Gluconeogenesis and Glucose Homeostasis

Gluconeogenesis is the process by which glucose is made de novo from some other molecule

amino acids (except lysine and leucine), glycerol (from adipose), lactate/pyruvate, and propionyl CoA

occurs during starvation, exercise, absense of carbohydrates, and diabetes – done only in liver and kidney cortex

Regulation of gluconeogenesis

substrate delivery: controlled by what’s coming out of muscle (ala, gln); controlled by TG
mass action on enzymes: ratio of NAD/NADH regulates direction of pathway
allosteric activation of key enzymes: via cAMP
change in mass of enzymes by gene expression: change in amount of enzyme in the liver (ie PEPCK via cAMP)

Glycolysis in Reverse

Gluconeogenesis is essentially reverse glycolysis, starting with pyruvate and ending with glucose

all glycolytic enzymes will run in reverse except three: pyruvate kinase, PFK1, and glucokinase
other pathways must be used to circumvent these obligatory forward reactions

(1) Reverse Pyruvate Kinase – i.e., pyruvate Þ phosphoenolpyruvate – uses two enzymes:

(a) Pyruvate Carboxylase – pyruvate + CO₂ Þ oxaloacetate (uses ATP, requires biotin)

(b) Phosphoenolpyruvate Carboxykinase (PEPCK) – oxaloacetate Þ phosphoenolpyruvate (uses GTP)

two forms – mitochondrial (regulated) and cytosol (constitutive)

any TCA intermediate (e.g., a -ketoglutarate from glutamate) can make glucose through oxaloacetate

(2) Reverse PFK1 – i.e., fructose-1,6-dP Þ fructose-6-P – fructose-1,6-diphosphatase

regulation: cAMP(+) – upregulate when fasting; AMP(-), fructose-2,6-dP(-) – downregulate when using energy

(3) Reverse glucokinase – i.e., glucose-6P Þ glucose – glucose-6-phosphatase – only in liver endoplasmic reticulum

no allosteric regulators known

Futile Cycling (reverse + forward reactions, i.e., going in circles) is prevented by coordinate regulation

e.g., fructose-2,6-dP activates PFK1 and downregulates fructose-1,6-diphosphatase

Gluconeogenesis in the kidney:

Starvation causes increased ala and gln (used for ammoniagenesis)

Ý H⁺ (Ý FA, Ý ketones, Ý lactate)

ammoniagenesis in the kidney helps maintain stable acid/base status

Regulation of Blood Glucose – Controlled by Insulin/Glucagon Ratio

Insulin/Glucagon ratio varies from 5.0 (fed state) to 0.5 (5-6 weeks starvation); diabetic ketoacidosis can be 0.1 or less

A low insulin/glucagon ratio (i.e., fasting state) will have seceral metabolic effects (opposite occurs when ratio is high due to effects of insulin):

(1) Decrease glucose transport in muscle and fat (through GLUT4)

(2) Inhibit glycogen synthase and Activate glycogen phosphorylase

(3) Stimulate liver gluconeogenesis

(4) Stimulate breakdown of protein for gluconeogenesis and Inhibit protein synthesis

(5) Inhibit lipogenesis and Stimulate fatty acid oxidation

When ratio falls to 1.5 (3 day starvation), ketone bodies will be formed to spare glucose

enables brain and muscles to continue to function, and eliminates the need for protein degradation for gluconeogenesis

creatine: made from arg and gly in kidney; shuffles between creatine and creatinine phosphate which both break down to creatinine (constitutive)

glucocorticoid hormone, cortisol (released from adrenal glands), is necessary for gluconeogenesis and for stimulating release of gluconeogenic precursors from muscle and other tissues (ie protein breakdown)

Glucose Homeostasis in Diabetes

elevation of blood glucose caused by relative or absolute deficiency in insulin

the inadequate release of insulin is aggravated by an excess of glucagon

Type I (10-20% of diabetics) – destruction of B cells Þ lack of insulin; accelerated rate of hepatic gluconeogenesis and decreased rate of glucose utilization

metabolic changes due to untreated Type I resemble those in starvation – more severe; insulin virtually absent with glucagon effects unopposed

Type II (non-insulin dependent) – pancreas retains some B cell capacity; insulin levels vary from below to above normal thus cannot maintain glucose homeostasis

Starvation

TG from adipose tissue Þ FA Þ liver Þ ketones

12 hr post absorptive

FA from adipose tissue Þ liver Þ acetyl CoA

amino acids from muscle Þ ala, gln Þ liver Þ gluconeogenesis

brain is using glucose coming out of liver

liver ATP rises due to FA Þ acetyl CoA Þ ketones Þ blood

liver cannot use ketones; muscle can use ketones; brain will use in ~ 3 days

when brain starts using ketones, protein degradation slows/stops

N excretion and gluconeogenesis go together – once ketones replace glucose, urea levels drop

Five Levels of Glucose Homeostasis

(1) Absorptive: blood glucose derived from exogenous CHO; insulin and glucose increase, glucagon decreases; excess glucose Þ liver and muscle glycogen and lipid

(2) Post-absorptive and early starvation: glucagon, glucose, insulin return to basal levels; liver: glycogen Þ glucose; brain is major user of glucose; oxidation of glucose is inhibited in muscle and adipose tissue at the PDC step, causing increased release of lactate, pyruvate, and alanine Þ gluconeogenesis

(3) Phase III and early Phase IV: hepatic and renal gluconeogenesis; decreased insulin and increased glucagon; increased release of gluconeogenic precursors; great demand for gluconeogenesis

(4) Phase V – prolonged starvation: beyond 3 weeks; hepatic gluconeogenesis diminishes; ketones are primary energy source for brain; muscle consumes FA and ketones