Proteins

Overview of Proteins

Proteins are essential macromolecules that perform a vast array of functions in living organisms. They are the most diverse biological molecules, involved in nearly every cellular process.

• Structure: Proteins provide structural support to cells and tissues (e.g., collagen in connective tissue).

• Nutrition: Some proteins serve as nutrient sources (e.g., casein in milk).

• Enzymes: Many proteins act as enzymes, catalyzing biochemical reactions.

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

• Communication: Proteins are involved in cell signaling and communication (e.g., hormones).

• Cellular Defense: Defensive proteins protect against disease (e.g., antibodies).

Examples: Hemoglobin (transport), insulin (regulation), antibodies (defense).

Amino Acids: Building Blocks of Proteins

Proteins are polymers made from amino acids. Each amino acid has a central carbon atom bonded to an amine group, a carboxyl group, a hydrogen atom, and a variable side chain (R group).

• Amine group:

• Carboxyl group:

• R group: Determines the chemical properties of the amino acid (e.g., hydrophobic, hydrophilic, acidic, basic).

The sequence and chemical nature of amino acids allow for the immense diversity of protein structures and functions.

Example: Glycine has a hydrogen as its R group, while glutamic acid has a carboxyl group, making it acidic.

Classification of Amino Acids

Amino acids are classified based on the properties of their R groups:

• Nonpolar (hydrophobic): e.g., leucine, valine

• Polar (hydrophilic): e.g., serine, threonine

• Acidic: e.g., aspartic acid, glutamic acid

• Basic: e.g., lysine, arginine

Additional info: The diversity of R groups allows proteins to fold into complex shapes and perform specific functions.

Protein Structure

The function of a protein is determined by its structure, which is organized into four levels:

• Primary Structure: The unique sequence of amino acids in a polypeptide chain, held together by peptide bonds. (between carboxyl and amine groups)

• Secondary Structure: Local folding of the polypeptide chain into alpha helices and beta sheets, stabilized by hydrogen bonds.

• Tertiary Structure: The overall three-dimensional shape of a single polypeptide, determined by interactions among R groups (hydrophobic interactions, disulfide bridges, ionic bonds).

• Quaternary Structure: Association of two or more polypeptide chains (subunits) to form a functional protein (e.g., hemoglobin).

Example: Hemoglobin has four polypeptide subunits and a heme group containing iron.

Protein Denaturation

Proteins only function properly when they maintain their correct three-dimensional shape. Denaturation occurs when proteins lose their shape due to changes in temperature, pH, or exposure to chemicals, resulting in loss of function.

• Causes: Heat, extreme pH, salts, detergents

• Consequences: Loss of biological activity, potential health effects

Enzymes: Biological Catalysts

Enzymes are proteins that accelerate chemical reactions by lowering the activation energy required. They are highly specific for their substrates.

• Substrate: The molecule upon which an enzyme acts

• Active site: The region of the enzyme where the substrate binds

• Enzyme-substrate complex: Temporary association during the reaction

Equation:

Factors Affecting Enzyme Activity

• Enzyme concentration: Increasing enzyme concentration increases reaction rate until substrate is limiting.

• Substrate concentration: Increasing substrate concentration increases rate until enzymes are saturated.

• Temperature: Higher temperatures increase activity up to an optimum; excessive heat denatures enzymes.

• pH: Each enzyme has an optimal pH; deviations can reduce activity or denature the enzyme.

• Activators: Molecules that increase enzyme activity (e.g., cofactors, coenzymes).

• Inhibitors: Molecules that decrease enzyme activity (competitive and noncompetitive/allosteric inhibitors).

Additional info: Feedback inhibition occurs when the product of a reaction inhibits the enzyme, preventing overproduction.

Nucleic Acids

Structure and Function of Nucleic Acids

Nucleic acids are polymers that store and transmit genetic information. The two main types are DNA and RNA.

• Monomer: Nucleotide

• Components of a nucleotide:

  • Pentose sugar (deoxyribose in DNA, ribose in RNA)

  • Phosphate group

  • Nitrogenous base (A, T, C, G in DNA; A, U, C, G in RNA)

Example: DNA contains the bases adenine (A), thymine (T), cytosine (C), and guanine (G).

Nucleotide Structure

• 5' carbon: Attached to the phosphate group

• 1' carbon: Attached to the nitrogenous base

• 3' carbon: Has a hydroxyl group, forms a bond with the phosphate of the next nucleotide

Phosphodiester bond: linkage between the 3' hydroxyl and 5' phosphate of adjacent nucleotides

DNA Structure

DNA consists of two antiparallel strands forming a double helix. The strands run in opposite directions (5' to 3' and 3' to 5').

• Base pairing: Adenine pairs with thymine (A-T), cytosine pairs with guanine (C-G)

• Hydrogen bonds: Hold complementary bases together (A-T: 2 bonds, C-G: 3 bonds)

• Phosphodiester bonds: Form the sugar-phosphate backbone

Additional info: Purines (A, G) have double rings; pyrimidines (C, T, U) have single rings. In RNA, uracil (U) replaces thymine.

Table: Types of Proteins and Their Functions