Glucose metabolism

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 importStarch/glycogen breakdown

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

hexokinase

D-Glucose

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

ADPATP

Metabolite feedback in glycolysis

G6p F6p F16p GAP

Gp2PEP

Pyr

Lac

AcCoA

NAD

NADH

ATP ADP

G3pG2p

PGI PFK ALD

GAPDH

PGKPGAMENO

PK

LDH(M)

PDH

NAD

NADH

NADH

ADP

ATP

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

Phosphofructokinase

• Substrate cooperativity

• Fructose 1,6-bisphosphate

Mansour & Ahlfors, 1968

+cAMP

PFK activity

Change in slope of concentration-activity curve

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--| DepolarizationCa

insulin+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