MCAT Biochemistry Chapter 9: Carbohydrate Metabolism I

Glucose Transport

• Driven by concentration and independent of sodium

• 4 glucose transporters (Glut 2 and Glut 4 most important)

Glut 2

• Low affinity transporter in hepatocytes and pancreatic cells 

• Captures excess glucose for storage 

• activated by insulin

• K_m for glucose is 15mM, when concentration drops below this, bypasses liver into peripheral circulation 

• Exhibits first order kinetic in liver, glucose picked up in proportion to its concentration in blood

• In pancreas, serves as glucose sensor for insulin release

Glut 4

• In adipose tissue and muscle, responds to concentration in peripheral blood 

• Increased by insulin, K_m is 5mM, close to normal blood glucose levels; is saturated easily 

Glycolysis

cytoplasmic pathway that converts glucose into two pyruvate molecules, releasing a modest amount of energy captured in two substrate-level phosphorylation and one oxidation reaction 

• MCAT focuses more on enzymes involved in glycolysis than its steps

Hexokinase and Glucokinase

• Glucose transported into cell via facilitated diffusion/active transport

• These kinases Phosphorylate glucose in to glucose-6-phosphate

Phosphofructokinase 1 (PFK-1)

• is rate limiting enzyme and main control point in glycolysis 

• Phosphorylates fructose-6-phosphate to fructose 1,6 bisphosphate

• Activated by AMP: low energy

• Inhibited by ATP and citrate: when there is sufficient energy

Phosphofructokinase (PFK-2)

• converts part of fructose-6-phosphate to fructose-2,6-biphosphate which activates PFK 1 

• Insulin activates PFK-2 to, in turn, activate PFK 1 indirectly

• Glucagon inhibits PFK-2 to, inturn, inhibit PFK-1 indirectly

• Overrides ATP inhibition to use glycolysis metabolite products for production of storage molecules

Glyceraldehyde 3-Phosphate Dehydrogenase

• Catalyzes an oxidation and addition of inorganic phosphate (P_i) to its substrate, glyceraldehyde-3-phosphate 

• Results in 1,3 biphosphoglycerate, a high energy intermediate, and oxidation NAD+ to NADH

3-Phosphoglycerate Kinase

• Transfers phosphate group from 1,3 biphosphoglycerate to ADP in an substrate level phosphorylation (ATP generation without oxygen) 

  • Forms ATP and 3-phosphoglycerate 

Pyruvate Kinase

• Activated by fructose 1,6 bisphosphate in a feed-forward activation (earlier product stimulates a later reaction)

• Catalyzes substrate level phosphorylation of ADP using high energy PEP substrate (phosphoenolpyruvate)

  • forms one pyruvate and one ATP

Fermentation

• Occurs in absence of oxygen 

• Lactate dehydrogenase oxidizes NADH to NAD+ and reduces pyruvate to lactate

Important Intermediates of Glycolysis

• dihydroxyacetone phosphate (DHAP)

• 1,3 biphosphoglycerate (1,3 BHG)

1,3 biphosphoglycerate (1,3 BPG) and phosphoenolpyruvate (PEP)

• High energy intermediates used to generate ATP by substrate level phosphorylation 

• Only ATP gained in anaerobic phosphorylation

Irreversible Enzymes

• Glycolysis used these enzymes to push forward the steps of glycolysis by catalyzing irreversible reaction 

  • Glucokinase or hexokinase

  • PFK-1

  • Pyruvate kinase 

Glycolysis in Erythrocytes

• Anaerobe glycolysis is only pathway for ATP production 

• Has bisphosphoglycerate mutase:

•  Produces 2,3 biphophoglycerate (2,3 BPG) from 1,3 BPG through movement of phosphate from 1 to 2 position 

  • Binds allosterically to beta chains of hemoglobin and decreases its affinity for oxygen 

  • Allows unloading of oxygen but still 100% saturation in lungs

Galactose Metabolism

• Sourced from hydrolysis of lactose; reaches liver via hepatic portal vein

• Phosphorylated by galactokinase and converted to glucose 1- phosphate by galactose-1-uridyl transferase and an epimerase (converts between epimers)

Fructose Metabolism

process primarily occurs in the liver and is crucial for energy production

• Fructokinase phosphorylates fructose which is cleaved into glyceraldehyde and DHAP 

  

Pyruvate Dehydrogenase

• irreversible complex of multiple enzymes (activated by insulin in liver) 

  • Requires multiple cofactors/enzymes

• helps convert pyruvate to acetyl CoA enzyme when it enters mitochonderia

  • enters citric acid cycle if ATP is needed

  • Enters fatty acid synthesis if ATP is present 

Glycogen

• branched polymer synthesized and degraded in skeletal muscle (reserve energy) and liver (stored for low blood sugar) primarily 

• Stored in cytoplasm as granules 

  • Has central protein core with chains radiating outwards 

Glycogenesis

• Synthesis of glycogen granules 

• Begins with core protein glycogenin and then the addition of glucose from a glucose-6-phosphate 

