Control of glycolysis & gluconeogenesis, glycogen metabolism

lecture 3

Key stages where the control of glycolysis occurs

transport of glucose into the cell

phosphorylation of glucose

phosphofructokinase-1

pyruvate kinase

transport of glucose into the cell

GLUT4 - muscle cells & adipocytes

GLUT2 - liver (non-insulin dependent)

phosphorylation of glucose - differences between hexokinase and glucokinase

glucokinase - present in liver, higher K_m prevents liver taking up low concentration of glucose it releases, not inhibited by glucose-6-phosphate, liver can still produce glucose-6-phosphate for glycogen synthesis.

hexokinase - present in muscle, K_m 100x lower, inhibited by glucose-6-phosphate so glycolysis stopped when concentration builds up - conserves glucose.

Phosphofructokinase-1 (PFK-1)

ATP both substrate and allosteric inhibitor

Inactive state (high citrate & ATP): glycolysis inhibited in muscle. AMP overcomes inhibition

Importance of regulation by AMP

adenylate kinase catalyses reaction: 2ADP ←→ ATP + AMP

large rise in AMP concentration during exercise

very sensitive indicator of energy status in the cell

What controls PFK-1 in muscle?

AMP

[AMP] increases, rate of glycolysis increases

What controls PFK-1 in liver?

fructose-2,6-bisphosphate

[fructose-2,6-bisphosphate] increases, glycolysis increases, gluconeogenesis decreases

potent activator of PFK-1

How is fructose-2,6-bisphosphate formed?

not a glycolytic intermediate

formed by phosphorylation of fructose-6-phosphate via PFK-2

Purpose of substrate cycles

important regulatory purpose of signal amplification in tissues like skeletal muscles

much more sensitive to small changes in [regulatory molecules].

hormonal control in liver (balance between glycolysis and gluconeogenesis)

glucose concentration is low

glucagon activates protein kinase A

PKA phosphorylates PFK-2

concentration fructose-2,6-bisphosphate decreases

gluconeogenesis favoured over glycolysis

hormonal control in muscle (balance between glycolysis and gluconeogenesis)

very little gluconeogenesis and fructose-1,6-bisphosphatase

cardiac - adrenaline causes PKA to phosphorylate PFK-2 on a different site, increasing its rate. fructose-2,6-bisphosphate concentration increases therefore glycolysis increases.

skeletal - PFK2 responds to increase in concentration of fructose-1,6-bisphosphate; concentration fructose-2,6-bisphosphate increases, reinforcing effect of AMP increase, glycolysis increases

Why is glucose stored as glycogen?

reduces osmotic potential - prevents cell damage

avoids glycosylation of proteins as occurs in diabetes

The structure of glycogen

glucose polymer

predominantly alpha-1,4 bonds, but alpha-1,6 every 10 glucose units as branch point

one end joined to glycogenin

Why us UTP in the synthesis of glycogen?

Glucose-1P not a powerful enough donor to form gluc-gluc bond

energy inout required from UTP for glycosylation of glucose

control of glycogen metabolism during exercise

[adrenaline] increases

glycogen phosphorylase activated

glycogen synthase inactivated

adrenaline binds to receptor

adenylate cyclase activated and cAMP produced

cAMP activates PKA

PKA activates phosphorylase kinase and inhibits glycogen synthase

Phosphorylase kinase activates glycogen phosphorylase b to make glycogen phosphorylase a

other regulators of glycogen breakdown

5’AMP allosterically stimulates phosphorylase b, but ATP opposes this

Ca²⁺ also activates phosphorylase kinase in muscle

Glycogen phosphorylation in the well fed state

cAMP hydrolysed to 5’AMP

protein phosphatases remove phosphates from proteins

insulin opposes action of adrenaline and glucagon

glycogen synthase kinase 3 inhibited

glycogen synthase turned on

glycogen is formed

Types of glycogenosis in dogs

type I-a

type II

type III

type IV

symptoms of type I-a (von Gierke)

glucose-6-phosphatase deficiency

usually found in Maltese puppies

may result in failure to thrive, mental depression, low blood sugar and eventually death by 60 day old.

type II (Pompe)

glycogen linkage

usually found in Lapland dogs

vomiting, progressive muscle weakness, cardiac abnormalities

death usually occurs before 2 years old

type III (Cori)

glycogen debranching enzyme (AGL)

German shepherds

depression, weakness, failure to grow, mild hypoglycemia

type IV

glycogen debranching enzyme (GBE1)

English Springer Spaniel

hemolytic anemia - red blood cells destroyed

hemoglobinuria - hemoglobin in abnormally high concentration in urine