Protein structure, non-enz protein function, lipids

Three-dimensional protein structure

Conformational stability

• Definition: A protein’s capacity to maintain its functional 3D shape.
• Stabilizing forces: Hydrogen bonds, ionic attractions, hydrophobic effects, and van der Waals forces.
• Denaturation: Loss of structure under stress (e.g., heat or extreme pH), causing the protein to unfold and lose biological activity.
• Solvation layer: Water molecules organize around hydrophobic regions, favoring a folded conformation by increasing net entropy when nonpolar residues cluster inside.

Quaternary structure

• Association of subunits: Many proteins achieve functional forms by assembling multiple polypeptide chains. These subunits interact through the same forces that stabilize tertiary structure. Disulfide bonds may also link separate chains.

Non-enzymatic protein function

Non-enzymatic proteins perform essential tasks that do not involve catalyzing chemical reactions. Instead, they rely on binding capabilities, immune defense, and motor functions to fulfill various physiological roles.

Binding

Many proteins serve as specialized transport or receptor molecules:

• Transport Proteins: Hemoglobin binds oxygen with high specificity, delivering it efficiently through the bloodstream.
• Receptors: Located on cell surfaces, receptors recognize hormones or neurotransmitters. This binding triggers signal transduction, leading to specific cellular responses. High-affinity interactions depend on a precise structural complementarity between the receptor site and its ligand.

Immune system

Certain proteins act as antibodies, recognizing and binding antigens—distinct molecular markers on pathogens or foreign substances:

• Neutralization and tagging: By binding these antigens, antibodies either neutralize potential threats or mark them for destruction by other immune cells.
• Specificity: Antibodies contain variable regions that fine-tune their shape to match an antigen, accounting for the vast diversity of immune responses.

Motor

Proteins can also convert chemical energy into mechanical work, vital for movement and transport in living systems:

• Myosin and actin: Myosin interacts with actin filaments to produce muscle contraction and enable cellular movement.
• Kinesin and dynein: These motor proteins move along microtubules, transporting cargo such as vesicles or organelles within cells.

Lipids—description, types

Lipids are a broad class of hydrophobic or amphipathic molecules playing diverse roles in energy storage, membrane structure, and signaling. They often feature long hydrocarbon chains or ring systems that make them insoluble or only partially soluble in water.

Storage

Free fatty acids: saponification

• Definition: Free fatty acids are the hydrolyzed form of fatty acids once cleaved from glycerol.
• Process: Saponification hydrolyzes ester bonds in fats, producing soap (the sodium or potassium salts of the fatty acids) and glycerol.

Structural

Phospholipids and phosphatids

• Major components of cell membranes, composed of two fatty acids attached to a glycerol backbone with a phosphate-containing head group.
• Their amphipathic nature forms bilayers, providing a selective barrier for cells.

Sphingolipids

• Derived from the amino alcohol sphingosine instead of glycerol.
• Found in the membranes of neurons (e.g., myelin sheaths), contributing to signal transduction and cell recognition.

Waxes

• Long-chain fatty acids esterified to long-chain alcohols.
• Highly hydrophobic and typically serve protective or structural functions (e.g., on plant leaves or animal fur).

Signals/cofactors

Steroids

• Structure: Steroids (e.g., cholesterol, testosterone, estrogen) share a multi-ring system derived from the cyclization of terpene precursors.
• Biosynthesis:
  • Begins with squalene, a triterpene containing six isoprene units.
  • The double bonds in squalene allow it to cyclize into the core steroid nucleus.
• Function:
  • Cholesterol modulates membrane fluidity and serves as a precursor for steroid hormones.
  • Testosterone and estrogen act as key signaling molecules in numerous physiological processes.

Prostaglandins

• Derived from arachidonic acid through the cyclooxygenase pathway.
• Function as local signaling molecules (e.g., regulating inflammation and smooth muscle contraction).

Additional notes on terpenes

• Terpenes: Built from isoprene ($C_5{H}_8$) subunits. Their classification depends on the number of isoprene units:
  • Monoterpenes: 2 isoprene units
  • Diterpenes: 4 isoprene units
  • Triterpenes: 6 isoprene units (e.g., squalene)
• Cyclization: Terpenes’ double bonds enable ring formation (e.g., steroid biosynthesis from squalene).