Carbohydrate Metabolism

• Carbohydrate metabolism will be discussed
• The digestion and absorption of carbohydrates will be covered
• The fate of absorbed carbohydrates will be identified
• The reactions, enzymes, and regulation of glycolysis will be explored
• The enzymatic reactions and energy produced from the Krebs cycle will be explained
• The biochemical pathway and importance of the HMP shunt will be covered
• The biochemical reactions of glycogenesis and glycogenolysis will be identified
• The reactions and importance of gluconeogenesis will be explained

Carbohydrate Digestion

• Starch, lactose, sucrose, and cellulose are digested in the mouth by α-amylase, which produces starch, dextrins, isomaltose, maltose, lactose, and sucrose
• Low pH in the stomach stops the action of salivary amylase
• In the small intestine, pancreatic α-amylase breaks down carbohydrates to Isomaltose, maltose, lactose, and sucrose
• Enzymes bound to the mucosal cell membrane (isomaltase, maltase, lactase, sucrase) further digest the carbohydrates
• Digested monosaccharides glucose, fructose, and galactose go through portal circulation to the liver
• Cellulose is not digested in humans and passes in the stool

Dietary Fibers

• Adds bulk to the diet, increasing bowel motility and decreasing the risk of constipation, hemorrhoids, and colon cancer
• Delays gastric emptying, resulting in a sensation of fullness and reducing the peak of blood glucose following a meal
• Dietary fibre lowers LDL cholesterol, reducing the risk of cardiovascular disease

Absorption

• Polysaccharides and oligosaccharides are not absorbable
• Monosaccharides are principally absorbed from the jejunum
• Glucose is absorbed at a rate of about 1g/kg B.W./h
• Absorbed sugar goes through the portal blood to the liver
• The absorption of hexoses is accelerated by thyroxine
• Insulin has no effect on the absorption of sugars

Fate of Absorbed Sugars

• Glucose uptake by tissues happens through facilitated diffusion, needing a glucose transporter (GLUT) carrier protein

Utilization by Tissues

• Liver converts fructose and galactose into glucose
• Oxidation occurs via glycolysis, followed by pyruvate oxidation to acetyl CoA in the Krebs cycle, the pentose phosphate pathway (HMP), or the uronic acid pathway
• Conversion into biologically important substances include ribose, fructose, galactose, glucuronic acid, aminosugars, and amino acids
• Storage is done in the form of glycogen (glycogenesis) or triacylglycerols (lipogenesis)
• Excretion occurs in the urine when the renal threshold is exceeded (180 mg%)

Glycolysis

• This is degradation of glucose via enzyme reactions:
  • Two molecules of pyruvate (aerobic glycolysis, oxygen present
  • Two molecules of lactate (anaerobic glycolysis or lactic fermentation, oxygen absent)
• Free energy from glucose is conserved as ATP and NADH
• Glycolysis takes place in the cytosol of cells

Important to Note

• Glycolysis occurs in RBCs, cornea and lenses
• Occurs in tissues with few (testis, leucocytes, retina, skin and GIT)
• Occurs in skeletal muscle during exercise
• Glycolysis occurs in ten reactions

Glycolysis reaction steps

• First five reactions are termed the preparatory phase
• Second five reactions are termed the payoff phase

Glycolysis reactions

• There are three types of chemical reactions:
  • Degradation of the carbon skeleton of glucose to yield pyruvate
  • Phosphorylation of ADP to ATP by high-energy phosphate compounds
  • Transfer of a hydride ion to NAD+, forming NADH
• Each NADH molecule entering the electron transport chain yields 3 ATP molecules; therefore, 6 ATP are yielded from 2 NADH.
• Aerobic glycolysis produces 8 ATP molecules (2 ATP + 6 ATP from 2 NADH)

Regulation of Glycolysis

• The rate of liver glycolysis is regulated to meet major cellular needs: ATP production, building blocks for biosynthesis, and lowering blood glucose
• It is stopped in the liver when glucose is low to allow gluconeogenesis, and vice versa

Regulation Sites

• The effectively irreversible reactions are regulatory control sites
  • Hexokinase
  • Phosphofructokinase
  • Pyruvate Kinase
• It is important to note hexokinase is inhibited by its product, glucose-6-phosphate

Enzymes of Glycolysis

• Glucokinase is found in the liver
• Glucokinase catalyzes at the same reaction of hexokinase
• Glucokinase has a high Km for glucose, while hexokinase has low Km so glucokinase is active only at high glucose levels
• Insulin in the liver activates transcription of the glucokinase enzyme
• Glucokinase is not subject to product inhibition by glucose-6-phosphate

