a stripped down figure of...

A stripped down Figure of Glycolysis

Fates of pyruvate

Other sugars (than glucose)

Energetics of glycolysis

Gluconeogenesis

Regulation of glycolysis/gluconeogenesis

Fates of Pyruvate

Stage 2: x21 oxidation2 substrate

level phos.

11a. Anaerobic Glycolysis – Reduction of Pyr to Lactate: Lactate DHpyruvate + NADH + H+ lactate + NAD+

- Pyruvate is reduced to lactate to recover NAD+ needed for glycolysis

- This is a reversible reaction – several isoenzyme forms of LDH

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Figure 17-24 Reaction mechanism of lactate dehydrogenase.Pa

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Reduction of pyruvate to lactate: lactate dehydrogenase

pyruvate + NADH + H+ lactate + NAD+

•reduced at expense of electrons originally donated by 3-phosphoglyceraldehyde, carried by NADH. Thus, no net oxidation occurs in glycolysis = fermentation; another organic serving as electron acceptor.

•lactate, end-product under anaerobic conditions, diffuses thru cell membrane as waste into blood - salvaged by liver and rebuilt to form glucose (gluconeogenesis). This occurs in skeletal muscle during periods of strenuous exertion: Cells use O2 faster than can be supplied by circulatory system; cells begin to function anaerobically, reducing pyruvate to lactate rather than further oxidation. Causes soreness due to decreased pH.

Lactate fermentation also important commercially since bacteria capable areresponsible for production of cheeses, yogurts, and other foods obtained byfermentation of lactose of milk.

Stage 2: x21 oxidation2 substrate

level phos.11

11b. Anaerobic Glycolysis – Reduction of Pyr to Ethanol: Pyr Carb. + ADH (in yeast, not humans)

pyruvate acetaldehydeacetaldehyde + NADH + H+ ethanol + NAD+

- Pyruvate is decarboxylated to form CO2 + acetaldehyde (TPP)- Acetaldehyde is reduced to ethanol to recover NAD+ needed for glycolysis

Figure 17-25 The two reactions of alcoholic fermentation.Pa

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Figure 17-26 Thiamine pyrophosphate.

Thiazole as an electron sink

+

Figure 17-27 Reaction mechanism of pyruvate decarboxylase.

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Figure 17-30 The reaction mechanism of alcohol dehydrogenase involves direct hydride transfer of the pro-R hydrogen of NADH to the re

face of acetaldehyde.

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In alcoholic fermentation, pyruvate is first decarboxylated to acetaldehyde that then serves as electron acceptor, giving rise to ethanol.

Commercially important in baking and brewing industries -Other less common fermentation processes (bacteria) yield propionic acid (swiss cheese); butyrate (rancid butter); acetone, isopropanol.

Reduction of pyruvate to ethanol: alcohol dehydrogenase

pyruvate + NADH + H+ ethanol + NAD+

Biochemical Regulation of Glycolysis

Energetics of Glycolysis

The glycolytic pathway is regulated at all three irreversible steps. The regulation is MOSTLY of a straight forward biochemical nature. We might consider it a sort of “primitive”regulation. The complete discussion, however, will require an integration of systems as part of a hormonal response. (More later).

Fig. 17-33 PFK activity vs. [F-6-P].

F6P

ATPRegulator site:ATP =IADP/AMP=Stim

The PFK tetramer (only dimer shown here) is allostericallyregulated. PFK is in an R to T equilibrium. ATP stabilizes T state and ADP or AMP the R state. Other effectors include F2,6BP and citrate.

Figure 18-23 Comparison of the relative enzymatic activities of hexokinase and glucokinase over the physiological blood

glucose range.

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Liver contains many insulin Independent GluT2 transporters. When blood glucose is high, and insulin is signaling glucose reduction, sugar enters the liver and glucokinasegenerates G6P which stimulates glycogen synthesis

Other sugars feed into glycolysisLactase expression can be repressed, depending on environment. This is basisfor lactose intolerance.

GalactoseGalactosemia

Gluconeogenesis1. What is the role of this pathway?

Convert 3-C lactate or pyruvate into 6-C glucose

2. What is the difference between glycolysis and gluconeogenesis?Need to by-pass the three irreversible steps

3. Where is the pathway located?Uses enzymes located in the cytosol and mito

The ability to synthesize glucose is important to mammals since certain tissues, particularly brain and RBC, are almost solely dependent on glucose as an energy source. In normal humans, under fasting conditions, 80% of glucose is consumed by brain. The glycogen reservoir in liver has only 1/2 day supply for the brain. In periods of dietary glucose deprivation, we must be able to make glucose from other sources.

Figure 23-9 The Cori cycle.Pa

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G-6-P

G-6-P

Gluconeogenesis/glycolysis

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1

2

3 HK

PFK

PK

Figure 23-7Pathways of gluconeogenesisand glycolysis.

Note that the ∆G values are given in the direction of gluconeogenesis.

Bypassing the PK step

The energy released from ATP hydrolysis is “stored” in carboxylated intermediate. CO2 release will help drive the next step.

By-Passing “PK”: Two-phase reaction mechanism of pyruvate carboxylase.

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By-Passing “PK”: Conversion of pyruvate to oxaloacetate and then to phosphoenolpyruvate.

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Step 2

GTP

The PK bypass uses OAA, which must be generated in the mitochondria. OAA cannot be transported out so it must be converted to PEP or malate (or Asp but lets ignore that).

Gluconeogenesis requires NADH so reducing equivalents must be generated for that purpose; cytoplasmic [NADH]/[NAD] is very low.

Pathways converting lactate, pyruvate, and citric acid cycle intermediates to oxaloacetate can all be used to generate glucose.

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TCA

The carbon skeletons of certain amino acids are readily made into OAA (glycogenic amino acids) and so protein can be sacrificed to make glucose.

It is clear that these two paths must be coordinately regulated to avoid wasteful futile cycles. When glycolysis is “on”, gluconeogenesis should be “off”

Important regulatory enzymes of opposing pathways are located atthose steps where the two pathways are not identical.

SUMMARY: Pathways from glucose to pyruvate and pyruvate to glucose are regulated by both the level of respiratory fuels and energy charge. Thus, whenever cell has ample ATP and respiratory fuels such as acetyl-CoA, citrate, or NADH glycolysis is inhibited and gluconeogenesis promoted.

Regulation of glycolysis and gluconeogenesis

1

2

3

1

2

3

In liver, PK inhibited by phosphorylation

These enzymes are in different cell compartments

Role of F2,6 BP in the Interconversionof F-1,6-BP to F-6-P.

Figure 18-24 Formation and degradation of β-D-fructose-2,6-bisphosphate is catalyzed by PFK-2 and FBPase-2.

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