LEHNINGER PRINCIPLES OF BIOCHEMISTRY Fifth Edition David L. Nelson and Michael M. Cox © 2008 W. H. Freeman and Company CHAPTER 15 Principles of Metabolic Regulation

Both the amount and the catalytic activity of an enzyme can be regulated 1.Extracellular signal: hormonal, neuronal, growth factors etc. 2.Transcription: activate or repress the transcription 3.The stability of mRNA 4.The rate of translation 5.The rate of protein degradation 6.Sequester the enzyme and its substrate in different compartments 7.By the concentration of substrate 8.The presence of allosteric effector 9.Covalent modification 10.Binding of regulatory protein

Adenine Nucleotides play special Roles in Metabolic regulation 1.It is important to maintain a constant supply and concentration of ATP: [ATP] drop  reaction rate is decreased. 2.AMP concentration is more sensitive indicator of cell’s energetic state than is [ATP] 3.AMP-activated protein kinase - regulated by [AMP] - A reduced nutrient supply or by increase exercise cause the rise in [AMP] - increase glucose uptake, activates glycolysis and fatty acid oxidation - suppress energy requiring processes such as fatty acid, cholesterol, and protein synthesis 4.NADH and NADPH : change in their mass action ratios have global effects on metabolism 5.Glucose

Coordinated Regulation of Glycolysis and Gluconeogensis

Hexokinase 1.Human have four isozyme, encoded by different genes Isozyme: Different proteins that catalyze the same reaction 2.In myocytes, hexokinase II: high affinity for glucose, inhibited by G-6-P 3.In liver, hexokinase IV (glucokinase) - low affinity for glucose: direct regulation by the level of blood glucose - not inhibited by G-6-P - is subjected to inhibition by reversible binding of a regulatory protein specific to liver - are transcriptionally regulated: [ATP] low or [glucose] high

Phosphofluctokinase-1 1.Higher enzyme activity: [ADP] or [AMP] high Low enzyme activity: [ATP] high 2. Citrate: High [citrate]  increases the inhibitory effect of ATP 3. F-2,6-BP: activates PFK-1

Fructose -1,6-bisphosphatase 1.Inhibited by AMP 2.Inhibited by F-2,6-BP 3.PFK-1 and FBPase-1 are gulated in a coordinated and reciprocal manner - energy high: gluconeogenesis - energy low: glycolysis

F-2,6-BP is potent allosteric regulator of PFK-1 and FBPase-1 1.The special role of liver in maintaining a constant blood glucose level requires additional regulatory mechanism to coordinate glucose consumption and production 2.Glucagon: the liver to produce glucose and to stop consuming it for own need 3.Insulin: the liver to use glucose as fuel and as a precursor for the synthesis and glycogen and triacylglycerol. 4. Hormonal regulation is mediated by F-2,6-BP 5.F-2,6-BP increase the PFK-1’s affinity for its substrate - in absence of F-2,6-BP, at physiological condition PFK-1 is inactive 6. F-2,6-BP reduce the FBPase-1’s affinity for its substrate.

F-2,6-BP is set by the relative rates of its formation and breakdown

1.Xylulose-5-P, a product of PPP, increase glycolysis 2.Activates PP2A  dephosphorylation of PFK-2/FBPase-2  activate PFK-2  increase F-2,6-BP  activate PFK-1  Inhibits FBPase-1  Stimulate glycolysis

Pyruvate kinase

1.Pyruvate carboxylase is activated by acetyl-coA 2.PEP carboxykinase is regulation by its synthesis and breakdown  Glucagon  cAMP  increase transcription of PEP carboxykinase  Insulin: opposite effect

Transcriptional regulation of glycolysis and gluconeogenesis

1.Carbohydrate response element binding protein (ChREBP) 2.Activates pyruvate kinase, fatty acid synthase 3.Cyclic AMP response element binding protein (CREB)  Glucogon  cAMP  PEP carboxykinase

1.Forkhead box other (FOXO1) stimulate the synthesis of gluconeogenic enzme  insulin  PKB activation  phosphorylation of FOXO1  Degradation  inhibition of gluconeogenesis

The PEP carboxykinase promoter region showing the complexity of regulatory input to this gene.

