BIOCHEMISTRY: A TEXTBOOK FOR MEDICAL UNIVERSITIES - E. S. Severin - 2004

CHAPTER 7. CARBOHYDRATE METABOLISM

X. Regulation of Glycolysis and Gluconeogenesis in the Liver

Compared to other Organs, the Liver exhibits the most complex glucose METABOLISM. In addition to a pair of opposing processes (Glycogen Synthesis and Breakdown), the liver can carry out two other oppositely directed processes—Glycolysis and Gluconeogenesis. Most other organs perform only glycolysis. The switching of the liver between glycolysis and gluconeogenesis is mediated by Insulin and Glucagon and is accomplished through:

✵ allosteric Introduction/15.html">Regulation of enzyme Activity;

✵ Covalent Modification of Enzymes via phosphorylation/dephosphorylation;

✵ induction/Repression of the synthesis of Key Enzymes.

Regulatory mechanisms target the enzymes that catalyze the irreversible steps of glycolysis and gluconeogenesis, the combination of which is referred to as "substrate" or "futile" cycles.

A. Regulation of the Rates of Glycolysis and Gluconeogenesis Reactions Constituting Substrate Cycles

"Substrate" cycles—paired combinations of metabolite synthesis and breakdown processes. As mentioned earlier, a combination of glycogen synthesis and breakdown processes, or the irreversible reactions of glycolysis and their corresponding irreversible reactions of gluconeogenesis, can form such a cycle. The term "substrate cycle" denotes the coupling of substrate synthesis and breakdown reactions. The term "futile" reflects the outcome of such a cycle, which results in the wasteful consumption of ATP. Although the existence of futile cycles seems counterintuitive, they nevertheless function. Moreover, these cycles can serve as targets for regulatory influences since the reactions comprising them are catalyzed by different enzymes. Reciprocal changes in The activity of these enzymes prevent the simultaneous occurrence of opposing processes.

The shift in the liver from a glycolytic pathway to gluconeogenesis and vice versa, occurring during the transition from the absorptive to the postabsorptive state or during starvation, is primarily driven by The regulation of enzyme activities that catalyze the reactions of substrate cycles. These cycles are designated by Roman numerals I, II, and III in Fig. 7-54, which illustrates the General Overview of glycolysis and gluconeogenesis regulation in the liver.

Class="center">Fig. 7-54. REGULATION OF GLUCOSE metabolism in the liver. BIF — bifunctional enzyme (fructose-2,6-bisphosphatase/Phosphofructokinase-2); BIF-OH — dephosphorylated enzyme; BIF-P — phosphorylated enzyme; PDC-OH — dephosphorylated Pyruvate dehydrogenase complex; PK-OH — dephosphorylated pyruvate kinase; GAP — glyceraldehyde 3-phosphate; DHAP — dihydroxyacetone phosphate; PEP — phosphoenolpyruvate. I-III — substrate cycles: boxed elements represent regulatory enzymes of glycolysis and gluconeogenesis.

The direction of the reaction in the first substrate cycle is regulated primarily by the concentration of glucose. During Digestion, Blood glucose concentration rises (up to 8–10 mmol/L). Under these conditions, glucokinase activity is at its maximum. Consequently, the glycolytic reaction producing glucose-6-phosphate is accelerated. In addition, insulin induces the synthesis of glucokinase, thereby accelerating glucose phosphorylation. Since hepatic glucokinase is not inhibited by glucose-6-phosphate (unlike Muscle hexokinase), the bulk of glucose-6-phosphate in the absorptive period is channeled toward glycogen synthesis and The Glycolytic Pathway.

