and the amino group of another.

• Condensation reaction: Water is released during bond formation.

Peptide bond formation equation:

Peptide bond structure:

Protein Structure: Levels of Organization

Proteins have four levels of structural organization, each contributing to the final shape and function of the molecule.

• Primary structure: Linear sequence of amino acids in a polypeptide chain.

• Secondary structure: Local folding into structures such as alpha helices and beta sheets, stabilized by hydrogen bonds between backbone atoms.

• Tertiary structure: Overall 3D shape of a single polypeptide, determined by interactions among R groups (side chains).

• Quaternary structure: Association of multiple polypeptide subunits to form a functional protein complex.

Summary Table: Levels of Protein Structure

Flexibility and Bond Rotation

• Protein backbones are flexible: Rotation can occur around single bonds, but peptide bonds are rigid (partial double bond character).

• Peptide bonds cannot rotate: This rigidity helps stabilize protein structure.

Forces Stabilizing Protein Structure

• Hydrogen bonds: Between backbone atoms or side chains.

• Hydrophobic interactions: Nonpolar side chains cluster away from water.

• Disulfide bonds: Covalent bonds between cysteine residues (–S–S–).

• Ionic bonds: Between charged side chains.

• Van der Waals forces: Weak attractions between all atoms.

Protein Shape and Function

• Shape determines function: Even a single amino acid change can alter protein function (e.g., sickle cell anemia).

• Examples: Normal vs. sickle cell hemoglobin differ by one amino acid, leading to different shapes and properties.

Protein Folding and Environment

• Folding: Proteins fold into their functional shapes spontaneously or with the help of chaperone proteins.

• Denaturation: Loss of structure (and function) due to changes in temperature, pH, or chemicals.

Summary Table: Types of Amino Acid Side Chains

Examples and Applications

• Hemoglobin: A quaternary protein that transports oxygen in blood; composed of four subunits.

• Enzymes: Catalyze metabolic reactions (e.g., DNA polymerase in DNA replication).

• Antibodies: Defend against pathogens in the immune system.

• Collagen: Provides structural support in connective tissues.

Additional info: Protein misfolding can lead to diseases such as Alzheimer's and prion diseases.