A Lesson in Metabolic Regulation Inspired by the Glucokinase Glucose Sensor Paradigm

Abstract

Special features of glucose metabolism in pancreatic β-cells are central to an understanding of the physiological role of these cells in glucose homeostasis. Several of these characteristics are emphasized: a high-capacity system for glucose transport; glucose phosphorylation by the high-K_m glucokinase (GK), which is rate-limiting for glucose metabolism and determines physiologically the glucose dependency curves of many processes in β-cell intermediary and energy metabolism and of insulin release and is therefore viewed as glucose sensor; remarkably low activity of lactate dehydrogenase and the presence of effective hydrogen shuttles to allow virtually quantitative oxidation of glycolytic NADH; the near absence of glycogen and fatty acid synthesis and of gluconeogenesis, such that intermediary metabolism is primarily catabolic; a crucial role of mitochondrial processes, including the citric acid cycle, electron transport, and oxidative phosphorylation with F_oF₁ ATPase governing the glucose-dependent increase of the ATP mass-action ratio; a Ca²⁺-independent glucose-induced respiratory burst and increased ATP production in β-cells as striking manifestations of crucial mitochondrial reactions; control of the membrane potential by the mass-action ratio of ATP and voltage-dependent Ca²⁺ influx as signal for insulin release; accumulation of malonyl-CoA, acyl-CoA, and diacylglycerol as essential or auxiliary metabolic coupling factors; and amplification of the adenine nucleotide, lipid-related, and Ca²⁺ signals to recruit many auxiliary processes to maximize insulin biosynthesis and release. The biochemical design also suggests certain candidate diabetes genes related to fuel metabolism: low-activity and low-stability GK mutants that explain in part the maturity-onset diabetes of the young (MODY) phenotype in humans and mitochondrial DNA mutations of F_oF₁ ATPase components thought to cause late-onset diabetes in BHE_cdb rats. These two examples are chosen to illustrate that metabolic reactions with high control strength participating in β-cell energy metabolism and generating coupling factors and intracellular signals are steps with great susceptibility to genetic, environmental, and pharmacological influences. Glucose metabolism of β-cells also controls, in addition to insulin secretion and insulin biosynthesis, an adaptive response to excessive fuel loads and may increase the β-cell mass by hypertrophy, hyperplasia, and neogenesis. It is probable that this adaptive response is compromised in diabetes because of the GK or ATPase mutants that are highlighted here. A comprehensive knowledge of β-cell intermediary and energy metabolism is therefore the foundation for understanding the role of these cells in fuel homeostasis and in the pathogenesis of the most prevalent metabolic disease, diabetes.