The direction of the reactions In the second substrate cycle depends on the activity of phosphofructokinase and fructose-1,6-bisphosphatase. The activity of these enzymes is governed by the concentration of fructose-2,6-bisphosphate. Fructose-2,6-bisphosphate—a metabolite formed in minor amounts from fructose-6-phosphate that performs exclusively regulatory Functions. The Formation of fructose-2,6-bisphosphate via the phosphorylation of fructose-6-phosphate is catalyzed by a bifunctional enzyme (BIF), which also catalyzes the reverse reaction (Fig. 7-55, A). However, The conversion of fructose-2,6-bisphosphate back into fructose-6-phosphate is not a reversible process. The synthesis of fructose-2,6-bisphosphate requires the expenditure of ATP, whereas The formation of fructose-6-phosphate from fructose-2,6-bisphosphate involves the hydrolytic Cleavage of inorganic phosphate.

Fig. 7-55. Reactions Catalyzed by the bifunctional enzyme (BIF) in the liver (A). Regulation of BIF activity (B).

In the phosphorylation reaction of fructose-6-phosphate, the enzyme exhibits kinase activity, whereas in the dephosphorylation of the resulting fructose-2,6-bisphosphate, it displays phosphatase activity. This dual characteristic gave rise to the name "bifunctional enzyme."

The kinase activity of BIF is manifested when the enzyme is in its dephosphorylated form (BIF-OH). The dephosphorylated form of BIF is characteristic of the absorptive period, when the insulin/glucagon ratio is high. During this period, the level of fructose-2,6-bisphosphate increases (Fig. 7-55, B).

At a low insulin-to-glucagon ratio, typical of prolonged fasting, BIF undergoes phosphorylation and functions as a phosphatase. The result is a decrease in the level of fructose-2,6-bisphosphate.

The kinase and phosphatase reactions are catalyzed by distinct active sites of BIF; however, in each of the two enzyme states (phosphorylated and dephosphorylated), one of the active sites is inhibited. The regulatory effect of fructose-2,6-bisphosphate lies in its allosteric activation of phosphofructokinase (a glycolytic enzyme). In this process, fructose-2,6-bisphosphate diminishes the inhibitory effect of ATP on this enzyme during the absorptive period and increases its affinity for fructose-6-phosphate. At the same time, fructose-2,6-bisphosphate inhibits fructose-1,6-bisphosphatase (an enzyme of gluconeogenesis). Thus, during the absorptive period, the level of fructose-2,6-bisphosphate rises, leading to the activation of phosphofructokinase and the acceleration of glycolysis.

The reduction in The amount of fructose-2,6-bisphosphate during the postabsorptive period results in decreased phosphofructokinase activity, slower glycolysis, and a metabolic shift from glycolysis to gluconeogenesis. The regulatory role of fructose-2,6-bisphosphate is illustrated in Fig. 7-56.

Fig. 7-56. Regulation of the second substrate cycle reactions by fructose-2,6-bisphosphate.

In the regulation of the third substrate cycle, the primary role belongs to pyruvate kinase, whose phosphorylated form is inactive, whereas its dephosphorylated form is active (Fig. 7-57).

Fig. 7-57. Regulation of liver pyruvate kinase.

During digestion, insulin activates phosphoprotein phosphatase, which dephosphorylates pyruvate kinase, converting it into its active state. Additionally, insulin affects enzyme levels in the liver by inducing the synthesis of pyruvate kinase and repressing the synthesis of phosphoenolpyruvate carboxykinase. Consequently, the glycolytic reaction phosphoenolpyruvate -> pyruvate is accelerated during digestion. This same reaction slows down in the postabsorptive state under The Influence of glucagon, which, acting indirectly via cAMP-dependent protein kinase, phosphorylates and inactivates pyruvate kinase.

During prolonged fasting, glucagon accelerates gluconeogenesis. This is achieved not only through the phosphorylation of pyruvate kinase and the subsequent reduction in The rate of glycolysis, but also by inducing the synthesis of key gluconeogenic enzymes: phosphoenolpyruvate carboxykinase, fructose-1,6-bisphosphatase, and glucose-6-phosphatase. It is known that glucagon indirectly phosphorylates Transcription factors, thereby altering their activity and inducing the synthesis of these gluconeogenic enzymes. Furthermore, the synthesis of phosphoenolpyruvate carboxykinase during prolonged fasting is induced by cortisol, although this occurs via a different MECHANISM OF ACTION typical of Steroid Hormones (see sections 5, 11).

