Coordinated regulation of glycolysis/gluconeogenesis

Regulation of glycolysis Hexokinase Phosphofructokinase-1 Pyruvate kinase

Hexokinase There are four isozymes (I, II, III and IV) of hexokinase encoded by four different genes. Hexokinase I and II are allosterically inhibited by their product, glucose 6-phosphate. Hexokinase IV is not inhibited by G-6-P.

Hexokinase Hexokinase I and II are the predominant forms existing in muscle. Hexokinase IV is the predominant form in liver. Hexokinase I and II will be half-saturated at about 0.1mM, but hexokinase IV will not be half-saturated until 10mM.

Hexokinase Hexokinase has different functions in liver and muscle. Muscle consumes glucose, using it for energy production. Liver maintains blood glucose homeostasis by removing or producing glucose.

Muscle hexokinase Because blood glucose concentration is about 4 to 5 mM, hexokinase in the muscle (which will be half saturated at 0.1mM) is always working at or near its maximal rate.

Liver hexokinase However, liver hexokinase (half-saturated at 10mM) will not ever reach its maximal rate even after meal.

Liver hexokinase Liver hexokinase (glucokinase) is also regulated by a regulatory protein. When glucokinase is bound by it (which is enhanced by F-6-P), it become nuclear-localized and inactive.

Liver hexokinase After meal, glucose enter hepatocytes by GLUT2 transporter and is converted to G-6-P. G-6-P competes with F-6-P for glucokinase, which relieves its inhibition by regulatory protein.

Phosphofructokinase-1 PFK-1 catalyze the committing step of glycolysis. This enzyme is regulated by ATP, AMP, ADP, citrate and fructose 2,6-bisphosphate.

ATP regulate the affinity of PFK-1 towards its substrate F-6-P Not only as a substrate, ATP is also one of the end product of the glycolytic pathway. ATP inhibit PFK-1 by binding to an allosteric site and lowering the affinity of the enzyme for F-6-P.

Other molecules regulate PFK-1 ADP and AMP relieve the inhibition by ATP. Citrate increases the inhibitory effect of ATP. F-2,6-BP is the strongest activator of PFK-1.

Pyruvate kinase Pyruvate kinase has at least three isozymes and one of them is liver-specific. The liver pyruvate kinase is being regulated differently than other tissue type.

Regulation of pyruvate kinase cAMP dependent

Regulation of gluconeogenesis Pyruvate carboxylase FBPase-1

Pyruvate carboxylase Pyruvate carboxylase is being positively regulated by acetyl-CoA. The accumulation of acetyl-CoA signals that cell’s energy demands are met. Acetyl-CoA also indirectly inhibit pyruvate dehydrogenase complex.

How acetyl-CoA regulate PDC Acetyl-CoA indirectly inhibit PDC by stimulating a protein kinase that inactivates the dehydrogenase.

FBPase-1 is inhibited by AMP

F-2,6-BP: a potent regulator

F-2,6-BP reciprocally regulate PFK-1 and FBPase-1

F-2,6-BP activates PFK-1

F-2,6-BP inhibit FBPase-1

The synthesis and breakdown of F-2,6-BP F-2,6-BP is synthesized by PFK-2 and brokedown by FBPase-2, which is a single, bifunctional protein. When it is phosphorylated, it is FBPase-2.

The bifunctional protein PFK-2/FBPase-2

Insulin and glucagon levels affect the balance between PFK-2/FBPase-2

A PP2A activated by xylulose 5-phosphate also activate FBPase-2

Regulation of glycogen metabolism Glycogen phosphorylase Glycogen synthase

Muscle glycogen phosphorylase Muscle glycogen phosphorylase has two forms: the active a form and the less active b form. The active form is phosphorylated.

Muscle glycogen phosphorylase Glucagon and epinephrine stimulate the kinase that phosphorylate phosphorylase b, therefore active the whole glycogen breakdown process.

How glucagon/epinephrine activate phosphorylase b kinase When epinephrine/glucagon is secreted, it started the whole enzyme cascade by activate a GTP-binding protein. Enzyme cascade allows for large amplification of the initial signal.

Muscle glycogen phosphorylase At resting stage, PP1 (phosphorylase a phosphatase) will dephosphorylate phosphorylase a, which will make it returning to the less active form (phosphorylase b).

Liver glycogen phosphorylase The dephosphorylated form (b) of liver glycogen phosphorylase is essentially inactive. Phosphorylation activates it, but when blood glucose is high, glucose will bind to the inhibitory allosteric site, induces a conformational change that will expose its phosphorylated Ser for PP1 to dephosphorylate (inactivate) this enzyme.

Glycogen synthase The activate form of glycogen synthase is not phosphorylated. To inactivate glycogen synthase, it must be phosphorylated by casein kinase II (CKII) first, then glycogen synthase kinase 3 (GSK3) will add phosphoryl groups to three Ser residues near the carboxyl terminus of this protein.

GSK3 inactivate glycogen synthase by phosphorylation

Glycogen synthase The activation of glycogen synthase requires PP1. Glucose 6-phosphate will bind to the allosteric site of glycogen synthase b, making the enzyme a better substrate for PP1.

GSK3 can be inactivated by phosphorylation Insulin triggers activation of a protein kinase B to phosphorylate GSK3 at a Ser residue near the amino terminus, converting that region of the protein to a pseudosubstrate, preventing GSK3 from binding the real substrate (glycogen synthase).

Insulin enhance glycogen synthesis by inhibiting the kinase that inactivate glycogen synthase

Phosphoprotein phosphatase 1 (PP1) PP1 can remove phosphoryl group from phosphorylase kinase, glycogen phosphorylase (inactivation), and glycogen synthase (activation) in response to glucagon/epinephrine. By activating PP1 and inactivating GSK3, insulin stimulates glycogen synthesis.

PP1 binds to glycogen-targeting protein (GM) and also other proteins

Muscle has a different glucose transporter GLUT2 is not present in myocyte. Instead, GLUT4 is present in myocyte and its expression is regulated by insulin.

Insulin regulated the externalization/internalization of GLUT4