Week 12 Gluconeogenesis and the Urea Cycle

Glycogenolysis

Breakdown of liver glycogen into glucose (limited supply of 1400 cals, catabolic process)

Gluconeogenesis

Production of glucose from non-carbohydrate sources like lactate, glycerol, and alanine

Location of gluconeogenesis

Primarily liver, sometimes kidneys

Lactate pathway

Lactate is converted to pyruvate and follows reverse glycolysis

Gluconeogenesis vs glycolysis

Gluconeogenesis uses different enzymes to bypass irreversible glycolysis steps

Pyruvate to oxaloacetate enzyme in gluconeogenesis

Pyruvate carboxylase

Oxaloacetate to PEP enzyme in gluconeogenesis

PEP carboxykinase

Fructose-1,6-bisphosphate to fructose-6-phosphate enzyme in gluconeogenesis

Fructose-1,6-bisphosphatase

Glucose-6-phosphate to glucose enzyme in gluconeogenesis

Glucose-6-phosphatase

Why ammonia is dangerous

Toxic to neurons and interferes with Krebs cycle

Deamination

Removal of nitrogen, producing ammonia and carbon skeleton to be dealt with by urea cycle

Transamination

Transfers nitrogen between species

Purpose of transamination

Allows cells to produce non-essential amino acids and reuse of carbon skeletons

Key transamination molecules

• Glutamate can donate amino groups

• Alpha-ketoglutarate can accept amino groups

Transaminase cofactor

Vitamin B6

Nitrogen transport form

Nitrogen transported as glutamine, alanine, or urea

Cahill cycle

• Purpose is to transport nitrogen from muscle to liver safely

• Allows muscle tissue to use proteins for energy without causing ammonia toxicity

Cahill cycle step 1 in the muscle

NH4 added to alpha-ketoglutarate to form glutamate in muscle

Cahill cycle step 2 in the muscle

Ammonia on glutamate is trans-aminated onto pyruvate by alanine aminotransferase to form alanine and alpha ketoglutarate

NH3 transaminated from L-glutamate to pyruvate ⇌ alpha-ketoglutarate and L-alanine

Cahill cycle step 3

Alanine travels to liver via blood

Cahill cycle step 4 in the liver

• L-alanine gives amino group to alpha-ketoglutarate ⇌ pyruvate and L-glutamate through alanine aminotransferase

• Glutamate then enters the urea cycle, used for protein synthesis

Why alanine is used

Biochemically close to pyruvate, non-toxic, polar and travels easily in the blood

Alternative nitrogen carrier (and when would they be used)

Glutamine can carry two ammonia molecules

For tissues without alanine aminotransferase or to increase capacity when protein catabolism is high

importance of Cahill cycle

• Important during fasting, low carbs, and exercise

Excess protein fate

• Converted into intermediates that enter Krebs cycle

• Can be used for gluconeogenesis, NT synthesis

Protein storage

Proteins are not stored; all are functional

what do high levels of nh3/nh4 in the blood indicate

Indicate liver dysfunction

High urea levels indicate what

problem with the kidneys because they are responsible for urea output

Urea cycle location

Occurs in liver across mitochdonrial matrix and cytosol through transporters

Hyperammonemia

Accumulation of ammonia due to urea cycle failure

Urea cycle energy use

Uses 3 ATP and produces 2 NADH (near energetically neutral)

Rate limiting step of urea cycle

Formation of carbamoyl phosphate

First step of urea cycle

NH3 enters mitochondrial matrix and forms NH4

Second step of urea cycle

• Mitochondrial NH4 is used to make carbamoyl phosphate

• Rate limtiing step

Third step of urea cycle

Carbomoyl phosphate combines with ornithine to create citrulline

Fourth step of urea cycle

Citrulline enters cytosol through ornithine-citrulline transporter

Fifth step of the urea cycle

The second nitrogen is attached to aspartate and reacts with citrulline forming arginosuccinate with both nitrogens

End product of urea cycle

Eventually, nitrogens are donated to form urea, a small water-soluble molecule with 2 NH3 groups

Urea cycle and Krebs link

• Ortnithinine is equivalent to oxaloacetate

• Both have fumarate, and oxaloacetate which is needed to form aspartate

Urea cycle regulation

Controlled by substrate availability and carbamoyl phosphate synthetase I (rate limiting enzyme that synthesizes carbamoyl phosphate)

OTC deficiency

Most common urea cycle disorder (X-linked recessive)

Effect of OTC deficiency

• ornithine is unable to move out of the mitochondria due to ornithine transcarbamylase (OTC) deficiency

• Urea cycle from NH4 cannot progress beyond OCT

Symptoms of OTC deficiency

Hyperammonemia, lethargy, ataxia, possible death

Treatment of OTC deficiency

Low protein diet, liver transplant, nitrogen scavengers like sodium benzoate