Carbohydrate metabolism

Lecture 2

2 divisions of metabolic pathway

catabolic

anabolic

catabolic pathways

larger molecule broken down into smaller units to generate energy

anabolic pathways

complex biomolecules are synthesised from simpler, smaller units

consume energy

Catabolic pathways required to completely oxidise glucose and release energy as ATP

Glycolysis

Citric acid cycle

Oxidative phosphorylation

Insulin independent tissues and the transporters used

brain, liver, erythrocytes

insulin independent transporters, like GluT1, GluT2, GluT3

Glycolysis short overview

Glucose/glycogen metabolised to pyruvate and 2 ATP

NAD reduce to NADH

2 fates of NADH

transported into mitochondria for oxidation

used to reduce pyruvate to lactate (regenerating NAD)

When is glycolysis used to generate energy?

absence of oxygen

lacking mitochondria (e.g. erythrocytes, cells within the retina)

burst of activity in fast-twitch (white) muscle

How is the ‘oxygen debt’ repaid?

increasing citric acid cycle rate to oxidise the lactate produced in the body

2 halves of glycolysis

chemical priming/energy consuming phase- up to glyceraldehyde 3-phosphate

energy-generating stages

1st reaction in glycolysis

Glucose + ATP ——> glucose-6-phosphate + ADP + H⁺

catalyst: hexokinase in muscle/glucokinase in liver

Type of reaction: Phosphorylation, reversible

*NB - ATP must form a complex with Mg²⁺, otherwise it acts as an inhibitor of hexokinase. 1st ATP has been used.

2nd reaction in glycolysis

Glucose-6-phosphate —→ fructose-6-phosphate

catalyst: phosphoglucose isomerase

Type of reaction: isomerisation, reversible

3rd reaction in glycolysis

Fructose-6-phosphate + ATP —→ Fructose-1,6-bisphosphate + ADP + H⁺

Catalyst: Phosphofructokinase

Type of reaction: phosphorylation, irreversible

*NB - ATP forms complex with Mg²⁺ again to prevent inhibition of PFK. 2nd ATP has been used.

4th reaction in glycolysis

Fructose-1,6-bisphosphate —→ Dihydroxyacetone phosphate + glyceraldehyde-3-phosphate

Catalyst: aldolase

Type of reaction: Cleavage, reversible

5th reaction of glycolysis

Dihydroxyacetone phosphate —→ enediol intermediate —→ glyceraldehyde-3-phosphate

Catalyst: Triose phosphate isomerase

Type of reaction: isomerisation, reversible

*NB - final step in stage 1 of glycolysis

6th reaction in glycolysis

Glyceraldehyde-3-phosphate + NAD⁺ + P_i —→ 1,3-Bisphosphoglycerate + NADH + H⁺

Catalyst: glyceraldehyde-3-phosphate dehydrogenase

Type of reaction: oxidation & phosphorylation, reversible

*NB - 1,3-BPG is first high-energy intermediate

7th reaction in glycolysis

1,3-Bisphosphoglycerate + ADP —→ 3-Phosphoglycerate + ATP

Catalyst: phosphoglycerate kinase

Type of reaction: Phosphorylation, reversible

*NB - 1st ATP is generated

8th reaction in glycolysis

3-Phosphoglycerate —→ 2-Phosphoglycerate

Catalyst: phosphoglycerate mutase

type of reaction: functional group movement, reversible

9th reaction in glycolysis

2-Phosphoglycerate —→ Phosphoenolpyruvate + water

catalyst: enolase

type of reaction: dehydration, reversible

*NB - 2nd high energy intermediate is formed

10th reaction in glycolysis

Phosphoenolpyruvate + ADP + H⁺ —→ Pyruvate + ATP

Catalyst: pyruvate kinase

Type of reaction: cleavage, phosphorylation, irreversible

*NB - 2nd ATP is generated. Last stage.

What is net reaction for glycolysis?

Glucose + 2P_i + 2ADP + 2NAD⁺ —→ 2 Pyruvate + 2ATP + 2NADH + 2H⁺ + 2H₂O

2 methods of NAD regeneration

1. NADH oxidised in mitochondria

2. NADH oxidised by lactate dehydrogenase during anaerobic glycolysis (pyruvate —→ lactate)

Production of lactate

Pyruvate + NADH —→ Lactate + NAD

Fate of lactate

Exported to liver or converted back to pyruvate for oxidation of NADH and pyruvate in mitochondria once oxygen concentrations increase

What type of glycolysis does fast twitch (white muscle) use?

anaerobic glycolysis

how is the oxygen debt repaid?

increasing citric acid cycle rate to oxidise the lactate

Pentose Phosphate Pathway

catabolic pathway

operates alongside glycolysis in the fed state

ensure supply of NADPH and other intermediates like ribose-6-phosphate.

opeartes in cell types heavily incolved in biosynthesis of fats and other biomolecules, e.g. mammary glands, adipose tissue, adrenal cortex, the liver

The mechanism of glucose synthesis from pyruvate (1st stage).

pyruvate + CO₂ —→ oxaloacetate

catalyst: pyruvate carboxylase

*NB - energy used from ATP

The mechanism of glucose synthesis from pyruvate (2nd stage).

oxaloacetate + (phosphate) —→ phosphoenolpyruvate + CO₂

catalyst: PEP carboxykinase

*NB - uses energy and phosphate from GTP hydrolysis. PEP leaves the cycle and becomes glucose through gluconeogenesis.

carbohydrate metabolism in the liver

major role in glucose homeostasis

maintains blood glucose concentration for brain and red blood cells

in fed state: converts glucose to glycogen and triacylglycerides

in fasting state: moblises glycogenolysis & gluconeogenesis.

carbohydrate metabolism in muscle

uses glucose

white (type II fast twitch muscle) - uses glucose in anaerobic respiration

red (type I slow twitch muscle) - uses glucose in aerobic respiration

glucose from blood supply or glycogen stores.

fate of pyruvate

ethanol (fermentation in yeast)

lactate

acetyl-CoA —→ citric acid cycle

glucose

dysregulation of glycolysis in ischaemia

lack of oxygen as reduced blood flow leads to anaerobic glycolysis

results in lactic acid build up, therefore lactic acidosis

where does gluconeogenesis take place?

primary location: liver

secondary location: kidneys

stages of gluconeogenesis different to glycolysis

most stages of glycolysis can proceed in reverse

BUT

once fructose-1,6-bisphosphate is reached, different enzymes used.

1. fructose-1,6-bisphosphate —→ fructose-6-phosphate

catalyst: fructose bisphosphatase

type of reaction: dephosphorylation, hydrolysis

2. glucose-6-phosphate —→ glucose

catalyst: glucose-6-phosphatase

type of reaction: hydrolysis, dephosphorylation