Biochemistry 1

CARBOHYDRATE BIOCHEMISTRY


Glycolysis

Energy investment stage: requires two ATPs then ends up with two molecules of glyco diphrephosphate.


Glyceraldehyde -3 phosphate (oxidation) is catalyzed by Glyceraldehyde-3-phosphate dehydrogenase (moving hydrogens) and forms bisphosphoglycerate

Pentose shunt

  • is a metabolic pathway parallel to Glycolysis. it takes place in the cytoplasm

  • two purposes are:

    • Production of NADPH

      • reducing agent needed for various cellular processes, including the synthesis of fatty acids and cholesterol and for neutralizing reactive oxygen species (ROS) that can be damaging to cells.

    • production of pentose (5 sugars)

      • It generates ribose-5-phosphate, a precursor for the synthesis of nucleotides (the building blocks of DNA and RNA) and other important molecules.

NAD is reduced

NADH gains hydrogen


ADP TO ATP (removing phosphate to add it to another to form ATP)


Enzymes

  1. Glucokinase specific to glucose (pancreas/liver)/Hexokinase (any 6 sugars) = glucose 6-phosphate

  2. Phosphoglucose/ isomerase = fructose 6- phosphate

  3. Phosphofructokinase -1 = fructose 1,6-bisphosphate

  4. Aldolase= dihydroxyacetone phosphate/ glyceraldehyde -3-phosphate

  5. Triose phosphate isomerase = glyceraldehyde -3-phosphate


Taking two molecules into the next step

Why phosphorylate? Destabilization of carbon-carbon bonds which are very strong bonds hard to break down.


ATP generation of glycolysis - Anaerobic

Anaerobic: no electron transport


Pathway

Molecules

1) HEXOKINASE

-1ATP

3) PHOSPHOFRUCTOKINASE

-1 ATP

7) PHOSPHOGLYCERATE KINASE

2 x 1 ATP

10) PYRUVATE KINASE

2 x 1 ATP

NET YIELD:

2 ATP

Sugar > glycolysis > ATP


Red blood cells rely on glycolysis to generate its energy in order not to use the O2 they carry








ATP generation of glycolysis - Aerobic

Aerobic: electron transport ( generate more ATP)




Pathway

Molecules

1) Hexokinase (A hexokinase is an enzyme that irreversibly phosphorylates hexoses)

-1 ATP

3) PHOSPHOFRUCTOKINASE (kinase enzyme that phosphorylates fructose 6-phosphate in glycolysis.)

-1 ATP

6) GLYCERALDEHYDE-3- PHOSPHATE DEHYDROGENASE (an enzyme of about 37kDa that catalyzes the sixth step of glycolysis and thus serves to break down glucose for energy and carbon molecules)

2 NADH> 2 x 2.5 ATP

7) PHOSPHOGLYCERATE KINASE (is an enzyme that catalyzes the reversible transfer of a phosphate group from 1,3-bisphosphoglycerate to ADP producing 3-phosphoglycerate and ATP)

2 x 1 ATP

10) PYRUVATE KINASE (the enzyme involved in the last step of glycolysis. It catalyzes the transfer of a phosphate group from phosphoenolpyruvate to adenosine diphosphate, yielding one molecule of pyruvate and one molecule of ATP.)

2 x 1 ATP

NET YIELD

7 ATP





Substrate level phosphorylation

Oxidative phosphorylation

Type of phosphorylation

Phosphate transferred from substrate to ADP

Phosphate is transferred from the energy of the electron transport chain to generate ATP

Where

Cytoplasm, mitochondrial matrix

Inner mitochondrial membrane

Phosphate source

From substrate by coupled reaction

Added by energy release

When

Glycolysis/ Krebs Cycle

Electron transport

Cofactors

NAD+ and FAD+ are reduced

NADH and FADH2 are oxidized

Amount of ATP

4 per glucose molecule

28 per glucose molecule

substrate oxidation

Partial Oxidation

Complete oxidation




G (Gibbs) = free energy of a chemical system



  1. Hexokinase ( glucose > glucose-6p)

Inhibited by glucose -6- phosphate


B) Phosphofructokinase-1 (fructose-6P > Fructose-1, 6bp)

Inhibited by ATP, Citrate, H+. Inhibited by the high energy state

When there are low ATP levels the reaction in velocity is quite high on the graph

When the ATP is high the production of fructose decreases




C) pyruvate kinase

Inhibited by ATP, Alanine

Activated by AMP, fructose-1,6- bisphosphate


Hexokinase

  • requires tissue ( e.g muscle)

  • Specificity: various six carbon sugars

  • Low Km - saturated at low glucose levels

  • Regulation: G6P negative feedback

  • Function: glycolysis/produce energy


Glucokinase

  • tissue: liver and pancreatic beta cells

  • Specificity: only glucose

  • Higher Km: higher capacity for glucose

  • Regulation: no feedback from G6P(responds to glucose normally)

  • Function: 1) glucose sensing in the pancreas 2) reduce glycolysis in the liver for glycogen synthesis (storage of glucose)


Glucokinase in pancreatic beta cells

  • glucose induces insulin release

Glucose into the cell, into Glucokinase (first step of glycolysis) phosphorylation of glucose into glucose -6- phosphate. Product go glycolysis will transport molecules to generate ATP.

The amount of ATP generated is the amount of glucose that enters the cell. (Via electron transport chain)

ATP stimulates the ATP-sensitive potassium (K) channels and blocks the channels at the membrane.

Build up of positively charged potassium. It depolarises the membrane and affects the calcium channel.

The calcium channel opens, and with calcium being present in the cell it releases Vesicles that are pre-primed with insulin.

Insulin gets released into pancreatic b cells.



Insulin

  • muscle & Liver: convert glucose to glycogen (storage)

  • Liver: decreases gluconeogenesis

  • Adipose: increases fat synthesis (storage), Acts on the hypothalamus to reduce our appetite


Glucagon

  • stimulates glycogen breakdown

  • Stimulates gluconeogenesis (new glucose)


The Cori Cycle

  1. glycolysis in the muscles converts glucose to lactate ( anaerobic) the fate of PYRUVATE

  2. Lactate is transported to the liver through the blood

  3. Gluconeogenesis in the liver reforms glucose from lactate

  4. Glucose is transported Back to the muscle through the blood


Lactic acidosis

Lactic acid ( lactate) builds. Up in the bloodstream (>4 mM)

Symptoms: nausea vomiting weakness


Caused by:

Decrease in O2 delivery to tissue

Decrease in lactate clearance

Increase lactate production