Regulation of Metabolism Pratt and Cornely Chapter 19

Regulation by Compartmentalization Form of reciprocal regulation Degradation vs biosynthesis Requires transporters

Specialization of organs

Fuel Storage Fuel Usage: About 7000 kJ/day minimum Storage: About 700,000 kJ Fats and muscle protein: 1-3 months Glucose: 7000 kJ (1 day) Glucose is essential for brain

Liver: Tissue Specific Functions Gluconeogenesis Ketogenesis Urea production Lactate recycling Alanine recycling

Liver in the Fed State Glucose uptake Glycogen synthesis Convert excess sugar, amino acids to fatty acid Make, transport TAG

Liver in the Fast State Glycogen breakdown Maintain blood sugar level Catabolize glucogenic amino acids to maintain glucose and citric acid cycle Catabolize fats and ketogenic amino acids for ketone body

Muscle Glucose trapped as glycogen (no blood sugar regulation) Source of energy in starvation

Muscle: Active State Immediate ATP/creatine Anaerobic muscle glycogen Aerobic liver glycogen Adipose fatty acids

Adipose Fed state: uptake of fats AND glucose (why?) Fast state: release of fats by hormone sensitive lipase (HSL)

Kidney Elimination of waste Maintenance of pH With liver, carries out gluconeogenesis

Cori Cycle

Alanine-Glucose Cycle

Metabolic Issues Starvation Alcoholism Metabolic Syndrome Obesity Diabetes

Starvation Early starvation: convert protein to glucose (cannot convert fat to glucose) Later starvation Preserve muscle Muscle uses fat as fuel; buildup of acetyl CoA shuts down pyruvate acetyl CoA Low [OAA] means acetyl CoA buildup Ketone bodies produced Brain uses KB, glucose is conserved

Metabolism of Ethanol Liver damage Too much NADH and acetyl CoA Shuts down citric acid cycle Fatty acid synthesis upregulated “fatty liver” Ketone bodies form acidosis

Obesity Hereditary, age, and environmental Set-point Leptin Brown fat Appetite suppressant Made in adipose Brown fat

Diabetes Type 1 (Juvenile onset) Type 2 Body feels like a fast Insulin dependent Type 2 Insulin resistance Body feels like a fast Gluconeogenesis increase Lower fat storage Increase in fat utilization ketogenesis

Hyperglycemia Non-enzymatic glycosylation Sorbitol production leads to tissue damage Drugs aimed at undoing metabolic problems Metformin Activates AMPK Suppress gluconeogenesis Activates glucose and fatty acid uptake in muscle

Review of Chemical Regulation Local vs hormone-level regulation Signal transduction pathways Allosteric effectors Covalent modification Product inhibition, feedback inhibition, feed forward activation Energy charge Reciprocal Regulation Isozymes Logic of regulation Know all purposes of pathway Know differences in tissue physiology

Hormone Regulation: Insulin Small protein hormome Released at high [glucose] Pancreatic b cells Release probably triggered by glucose metabolism, not cell surface glucose receptor May be mitochondrial difference, explaining why diabetes changes with age May be difference between hexokinase and glucokinase isozyme in pancreas

Hexokinase Most tissues except pancreas and liver First irreversible reaction Linked to glucose uptake Locks glucose in cell Many isozymes Most inhibited by glucose-6-phosphate Product inhibition

Glucokinase Isozyme in liver and pancreas Higher Km Hexokinase always saturated, but glucokinase sensitive to [glucose] Not inhibited by glucose-6-P Why? Liver serves to modulate blood sugar

Isozyme kinetics Looks allosteric, but this is monomeric enzyme May be due to conformational change upon product release—stays in active state at high concentration of glucose

Insulin Signal Transduction

Glucagon and Epinephrine Glucagon released with low blood sugar (pancreas a cells) Epinephrine released by adrenal glands Oppose insulin Activates glycogen breakdown Activates gluconeogenesis Activates hormone sensitive lipase

Hormone Summary “Insulin signals fuel abundance. It decreases the metabolism of stored fuel while promoting fuel storage.” “Glucagon stimulates the liver to generate glucose by glycogenolysis and gluconeogenesis, and it stimulates lipolysis in adipose tissue.”

Some Major Points of Regulation Entry of glucose into cell Glycolysis/gluconeogenesis Fatty acid synthesis/breakdown Glycogen synthesis/breakdown Urea:

Glucose Entry into Cells Tissues have unique function Isozymes of glucose transporter, GLUT Insulin dependent in muscle Higher [glucose] required for liver uptake

Glycolysis/Gluconeogenesis Role of citrate in multiple pathways Regulation by energy charge (ATP, AMP ratio) [ATP] does not change much AMP-dependent protein Kinase (AMPK) acts as energy sensor High [AMP] activates kinase to switch off anabolism and switch on catabolism Boosts production of F-2,6-bP

Hormone Regulation of Glycolysis/Gluconeogenesis and AMPK activates phosphoprotein phosphatase

Glycogen Metabolism

Glycogen Phosphorylase Dimeric Allosteric control Hormone level control Tissue isozymes Muscle: Purpose it to release fuel for itself Liver: Purpose is to release fuel for whole organism

Covalent Modification Phosphorylase a Phosphorylated “usually active” Default liver isozyme Phosphorylase b Dephosphorylated “usually inactive” Default muscle enzyme

Liver Activity Physiological purpose: release of glucose Default setting High glucose concentration favors T state in Phosphorylase a Turns off active glycogen degradation

Muscle Activity Physiological purpose: conserve glycogen until a burst is needed Detection of energy charge AMP shifts equilibrium to relaxed state

Glucagon/Epinephrine Regulation through Phosphorylase Kinase Activation of cascade leads to active degradation of glycogen Epinephrine affects liver through IP3 pathway

Regulating regulators Influx of calcium in active muscle partially activates kinase Hormone response fully activates

Reciprocal Regulation Glucagon

Protein Phosphatase 1 Opposite of PKA Deactivates phosphorylase Activates glycogen synthase Active in cell unless epinephrine signals PKA PKA activates its inhibitors

Insulin stimulates glycogen synthesis Insulin blocks the “turn off” switch for glycogen synthase Allows PP1 to “turn on” glycogen synthase

Fatty Acid Regulation Carnitine Transporter Acetyl CoA carboxylase Matrix malonyl CoA Error in this picture Actually produced by acetyl CoA carboxylase isozyme in matrix Acetyl CoA carboxylase Local AMP level Citrate and Fatty Acids Hormones

AMP level AMP-activated protein kinase Fuel sensor Inactivates acetyl CoA carboxylase under low energy conditions in cell

Citrate and Fatty Acids [Citrate] high in well fed state Lots of OAA and acetyl CoA Carboxylase forms active filaments If [fatty acids] is high, no need to synthesize Fatty acids break down filaments

Hormone-level control Glucagon and epinephrine Suppress acetyl CoA carboxylase by keeping it phosphorylated Insulin—activates storage Leads to dephosphorylation of carboxylase

Review Principles of Metabolism Central molecules Enzyme classes Relate to reactions Enzyme classes Cofactors Basic reactions Redox Decarboxylation energetics Reaction motifs

Central Molecules

Enzyme classes Problem 6.14. Propose a name for the enzyme, and indicate metabolic purpose of reaciton.

Cofactors

Problem 12.26-27 Identify the metabolic pathway. Indicate which redox cofactor is necessary.

Problem 33: Identify the necessary cofactors

Reaction Motifs