carbohydrates 1+2

What is glycolysis?

the breakdown of glucose by enzymes, releasing energy and pyruvic acid

Where does glycolysis occur?

cytosol

2 phases of glycolysis

1. energy investment phase 2 ATP used

2. energy payoff phase - Net 2 ATP gain (4 gained altogether)

Step 1: Glycolysis

what is formed?

Enzyme used?

ATP used?

irreversible or reversible

Glucose → G6P (phosphorylation)

hexokinase enzyme

ATP used

IRREVERISBLE

step 2 of glycolysis

what is formed

enzyme used

ATP used?

irreversible or reversible

G6P→F6P (conversion)

phosphohexase isomerase

REVERSIBLE due to free energy

step 3 of glycolysis

what is formed

enzyme used

ATP used?

irreversible or reversible

F6P →F1,6BP (phosphorylation)

phosphofructokinase-1 (PFK-1)

ATP

IRREVERSIBLE - first committed step of glycolysis - cannot go back after this

step 4 of glycolysis

what is formed

enzyme used

ATP used?

irreversible or reversible

cleavage of F1,6BP → DHAP + G3P (splitting part of glycolysis - 1 Glc into 2 triose sugars)

F1,6BP aldoase (aldoase for short)

none

REVERSIBLE

step 5 of glycolysis

what is formed

enzyme used

ATP used?

irreversible or reversible

interconversion of triose sugars to form 2 G3P

triose phosphate isomerase

none

REVERSIBLE

what triose can only participate in glycolysis

glyceraldehyde-3-phosphate

step 6 of glycolysis

what is formed

enzyme used

ATP used?

irreversible or reversible

oxidation of G3P to 1,3-biphosphate

dehydrogenase

2 NADH produced

REVERSIBLE

first energy payoff phase

step 7 of glycolysis

what is formed

enzyme used

ATP used?

irreversible or reversible

transfer from 1,3-BiPG to ADP forms 3-phosphoglycerate

phosphoglycerate kinase

2 ATP produced

spontaneous

REVERSIBLE

substrate level phosphorylation

step 8 of glycolysis

what is formed

enzyme used

ATP used?

irreversible or reversible

conversion of 3-PG to 2-PG

phosphoglycerate mutase

REVERSIBLE

step 9 of glycolysis

what is formed

enzyme used

ATP used?

irreversible or reversible

dehydration of 2-PG to PEP

enolase

none

REVERISBLE

step 10 of glycolysis

what is formed

enzyme used

ATP used?

irreversible or reversible

transfer of PEP to ADP

pyruvate kinase

2 ATP produced

IRREVERSIBLE

final step - produces pyruvate

No NAD+ has what effect on glycolysis?

Inhibits it, NAD+ is required for glycolysis.

Fate of Pyruvate

Depends on oxygen availability.

- oxygen is present, pyruvate oxidised to acetyl-CoA, enters the citric acid cycle

- Without oxygen, pyruvate reduced in order to oxidise NADH back to NAD+ (2 lactate or 2 ethanol + CO2)

function of pyruvate dehydrogenase

converts pyruvate to acetyl-CoA so it can enter citric acid cycle

Function of lactate dehydrogenase

changes pyruvate to lactate to regenerate NAD+

Fate of blood lactate

cori cycle

- ATP made via substrate level phosphorylation producing lactate - converted to glucose in liver via gluconeogenesis

- liver repays O2 debt

What is gluconeogenesis?

production of glucose from pyruvate

is glycolysis irreversible or reversible

three steps are irreversible

bypass reactions A + B of gluconeogenesis

pyruvate --> into mitochondria --> oxaloacetate --> malate --> out of mitochondria --> oxaloacetate --> PEP

pyruvate carboxylase

malate dehydrogenase

PEP carboxykinase

reaction C of gluconeogenesis

F-1,6-BiP + H2O --> fructose-6-phosphate + Pi

checkpoint

irreversible

enzyme - fructose-1,6-phosphatase

bypass reaction D of gluconeogenesis

glucose-6-phosphatase + H2O --> glucose + Pi

dephosphorylation

PEP --> pyruvate generates how many ATP

1 but in glycolysis there are two PEP molecules for one glucose

where is fructose metabolised

liver

What are carbohydrates?

sugars and starches

functions of carbohydrates

quick energy

energy storage

structure

cell-cell communication

3 important monosaccharides

glucose, galactose, fructose

How are disaccharides formed?

condensation reaction

bonds found in disaccharides

glycosidic bonds

3 important disaccharides

sucrose, lactose, maltose

What are polysaccharides?

large macromolecules formed from monosaccharides

glycogen

Storage form of glucose

alpha 1-4 linked subunits with alpha 1-6 branches

glycogen in liver

replenishes blood sugar when fasting

Glycogen in skeletal muscle

catabolism of glycogen produces ATP for contraction

what carbohydrates are in our diets (9)

starch - cereals potatoes rice

glycogen - meat

cellulose - plant cell walls

oligosaccharides - short - peas, beans, lentils - not digested

lactose - milk

sucrose - sugar

maltose - beer

glucose - fruit

fructose - honey

how are carbohydrates digested - describe the process (4)

mouth - amylase breaks down starch

stomach - no carbohydrate digestion here but need to bypass to get to intestines

duodenum - pancreatic amylase breaks down starch - same as salivary amylase

jejunum - final digestion by mucosal cell surface enzymes

what enzymes digest carbohydrates in jejunum

isomaltase - hydrolyses alpha 1-6 bonds

glucoamylase - removes glucose sequentially from non-reducing ends

sucrase - hydrolyses sucrose

lactase - hydrolyses lactose

How is glucose absorbed?

Na+ glucose symporter into epithelial cell
GLUT2 uniporter into blood
meanwhile ATPase is always pumping Na out of cell so symporter works

fate of absorbed glucose

glucose diffuses through epithelial cells into portal blood and to the liver

glucose immediately phosphorylated into glucose-6-phosphate by hepatocytes

G-6-P cannot diffuse out of cell as GLUT transporters will not recognise it

enzyme catalysts - glucokinase (liver) and hexokinase (other tissues)

glucokinase Km and Vmax

High Km and high Vmax

hexokinase Km and Vmax

Low Km and low Vmax

synthesis of glycogen step 1

glycogenin begins process - covalently binds glucose from uracil diphosphate (UDP)

glycogenin forms chains of around 8 glucose residues

glycogen synthase takes over

synthesis of glycogen step 2

chains formed by glycogen synthase are broken by glycogen branching enzyme and reattached via alpha 1-6 bonds to give branch points

degradation of glycogen

this process produces glucose 1-6 phosphate from breaking alpha-1,4 and alpha-1,6 glycosidic bonds