Glucose metabolism

• Processes– Glycolysis– Glycogenolysis– Gluconeogenesis

• Substrate level regulation

• Hormone level regulation

Carbohydrate metabolism

• Glycolysis– Breakdown of glucose to pyruvate– Provides substrate for TCA cycle

• Gluco-/glyco-neogenesis– Synthesis of glucose or glycogen– Storage of excess substrate

• Regulatory mechanisms– Allosteric– Phosphorylation

Glycolysis

• Convert Glucose to Pyruvate– Yield 2 ATP + 2 NADH per glucose– Consume 2 ATP to form 2x glyceraldehyde

phosphate– Produce 2 ATP + 1 NADH per GAP

• Carefully controlled– 12 different enzyme-catalyzed steps– Limited by phosphofructokinase– Limited by substrate availability

Glycolysis/Gluconeogenesis

-D-Glucose-1P

-D-Glucose-6P

-D-Fructose-6P

-D-Fructose-1,6,P2

Glyceraldehyde-3P

Glycerate-1,3P2

Glycerate-3P

Phosphoenolpyruvate

Pyruvate

phosphoglucomutase

glucose-6-phosphate isomerase

6-phosphofructokinase

fructose-bisphosphate aldolase

fructose-1,6-bisphosphatase

Glycerate-2P

GAPDH

phosphoglycerate kinase

phosphoglycerate mutase

enolase

pyruvate kinase

Hexose import

Starch/glycogen breakdown

Except for these steps, glycolysis happily runs backward. Backwards glycolysis is gluconeogenesis

Glycolysis: phosphorylation

• ATP consuming– Glucose phosphorylation by hexokinase– Fructose phosphorylation by

phosphofructokinase

• Triose phosphate isomerase

Glycolysis: oxidation

• Pyruvate kinase– Transfer Pi to ADP– Driven by oxidative

potential of 2’ O

• Summary– Start C6H12O6

– End 2xC3H3O3

– Added 0xO– Lost 6xH– Gained 2xNADH, 2xATP

NADHATP

pyruvate kinase

GAPDH

phosphoglycerate kinase

Pyruvate

• Lactic Acid– Regenerates NAD+– Redox neutral

• Ethanol– Regenerates NAD+– Redox neutral

• Acetyl-CoA– Pyruvate import to mitocondria– ~15 more ATP per pyruvate

pyruvate2-Hydroxyethyl-

Thiamine diphosphate

S-acetyldihydro-lipoyllysine Acetyl-CoA

Carbohydrate Transport

• H+, pyruvate cotransporter

Halestrap & Price 1999

Major Facilitator SuperfamilyMonocarboxylate transporter

Competition between H+ driven transport to mitochondria and NADH/H+ driven conversion to lactate

Cytoplasmic NADH is also used to generate mitochondrial FADH2, coupling transport to ETC saturation “glycerol-3P shuttle”

Gluconeogenesis

• Regenerate glucose from metabolites– Mostly liver– Many glycolytic enzymes are reversible

• Special enzymes– Pyruvate carboxylase

• Generate 4-C oxaloacetate from 3-C pyruvate

– Phosphoenyl pyruvate carboxykinase• Swap carboxyl group for phosphate• Generates 3-C phosphoenolpyruvate from OA

– Fructose-1,6-bisphosphatase• Generates fructose-6-phosphate

Mitochondrial

Glycogen

• Glucose polysaccharide– Intracellular carbohydrate store– Easily converted to glucose

• Glycogenolysis– Phosphorylase generates glucose-1-P

from glycogen

• Glycogenesis– Glycogen synthase adds UDP-glucose-1-P

to glycogen

Substrate control of CHO metabolism

• Kinetic flux balance

• Competition for energy-related molecules– Oxaloacetate: endpoint of TCA– Pyruvate

• Allosteric regulation by energy-related molecules– ATP/AMP: PFK/PFP– F-1,6-BP: pyruvate kinase– Fatty acids

Substrate competition

• Oxaloacetate– Oxa + AcCoA citrate– Oxa + GTP GDP + PEP

• Acetyl-CoA– Oxa + AcCoA citrate– AcCoA + HCO3 MalonylCoA fatty acids– Amino acid synthesis

Oxaloacetate Citrate

=

Phosphoenylpyruvate

Adenine nucleotides balance glucose breakdown

• PFK activity depends on ATP/AMP– Competitive binding to regulatory domain

• PFP activity depends on AMP/citrate

ATPAMPPFK PFP

Glycolysis

PFK Glycolysis ATP

AMP

PFP Glycolysis AMP

Pyruvate kinase

• Substrate cooperativity

• Fructose 1,6-bisphosphate

Mansour & Ahlfors, 1968

+cAMP

Hormonal control of CHO metabolism

• Liver/periphery (liver/muscle)– Glucagon – glucose release– Insulin – glucose uptake

• System wide response– Distribution of receptors– Tissue specialization

• Effector systems– Glucose uptake– PFK/PFP balance

Systemic Regulation of Blood Sugar

• Pancreas– -cells:GlucoseATP--|KATP--|

depolarizationCainsulin+GABA release– -cells:GABACl- --|glucagon

• Peripheral tissues– Insulin IRPI3KGLUT4 translocation glucose uptake– PI3KPKB--|GSK--|GS

• Liver– GlucagonGRGsACPKA--|GS

GlucagonGlycogenolysis(Liver)

Blood glucose

InsulinGlucose uptake, glycogenesis (muscle)

Glucagon

• Endocrine factor, Gs coupled receptor

• PLC, AC enhance glycogenolysis– Rapid secretion of glucose from liver

Jiang, G. et al. Am J Physiol Endocrinol Metab 284: E671-E678 2003;doi:10.1152/ajpendo.00492.2002

PLC AC

Tiedgen & Seitz, 1980

Insulin/Glucagon ratio

Hep

atic

cA

MP

Glucagon:Insulin

• Glucagon– Liver only– GPCR

• PLC• Adenylate cyclase

– Activates GP– Inhibits GS– Stimulates

gluconeogenesis

• Insulin– Most tissues– RTK

• PI-3K• PP1

– Activates GS– Inhibits GP– GLUT-4 translocation

Glucose storage(muscle)

Glucose distribution(liver)

Phospho-regulation of glycogenThe straight activity version

• PKA+GP via phosphorylase

kinase

-GS

-PP1 via G-subunit

• PKB+GS via GSK

+PP1 via G-subunit

•PP1+GS-GP

PKA PKB

PK

PP1-G

GS

PP1-G

GS

GP

PP1 PP1

GSK3

GlycogenSynthesis

GP

Activates

Inhibits

Phospho-regulation of glycogenThe phosphorylation story

• PKA+GP via phosphorylase

kinase

-GS

-PP1 via G-subunit

• PKB+GS via GSK

+PP1 via G-subunit

•PP1+GS-GP

PKA PKB

PK

PP1-G

GS

PP1-G

GS

GP

PP1 PP1

GSK3

GlycogenSynthesis

GP

Phos/Increase

Dephos/Decr

Active Inactive