Metabolic Pathways, Isozymes, Fermentation, and Aquaporins

Vocabulary flashcards covering metabolic pathways, enzyme regulation, fermentation, isozymes, and aquaporin transport.

Aquaporins

Membrane transport proteins that selectively allow water molecules across the biological membrane while blocking protons to preserve the transmembrane proton gradient.

Arginine residue (Aquaporin filter residue)

A positively charged amino acid residue located at the entrance of the $\alpha$-helical protein in aquaporins that repels protons to maintain the proton gradient.

Hydrogen bonding donor/acceptor interactions

Specific hydrogen bonding interactions within aquaporin channels that draw $H_2O$ molecules through the membrane.

Fermentation

A pathway that serves as a safety valve to keep glycolysis running and provide emergency ATP production when cellular $O_2$ levels do not keep up with demand.

Purpose of pyruvate fermentation

The conversion of pyruvate into lactate to regenerate $NAD^+$ from $NADH$, providing the steady supply of $NAD^+$ required by glycolysis.

Total cellular nicotinamide dinucleotide concentration

The conserved pool of electron carriers in fermentation where $[NAD^+] + [NADH] = 10\,\text{mM}$.

Pyruvate decarboxylase regulation

The main site for Pyruvate Dehydrogenase (PDH) regulation, where calcium and magnesium act as allosteric activators to shift the complex into its dephosphorylated state.

Cori Cycle

The metabolic process in which lactate generated during fermentation is exported to the blood and carried to the liver for gluconeogenesis.

Isozymes

Different structural forms of the same enzyme that catalyze the exact same chemical reaction but differ in amino acid sequence, physical properties, and kinetic parameters.

Lactate Dehydrogenase (LDH)

An enzyme that catalyzes the conversion of pyruvate to lactate.

Glucokinase

A liver isozyme whose activity increases proportionally as blood glucose spikes after a meal; because it is not inhibited by its product, it allows the liver to continuously trap massive amounts of glucose to safely lower blood sugar levels and store energy.

Reciprocal regulation

A regulatory mechanism operating in the liver that coordinates glycolysis and gluconeogenesis to ensure glucose is broken down when energy is needed and synthesized when supplies are low.

Enzyme and substrate concentration regulation

A regulatory mechanism in metabolic pathways where enzyme or substrate availability controls catalytic activity, such as glucose controlling Hexokinase activity.

Allosteric Regulation

Enzyme regulation where a modulator molecule binds to activate or inhibit activity, such as ATP inactivating PFK-1 at step 3, or F-2,6-BP activating PFK-1 and inhibiting F-1,6-BPhosphate.

Post-Translational Modification

A regulatory mechanism that functions as a molecular turn-on/off switch for enzyme activity, such as the regulation of Pyruvate kinase in the liver.

Hormone Signaling regulation

A high-level regulatory system using signals like Insulin and Glucagon to guide gene expression, post-translational modifications, and allosteric activators or inhibitors.

Thermodynamics vs. Kinetics in metabolic control

The cellular strategy of managing metabolism by separating the direction of a pathway (thermodynamics, determined by $\Delta G$) from the speed at which it occurs (kinetics, controlled by enzymes).