Phosphpfructokinase

• Rate-limiting step of glycolysis
• Allosterically inhibited by ATP
• ATP binds only at the active site if at low concentrations
• ATP binds also at a low-affinity to activate the enzyme
• ATP binds also at a low-affinity to regulatory site if at high concentrations
• Fructose 2,6-bisphosphate (F2,6BP) is a potent activator of phosphofructokinase (PFK-1)
• AMP is an allosteric activator for PFK-1 enzyme
• Citrate inhibits phosphofructokinase as it is Converted to acetyl-CoA for fatty acid and cholesterol synthesis in the cytosol

Pyruvate Kinase

• PEP and fructose 1,6-bisphosphate enhance its activity
• The enzyme increases glycolosis when there is substrate
• ATP is a negative allosteric inhibitor, for parallel regulation with PFK 1
• Alanine a negative allosteric modulator (inhibitor)

Fates of Pyruvate

• Becomes Ethanol + 2CO₂ under hypoxic or anaerobic conditions
• Becomes 2 Acetyl-CoA under aerobic conditions
• Becomes 2 Lactate under anaerobic conditions

Other important details

• Fermentation to ethanol in yeast.
• Lactate fermentation occurs in vigorously contracting muscle, in erythrocytes and in some other cells, and in some micro-organisms
• 4CO2 + 4H2O in animal, plant, and many microbial cells under aerobic conditions

Lactate Dehydrogenase

• Catalyzes the reduction of keto in pyruvate to a hydroxyl, yielding lactate, as NADH is oxidized to NAD+
• Cell membranes contain special proteins that transport of lactate
• Skeletal muscles carry out glycolysis in lactate during brief/intense exercise

Anaerobic Organisms

• Metabolize pyruvate to ethanol, which is excreted as a waste product
• Lactate released to the blood may be taken up by muscle after exercise, and converted back to pyruvate (using Lactate Dehydrogenase)
• Pyruvate may be oxidized in the Krebs Cycle (in liver) or converted back to glucose via gluconeogenesis

Cori Cycle

• Glucose is made from glucose in the Red cells or muscles
• Glucose becomes Pyruvate via Glycolysis
• Pyruvate becomes Lactate via LDH ( Lactate Dehydrogenase)
• The lactate becomes the Glucose in the blood

Krebs Cycle (Citric Acid Cycle)

• The final common pathway where carbohydrates, amino acids, and fatty acids are oxidized into CO2 and H2O, and thus Acetyl-CoA undergoes oxidation
• Oxidization provides energy for the production of the majority of ATP in most animals, including humans
• All this occurs in the mitochondria, where the reduced coenzymes (NADH & FADH2) are oxidized
• Oxygen is required as the final electron acceptor

Important aspects of the cycle

• The cycle participates in synthetic reactions
  • The formation of glucose from the carbon skeletons of some amino acids
  • Provides building blocks for the synthesis of some amino acids and heme
• Steps involved in the cycle:
  • Pyruvate is oxidized, becoming acetyl-CoA , releasing carbon dioxide via the Pyruvate dehydrogenase (PDH) complex
• This oxidative decarboxylation step has an irreversible oxidation process, removing carboxyl and molecule of carbon dioxide to product acetyl coa.
• NAD+ Coenzyme reduces, gives up hydride and becomes NADH.
• Pyruvate dehydrogenase complex require 5 important coenzymes: NAD+, FAD+, CoA, thyamine pyrophosphate (TPP), Lipoate.

TCA Cycle Steps

• Condensation
• Dehydration
• Hydration
• Oxidative decarboxylation
• Substrate-level phosphorylation
• Dehydrogenation
• Hydration
• Dehydrogenation

Energy Production and the Coenzymes

• Oxidation of NADH+H results in the release of 3 ATP molecules
• Oxidation of FADH results in the release of 2 molecules
• There is one ATP for each acetyl CoA cycle, thus there is 12 total ATP/ acetyl CoA cycle

Regulations and Enzymes of the Cycle

• Isocitrate dehydrogenase, α-Ketoglutarate dehydrogenase, malate dehydrogenase are important enzymatic reactions
• The pyruvate dehydrogenase enzyme complex (PDH) activity is turned off (inhibited) when fuel is available in the form of fatty acids
• Also turned of by concentrations of acetyl-CoA and high concentrations of [ATP]/[ADP] and [NADH]/[NAD+] ratios.
• If the demands are highly, energy is reuiqred, which is an important requirement for acetyl-CoA into the citric acid
• Has 2 regulation levels with a Covalent protein with a Ser residue
  • This is is inhibited by reversible phosphorylation of a specific Ser residue.
• Strong regulation of enzymes is very common and there factors governing: flux through the Krebs cycle:

1. Substrate availability.
2. Inhibition by accumulating products.
3. And allosteric feedback, the rate limiting enzymes important enzyme: synthase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase.

