metabolism in the liver

what is metabolism?

sum of all chemical reactions in the body

what is catabolism?

breaking down fuel into simple molecules

what is anabolism?

building up complex molecules

what are the digestive products of carbohydrates?

- monomer sugars (fructose, glucose and galactose)
- disaccharides (maltose and lactose)

what 3 things are blood glucose dependent on?

- glucose absorption from intestine
- glucose production by liver
- glucose uptake and metabolism by all tissues in body

how much hepatic glycogen is stored in the liver?

50-100g

how long does the hepatic glycogen last?

maintains blood glucose for up to a 12-24 hour fast

what is glycogenolysis?

the breakdown of glycogen to glucose via phosphorylation

when does gylcogenolysis in the liver occur?

- when there is a fall in blood glucose concentration
- very important for providing glucose to brain and erythrocytes between meals

what stimulates glycogenolysis?

glucagon and adrenaline

what inhibits glycogenolysis?

insulin

what 4 enzymes are involved in glycogenolysis?

- glycogen phosphorylase - breaks long chains of glucose molecules
- debranching enzyme - removes branches
- phosphoglucomutase - converts G1P to G6P
- glucose-6-phosphotase - coverts G6P to glucose

whats the first step in glycogenolysis?

- removal of terminal glucose by breaking the α1-4 bonds using glycogen phosphorylase or a debranching enzyme
- this releases G1P

what is the second step of glycogenolysis?

- G1P is converted into G6P by enzyme phosphoglucomutase

what is the third step of glycogenolysis?

- G6P is converted to glucose by glucose-6-phosphotase
- glucose is transported out of the cell by GLUT2 straight into the bloodstream

what causes an increased demand for glucose?

- physiological stress (e.g. prolonged exercise)
- pathological stress (e.g. severe shock from blood loss)
- psychological stress (e.g. in fear and prep for flight or fight)

what is gluconeogenesis?

synthesising glucose from non-carbohydrate sources

what stimulates gluconeogenesis?

glucagon and cortisol

what inhibits gluconeogenesis?

insulin

where does lipogenesis take place in cells?

in the cytosol

what is the first stage of lipogenesis?

Acetyl-CoA is converted to Malonyl-CoA by Acetyl-CoA carboxylase (ACC)

why is the first stage important?

its important in the regulation of lipogenesis:
- activated by citrate
- inhibited by AMP

what is the second stage of lipogenesis?

- fatty acid synthase adds Malonyl-CoA to a growing fatty acid
- this fatty acid is then linked to a carrier protein

what stimulates lipogenesis?

insulin

what inhibits lipogenesis?

glucagon and adrenaline

what is lipolysis?

β-oxidation (breakdown of fatty acids)

where does the β-oxidation of fatty acids take place?

in the mitochondrial matrix

what happens in the β-oxidation of fatty acids?

it is a chain reaction:
- the long fatty acid chains are broken down into 2 carbon acetate units
- the acetate units combine with co-enzyme A to form acetyl-CoA
- acetyl-CoA combines with oxaloacetate to form citrate at the beginning of the TCA cycle

what stimulates lipolysis?

glucagon and adrenaline

what inhibits lipolysis?

insulin

what does the chain reaction of β-oxidation release?

- acetyl CoA
- (FADH2)
- (NADH)

what can the absorbed amino acids be used for in the liver?

protein synthesis

what stimulates protein synthesis?

insulin and growth hormone

what proteins are synthesised in the liver?

- albumin
- blood clotting factors
- thrombopoietin
- angiotensinogen

what happens to excess amino acids?

deamination or transamination

what happens in transamination?

- transferring the amino group to ketoacids through alanine aminotransferase (ALT) and aspartate aminotransferase (AST)
- this is more ideal than deamination because it does not leave toxic ammonia (NH₃) as the product

what happens in deamination?

- the amino group is removed from the aa to produce ketoacid and ammonia
- ammonia is converted into an ammonium ion which needs to be removed as its toxic
- it is removed via glutamine or the urea cycle