Coordination of the rates of the second and third substrate cycles is achieved through fructose-1,6-bisphosphate—the product of the second substrate cycle (the glycolytic pathway)—which acts as an allosteric activator of pyruvate kinase. During digestion, as the Initial Stages of glycolysis accelerate, the concentration of fructose-1,6-bisphosphate increases, leading to additional activation of pyruvate kinase.

An overall overview of the regulation of processes constituting Glucose metabolism in the liver is presented in Fig. 7-54.

It should be noted that opposing reactions of each substrate cycle can proceed simultaneously. Accordingly, glycolysis and gluconeogenesis in the liver may also occur simultaneously to some extent, although their relative rates change. For instance, the glycolytic pathway predominates during digestion, whereas The gluconeogenesis pathway prevails in the postabsorptive state. However, the gluconeogenic reaction pyruvate -> oxaloacetate can take place under any physiological conditions. This is necessary to maintain a constant concentration of oxaloacetate, as it is utilized not only in gluconeogenesis but also in other pathways, such as The Citric Acid Cycle, transmembrane transport, and Amino acid synthesis.

B. The Role of hepatic glycolysis in lipid synthesis

The primary significance of accelerated glycolysis in the liver during digestion is The production of dihydroxyacetone phosphate and acetyl-CoA, which serve as precursors for fat synthesis. The Formation of Acetyl-CoA from pyruvate via the PDH-catalyzed reaction is regulated through various mechanisms, as described in detail in section 6.

In the absorptive state, the PDH complex is in its dephosphorylated (active) form, and consequently, pyruvate decarboxylation is accelerated. The resulting acetyl-CoA is primarily utilized in two pathways: fatty acid synthesis and The Citric Acid cycle. During digestion, both the production of acetyl-CoA and its utilization for fatty acid synthesis are upregulated. The α-glycerophosphate required for fat synthesis is produced via the reduction of dihydroxyacetone phosphate (Fig. 7-58). This process is covered in detail in section 8.

Fig. 7-58. Synthesis of fat from CARBOHYDRATES. 1 — glucose oxidation to pyruvate and Oxidative Decarboxylation of pyruvate lead to the formation of acetyl-CoA; 2 — acetyl-CoA serves as the building block for fatty acid synthesis; 3 — Fatty acids and α-glycerol phosphate, produced by the reduction of dihydroxyacetone phosphate, participate in triacylglycerol synthesis.

C. Allosteric Regulation of aerobic Glucose Catabolism and gluconeogenesis in the liver by the cellular energy status

The allosteric Regulation of Glycolysis rates, driven by Changes in the ATP/ADP ratio, serves to modulate the rate of glucose utilization directly within liver Cells. Hepatocytes use glucose not only for glycogen and lipid synthesis but also as an energy source for ATP production. The primary consumers of ATP in hepatocytes include transmembrane transport processes, as well as the synthesis of Proteins, glycogen, Lipids, and glucose. The rate of ATP synthesis depends on its utilization rate in these processes. ATP, ADP, AMP, as well as NAD⁺ and NADH, act as allosteric effectors for several glycolytic and gluconeogenic enzymes. Specifically, AMP activates phosphofructokinase and inhibits fructose-1,6-bisphosphatase. ATP and NADH inhibit pyruvate kinase, whereas ADP inhibits pyruvate carboxylase.

Consequently, when ATP consumption increases and its concentration drops while AMP levels rise, glycolysis and ATP production are stimulated, whereas gluconeogenesis is inhibited. Furthermore, the rates of reactions in the common catabolic pathway depend on the ratios of ATP/ADP, AMP, and NAD⁺/NADH (see section 6).