Biochemistry — High-Yield Summary Macromolecules & Metabolism Three calorie-yielding macromolecules: carbohydrate (→ monosaccharides), protein (→ amino acids)…

Biochemistry — High-Yield Summary

Macromolecules & Metabolism

  • Three calorie-yielding macromolecules: carbohydrate (→ monosaccharides), protein (→ amino acids), lipid (→ fatty acids + glycerol). Vitamins, minerals, and water are essential but yield no calories.
  • Catabolism breaks nutrients down to CO2/H2O/NH3, releasing energy as ATP; anabolism spends that ATP to build macromolecules back up from small precursors.
  • Three stages of catabolism: Stage I breaks macromolecules into building-block units; Stage II converges them on pyruvate and acetyl-CoA; Stage III is the TCA cycle.
  • Glucogenic amino acids feed pyruvate or a TCA intermediate (can make new glucose); ketogenic amino acids feed acetyl-CoA/acetoacetyl-CoA (cannot). Leucine and lysine are purely ketogenic; several others are both.

Protein Structure & Function

  • Peptide bond: -COOH of one amino acid + -NH2 of the next → amide bond + H2O released.
  • Four structure levels: primary (sequence) → secondary (α-helix/β-sheet) → tertiary (single-chain 3D fold) → quaternary (multiple folded chains assembled, e.g. hemoglobin).
  • Simple proteins = amino acids only; complex/conjugated proteins add heme (hemoglobin), lipid (lipoprotein), or carbohydrate (glycoprotein).
  • Seven functional classes: regulator (hormones), biocatalyst (enzymes), transport (Hb, albumin, transferrin), contractile (actin/myosin), structural (collagen, keratin), protective (immunoglobulins, interferon), and receptor.

The Cell Membrane

  • Composition by mass: ~49% protein, ~43% lipid (phospholipid/glycolipid/cholesterol), ~8% carbohydrate.
  • Integral proteins span the bilayer (most receptors/channels); peripheral proteins attach loosely to the surface.
  • Three functions: maintains cell shape, protects the cell under mechanical stress, and transports substances (passive: diffusion/osmosis; active: ion pumps, needs ATP).
  • Clinical link: SARS-CoV-2's spike protein (ligand) binds the host membrane enzyme ACE2 (receptor); TMPRSS2 then activates the spike for membrane fusion — a textbook case of receptor–ligand specificity gone wrong.

Plasma Proteins — Albumin & Globulin

  • Normal ranges: albumin 35–50 g/L (~58% of total plasma protein), globulins 65–85 g/L (α1/α2/β/γ fractions), fibrinogen 2–4 g/L.
  • On electrophoresis, albumin runs fastest/farthest (smallest protein); globulin bands stay closer to the origin.
  • Albumin's two jobs: maintains plasma oncotic pressure (prevents edema/ascites/pleural effusion) and transports ligands (bilirubin, fatty acids, drugs like furosemide and methotrexate).
  • Hypoalbuminemia causes: nephrotic syndrome, cirrhosis, kwashiorkor, pre-eclampsia.
  • Gamma globulin = antibodies: IgA (mucosal, dimer), IgD (B-cell receptor), IgE (allergy/parasites), IgG (chronic/late, crosses placenta), IgM (acute/early, pentamer — rises first in new infection, followed by IgG).
  • Protein synthesis = transcription (DNA→mRNA, nucleus) + translation (mRNA→protein, cytoplasm). Antibiotics exploit this: aminoglycosides/tetracycline hit the 30S subunit, chloramphenicol/erythromycin hit the 50S subunit.

Nucleotides

  • A nucleotide = nitrogenous base (purine: A, G; pyrimidine: C, T, U) + five-carbon sugar (ribose/deoxyribose) + phosphate.
  • Base attaches to the sugar's C1' by an N-glycosidic bond; the DNA backbone is built from phosphodiester bonds linking each sugar's C3' to the next sugar's C5' phosphate.
  • A nucleoside is base + sugar only (no phosphate); ATP is the triphosphate nucleotide that also serves as the cell's energy currency.