The metabolism of glycogen in animal 1.In vertebrates, glycogen is found primarily in the liver and skeletal muscle 2.The glycogen in muscle provide a quick source of energy for aerobic or anaerobic metabolism 3.Liver glycogen serves as a reservoir of glucose for other tissue when dietary glucose is not available.

Glycogen breakdown is catalyzed by glycogen phosphorylase 1.phosphorylase, debranching enzyme, phosphoglucomutase  glycosidic bond between nonreducing end by inorganic phosphoate 3.Formation of glucose-1 –phosphoate 4.Called phosphorolysis

1.Debranching enzyme: oligo  to  glucan-transferase 2.Phosphorylase reaches a point 4 glucose residues assay from an  branch point 3.branches are transferred 4.The glucosyl residue at C-6 is hydrolyzed.

1.G-1-P to G-6-P 2.In muscle, G-6-P can enter glycolysis 3.In liver, to release glucose to the blood,G-6- phosphatase present in liver and kidney but not in other tissue

1.G-1-P to G-6-P 2.In muscle, G-6-P can enter glycolysis 3.In liver, release glucose to the blood 4.G-6-P enter ER lumen by T1 and then convert into G and Pi 5.G and Pi are transported by T2 and T3, repectively. 6.G is transported by GLUT2 into blood.

Glycogen synthesis 1.Sugar nucleotide (UDP-glucose) are the substrates for the polymerization of monosaccharides 2.Properies of sugar nucleotide for biosynthesis - Their formation is metaboically irreversible - nucleotide moiety has many groups that can undergo noncovalent interaction with enzyme - nucleotidyl group are excellent leaving group - cell can set sugar nucleotide aside in a pool for one purpose.

1.Glycogen branching enzyme: amylo (1->4) to (1->6) transglycosylase 2.Transfer a terminal fragments of 6 or 7 glucose residues to C-6 hydroxyl group 3.The biological effect of branching is to make the glycogen molecule more soluble and to increase the number of non-reducing end for more accessible phosphorylase and synthase.

1.Glycogen synthase can not initiate a new glycogen chanin de novo (at least 4 glucose residues). 2.Glycogenin - the primer on which new chains are assembled - catalyzed their assembly

Glycogen phosphorylase is regulated allosterically and hormonally

1.Ca2+ binds to and activates phosphorylase b kinase  activate phosphorylase b 2.AMP bind to and activate phosphorylase, speeding up the release of G-1-P from glycogen.

Glycogen phosphorylase of liver as a glucose sensor. 1.In liver, when blood glucose levels return to normal, glucose enters hepatocytes and binds to an inhibitory allosteric site on phophorylase a. 2.This binding produces a conformational change that exposes the phosphorylated Ser residues to PP1 3.And then dephosphorylated and inactivated.

Glycogen synthase is regulated by phosphorylation and dephosphorylation

The path from insulin to GSK3 and glycogen synthase

Protein phosphatase I is central to glycogen metabolism 1.PP1 can remove phosphoryl groups from all three enzymes in response to glucagon (liver) and epinephrin (liver and muscle); phosphorylase kinase, glycogen phosphorylase, and glycogen synthase 2.Glycogen-targeting protein bind PPI, glycogen, and each of three enzyme 3.PPI is inactivated when phosphorylated by PKA and is activated by G-6-P

Regulation of carbohydrate metabolism in the liver

Difference in the regulation of carbohydrate metabolism in liver and muscle 1.Myocyte lacks receptors for glucagon and the enzymatic machinery for gluconeogenesis 2.Pyruvate kinase is not phosphorylated by PKA, thus glycolysis is not turned off when [cAMP] is increased. 3.In fact activated PKA phosphorylates phophorylase kinase and increase glycogen breakdown 4.Insulin triggers increase glycogen synthesis in myocyte: activating PPI and inactivating GSK3, GLUT4 targeting to membrane