• Indirect regulation occurs through phosphorylation

Energy Products from Krebs Cycle:

• Glycolysis from glucose: is 2 + (2x3) = 8 ATP
• 2 Acetyl CoA: 2x12= 24 ATP

Pentose Phosphate Pathway

• It is an oxidative process for producing glucose, and occurs in the cytoplasm of cells

• An alternative pathway for glucose oxidation where ATP is neither produced or utilized.

• Production of NADPH + H+

  • Synthesis of substrates e.g. FA, cholesterol
  • Reduction of glutathione.
  • Hydroxylation of aromatic compounds. Phagocytosis and respiratory burst.

• Pathway as has 2 components.

  • Non-oxidative
  • Oxidative

• NADPH and the oxidative phase are very important in glutathione reductase as by product helps release energy.

• Non-oxidative phase involves ribose to DNA/RNA synthesis.

• Transketolase and Transaldolase reactions

Significant aspects of the pathway:

• Transketolase enzyme: catalyzes the transfer of a two-carbon fragment from a ketose donor to an aldose acceptor.
• Transaldolase enzymes: catalyzes the removal of three-carbon fragment from ketose enzymes.
• NADP is important co-enzyme, with 28ATP produced.

Location of oxidation:

• Glycolysis occurs in all cells, HMP is only in certain cells

Glycogen Synthesis (Glycogenesis)

• Occurs in animal tissue, prominent in liver and skelatal muscles.
• Point starts from glucose 6-phosphate derived from free glucose using hexokinase enzyme:
• D-Glucose + ATP → D-glucose 6-phosphate + ADP
• Then the product is converted to glucose 1-phosphate, enzyme phosphoglucomutase:
• Glucose 6-phosphate ↔ glucose 1-phosphate
• The product Glucose 1-phosphate is converted to UDP-glucose, with enzyme UDP-glucose pyrophosphorylase
• Glucose 1-phosphate + UTP → UDP-glucose + PP .
• 1Keyrate limiting enzymes used are
• Glycogen syntase: catalyles transfer glucose
• Udp enzyme

The process:

• Branches are by Glycogen synthase. fragment of 6 or 7 glucose is used where nonreducing end with C-6 hydroxyl for new residue branch.

Glycogen Breakdown (glycogenolysis)

• Breakdown happens in skelatal muscle and liver. -Action occurs between glycogen is 3 important enzymes: Glycogen phosphorylase, enyme/ phosphoglucomutase.
• Cleavage step(α1-4) glycosidic linkage is crucial, between 2 residues and a a-D-glucose 1- phosphate.
• Enzyme worka until there is four enzyme residues.
• enzyme catalyes the transfer of 3 glucose the next.
• Glycogen is recycled.

Regulation of glycogenesis and glycogenolysis:

• Steps are very corodiated

B. During fasting, glycogenolysis is stimulated and glycogenesis is inhibited. This provides blood glucose.

C. After eating the liver produces excess glycogen production, to get ready for fasting.

D. Glycogen synthase important for metabolism and breakdown.

Liver Glycogen Vs Muscle Glycogen:

• Liver can create the molecules, muscle glycogen is dependant on blood glucose.
• Liver maximum intake: 110/120 , muscle maximum: 330/350
• Hormones affect:Insulin, Epinephrine, Glucagon.

Gluconeogenesis

• Occurs in reverse but steps must perform using enzyme: to over come over from the irreversible the 3 enzymes.
• Formation of glucose for creating non carb source:
• Lactate is converted to amino acid , pathway consumes energy.
• Steps involve the reversal of glycolysis but the following enzyme responsible to reverse of the 3 irreversible glycolysis enzymes.
• NADH is energy used

Enzymes and Reaction Steps

• Glucose-6-phosphatase: is very important enzyme here!
• Pyruate:
• Two important Hormonal regulations:Glucocorticocoids , Glucagon Also, there are multiple step in the cycle to consider with and increase in ATP to synthesis.