Lipids and Proteins

Lipids

  • Function: insulation of organs; energy storage; cell membrane components; hormones and hormone precursors; transport of lipid vitamins (A, E, K) expressed as retinol, tocopherol, phylloquinone.

  • Fatty acids

    • Chain length 12−2412-24 carbons

    • Saturated: no double bonds; Monounsaturated: one double bond; Polyunsaturated: two or more

    • Double bonds usually cis (kinked)

  • Glycerides

    • Glycerol backbone with ester linkages to fatty acids (−O−C(=O)−-O-C(=O)-)

    • 1 fatty acid

  • Glycerides E.G. Phospholipids

    • Glycerol at one end; two fatty acids; phosphate group with an organic moiety (e.g. choline)

    • Amphipathic; major cell membrane components

  • Non-glycerides E.G. Steroids

    • Variations at substituent sites; ring system: 1 cyclopentane (D) and 3 cyclohexane rings (A, B, C)

    • Steroids (e.g. Cholesterol)

    • Present in all animal cells; brain and spinal cord

    • Maintains membrane fluidity; precursor for vitamin D and steroid hormones (e.g. progesterone, testosterone)

Proteins

  • Protein functions (examples):

    • Enzymatic: selective acceleration of reactions (e.g. digestive enzymes)

    • Storage: amino acids reserve (casein in milk; ovalbumin in egg white)

    • Transport: carriers like hemoglobin; membrane transporters

    • Receptor proteins: detect chemical signals

    • Hormonal: insulin

    • Defensive: antibodies

    • Structural: keratin, collagen, elastin; silk

  • Amino acids (general)

    • Structure: extH2extN−extCH(extR)−extCOOHext{H}_2 ext{N}- ext{CH}( ext{R})- ext{COOH} with an

      • α\alpha -carbon linked to an H, amino group, carboxyl group, and side chain R

    • 20 amino acids encoded by the genetic code; classified by R group properties

  • Classes of amino acids

    • Non-polar, uncharged: hydrophobic (examples: Ile, Leu, Val, Phe, Met, Pro, Ala, Gly)

    • Polar, uncharged: hydrophilic (examples: Ser, Thr, Asn, Gln, Tyr, Cys)

    • Electrically charged (hydrophilic at pH 7):

    • Basic (positively charged): Lys, Arg, His

    • Acidic (negatively charged): Asp, Glu

  • Protein structure (4 levels):

    • Primary structure

    • Linear amino acid sequence; determined by genetic information

    • A single amino acid change can cause malfunction (e.g. sickle cell: extGlu→extValext{Glu} \rightarrow ext{Val})

    • N-terminus and C-terminus definitions

    • Secondary structure

    • Hydrogen bonds between the backbone amide and carbonyl groups

    • Forms α-helix\alpha\text{-helix} and β-pleated sheet\beta\text{-pleated sheet}; proteins can have both

    • Tertiary structure

    • 3D folding of a single polypeptide; driven by interactions between R groups

    • Example: myoglobin

    • Interactions stabilizing tertiary structure

      • Ionic bonds; hydrogen bonds; disulfide bridges (−S−S−-S-S- between cysteine residues)

      • Hydrophobic interactions; hydrogen bonds between polar side chains

    • Quaternary structure

    • Spatial arrangement of two or more polypeptide subunits; native conformation

    • Example: hemoglobin (four subunits)

  • Fibrous vs Globular proteins

    • Fibrous: structural, insoluble (e.g. keratin, collagen, myosin)

    • Globular: soluble, functional (e.g. antibodies, enzymes like cytochrome oxidase, hormones like insulin, transport proteins like hemoglobin)

  • Protein denaturation

    • Loss of native conformation; loss of biological activity

    • Triggered by pH changes, salt concentration, temperature, etc.

  • Protein renaturation

    • Some proteins can refold (renature) after denaturation

    • Demonstrated when disulfide bonds reform after removal of denaturants (e.g. urea, mercaptoethanol)

  • Protein folding in the cell

    • Chaperonins (chaperones) assist folding; keep new polypeptides from misfolding in the cytoplasm