GLOSSARY

Phosphoenolpyruvate Carboxykinase (PEPCK) converts oxaloacetate to phosphoenolpyruvate during gluconeogenesis.

Gluconeogenesis makes glucose from lactate, glycerol and some amino acids, with major hepatic and context-dependent renal contributions. It costs energy and is not a simple reversal of all glycolysis. Even-chain fatty-acid acetyl units do not normally provide direct net human glucose; glycerol and other routes require separate treatment. Endogenous production does not automatically eliminate exercise fueling needs, particularly at high demand, and precursor availability is only one part of pathway regulation.

Endogenous glucose production includes glycogen breakdown and new synthesis rather than gluconeogenesis alone. Cellular production and release into blood differ because local use occurs. Contributions change with fasting and activity. Tracers and pool definitions matter; a stable blood glucose can coexist with high turnover. It does not mean fuel is unnecessary or glucose use has stopped, nor does one circulating concentration isolate hepatic versus renal contributions to supply.

Allosteric regulation changes activity through communication between regulatory binding and functional regions; it can activate or inhibit. Feedback carries downstream-product or state information upstream, whereas feedforward conveys input or upstream information onward. Both can operate in one network. Regulation coordinates supply and use rather than keeping every enzyme maximally active, so more activation is not automatically a better nutritional or training outcome.

A cofactor is a nonprotein component needed by some enzymes and may be an inorganic ion or an organic molecule. A coenzyme is organic, while a prosthetic group is generally tightly associated. Apoenzyme denotes protein lacking a required cofactor, and holoenzyme the complete active form. Binding strength and role must be specified: not every substance near an enzyme is its essential cofactor.

For example, after training changes an enzyme result, first ask whether it was measured in blood, tissue or purified protein. Then assess assay conditions, nutrition and time since the last session alongside actual performance; this comparison helps distinguish a regulatory response from a nutritional deficiency or a presumed supplement need.

Measured activity is conversion per time under assay conditions rather than simply protein abundance. A substrate-rich assay may reveal potential capacity, while living tissue has different constraints and regulation. Units, tissue normalization and preparation methods are needed for comparison. Blood activity is not identical to intramuscular activity; laboratory changes require exercise timing, symptoms and tissue origin rather than being read as direct performance measures or a standalone diagnosis.

Training can change enzyme abundance, regulation or activity capacity depending on mode, intensity, volume and training history. An acute session response is not the same as chronic adaptation. Oxidative or glycolytic markers describe only part of performance; oxygen delivery, fuel stores and neural coordination also matter. Practical assessment therefore combines performance and recovery outcomes with adequate nutrition and the actual training program rather than relying on one enzyme marker.