  • See process of addition in diagram

Glycogen Synthase

• Rate-limiting enzyme of glycogenesis that forms alpha-1,4 glycosidic bond found in the linear glucose chains of the granule 

  • Stimulated by glucose-6-phosphate and insulin

  • Inhibited by epinephrine and glucagon

Branching Enzyme (glycosyl 𝞪 1,4: 𝞪 1,6 transferase)

• Responsible for introducing 𝞪 1,6 linked branches into the granule as it grows 

1. Hydrolyzes one of the 𝞪 1,4 bonds to release a block of oligoglucose 

2. Forms an 1,6 bond to create a branch 

Glycogenolysis

• The process of breaking down glycogen 

Glycogen Phosphorylase

• Rate limiting enzyme of glycogenolysis 

• Breaks 1,4 glycosidic bonds, releasing glucose-1-phosphate form the periphery of the granule 

• Cannot break alpha 1,6 bonds so stops when it reaches outermost branches

  • Activated by glucagon, AMP, epinephrine

  • Inhibited by ATP 

Debranching Enzyme (glucosyl 𝞪1,4; 𝞪1,4 transferase and 𝞪1,6 glucosidase)

• Two enzyme complex that deconstructs branches in glycogen exposed by glycogen phosphorylase 

1. Breaks an 𝞪1,4 bond adjacent to branch and moves oligoglucose chain that's released

2. Forms new 𝞪1,4 bond

3. Hydrolyzes 𝞪1,6 bond, releasing single residue at the branch point as free glucose 

Glycogen Storage Diseases

• Depend on enzyme affected, degree of decreased enzymatic activity, which isoform of enzyme is affected

  • Isoform: slightly different version of the same protein 

    • Different isoforms in liver and muscle 

• Characterized by accumulation or lack of glycogen in one or more tissues

Gluconeogenesis

• Promoted by glucagon and epinephrine 

• Inhibited by insulin 

• Carried out by liver and kidneys 

  • Glucose produced by hepatic gluconeogenesis is not an energy source for liver 

  • Rather utilizes ATP produced from beta oxidation of fatty acids  

Important Substrates of Gluconeogenesis

• Glycerol-3-phosphate

• Lactate

• Glucogenic amino acids

Glucogenic Amino Acids

• All amino acids except leucine and lysine 

• Can be converted into intermediates that feed gluconeogenesis 

  • Most converted to citric cycle intermediates  (via individual pathways) then to malate, following the same path from there to glucose 

Imporant Enzymes of Gluconeogenesis

• Four enzymes circumvent irreversible steps of glycolysis 

  • Pyruvate Carboxylase

  • Phosphoenolpyruvate Carboxykinase (PEPCK)

  • Fructose -1,6 biphosphatase

  • Glucose-6-phosphatase

Pyruvate Carboxylase

Mitochondrial enzyme activated by acetyl-CoA 

• Produces oxaloacetate(OAA), citric cycle intermediate, that is reduced to malate 

  • a high level of acetyl-CoA implies that the cell is energetically satisfied and neesd to generate sugars

• Fatty acids in the liver are the source burned for glucose production during gluconeogenesis 

Phosphoenolpruvate Carboxykinase (PEPCK)

• Induced by glucagon and cortisol, to raise blood sugar levels 

• Converts OAA to phosphoenolpyruvate using GTP

• Works with pyruvate carboxylase to circumvent action of pyruvate kinase

Fructose-1,6-Biphosphatase

• Key control point and rate limiting step of gluconeogenesis 

• Reverse the action of phosphofructokinase-1 

  • Removes phosphate from fructose 1,6 bisphosphate

 to produce fructose 6-phosphate 

• is activated by ATP and inhibited by AMP and fructose 2,6 bisphosphate 

Glucose-6-Phosphatase

• Glucose 6- phosphate is found only in lumen of endoplasmic reticulum in liver cells

• Used to circumvent glucokinase and hexokinase, which convert glucose to glucose 6-phosphate 

• free glucose is transported back into cytoplasm

The Pentose Phosphate Pathway (PPP)

• Occurs in the cytoplasm of all cells 

  • Produces NADPH and serves as source of ribose 5-phosphate for nucleotide synthesis 

Pentose Phosphate Pathway: Part 1

• NADPH producing part

  • is irreversible and uses glucose-6-phosphate dehydrogenase (G6PD) as its rate limiting enzyme 

    • G6PD is induced by insulin 

Pentose Phospahte Pathway: Part 2

• Series of reversible reactions that produce a pool of sugars for biosynthesis 

  • Fructose-6-phosphate and glyceraldehyde 3-phosphate intermediates can feed back into glycolysis 

  • Pentoses can be made grom glycolytic inermediates

Functions of NADPH

• Acts as an electron donor in many biochemical reactions 

  • Biosynthesis of fatty acids and cholesterols 

  • Assisting in cellular bleach production in some white blood cells

  • Maintenance of glutathione to protect against reactive oxygen species  caused by peroxides