Cell Structure and Membrane Transport

Organelle-specific enzyme localization

Enzymes are not spread evenly through the cell. Each is usually confined to the compartment where its substrates, cofactors and membrane environment are available, so knowing the enzyme map of each organelle is a core topic in cell biology.

In hepatocytes, drug detoxification takes place mainly in the smooth endoplasmic reticulum (SER). Cytochrome P450 monooxygenases are anchored in the SER membrane, and they oxidize lipophilic drugs into more polar products that can then be conjugated and excreted. The rough ER, by contrast, is studded with ribosomes and specializes in the synthesis of secretory and membrane proteins, while the Golgi apparatus sorts, modifies and packages proteins and lipids. The nucleolus is the site of rRNA synthesis and ribosome subunit assembly.

Mitochondrial enzymes are also distributed by sub-compartment.

• Outer membrane enzymes — anchored to the outer mitochondrial membrane. Monoamine oxidase is the classic example, oxidizing monoamines such as serotonin and dopamine.
• Matrix enzymes — soluble, and they include the TCA cycle enzymes citrate synthase and fumarase.
• Inner membrane enzymes — integral proteins, such as succinate dehydrogenase, which is Complex II of the electron transport chain.

A common exam trap is to assign an enzyme to a compartment because it belongs to the same pathway, while forgetting that the pathway may be split across several organelles.

Nernst equilibrium potential

The Nernst equilibrium potential is the membrane potential at which the electrical force exactly balances the chemical (concentration) force acting on a single ion. For an ion of valence z it is written as E_{\text{ion}} = \dfrac{RT}{zF}\ln\dfrac{[\text{X}]_o}{[\text{X}]_i}, where R is the gas constant, T the absolute temperature in kelvin, F the Faraday constant, and [\text{X}]_o and [\text{X}]_i the extracellular and intracellular concentrations. Temperature must be converted to kelvin by adding 273.15 to the Celsius value.

The equation applies only when the membrane is permeable to that one ion. For a monovalent cation, z = +1, so the sign of E_{\text{ion}} is positive when the extracellular concentration is higher. For example, at 37 ºC, RT/F \approx 0.0267 V, so a fivefold higher extracellular Na^+ concentration gives E_{\text{Na}} \approx 0.0267 \times \ln 5 \approx +43 mV.

A concentration difference is not the same thing as a membrane potential. A potential appears only when there is a permeant pathway that allows net ion movement. Phospholipid vesicles prepared in a KCl solution keep the high internal K^+ concentration when they are diluted, so a K^+ gradient exists across the membrane. Without a K^+ channel or ionophore such as valinomycin, however, a pure lipid bilayer is essentially impermeable to K^+, so no ion flux occurs and no membrane potential develops.

Membrane ion channel antibiotics

Some antibiotics kill bacteria by damaging the membrane itself rather than by inhibiting an enzyme. Gramicidin A is a linear pentadecapeptide with alternating D- and L-amino acids. Its alternating configuration lets two monomers meet head to head across the bilayer and form a beta-helical transmembrane channel. This channel is selective for monovalent cations, and it collapses the ion gradients that the cell needs.

The other common antibiotic classes act on different targets, which is a frequent source of confusion.

• Beta-lactams such as ampicillin block the cross-linking of peptidoglycan by binding penicillin-binding proteins. They do not form ion channels.
• Aminoglycosides such as gentamicin bind the 30S ribosomal subunit and cause mistranslation.
• Rifamycins such as rifampicin inhibit bacterial RNA polymerase by binding its beta subunit.

Membrane lipid charge classes

Membrane lipids are classified by the net charge of their head groups at physiological pH.