Cataracts and Biochemistry of the Lens

 

Original questions:

 

http://biochemistryquestions.wordpress.com/2008/11/03/q-about-cataracts-in-diabetes/

 

http://biochemistryquestions.wordpress.com/2008/10/20/about-a-baby-with-cataracts/

 

The light should pass through the cornea, aqueous humor, lens and vitreous humor before reaching the retina for triggering the process of vision. These structures should be transparent in order to allow the path of light.

 

 

The lens (m in the figure) is bathed in its anterior side by the aqueous humor (i) and in its posterior side by the vitreous humor (o). It has no blood capillaries (that would interfere with light path), so the aqueous humor is responsible of the nutrition of the lens and the disposal of metabolic products.

 

The energy necessary for the lens is provided mainly through anaerobic glicolysis; the Krebs cycle, located in peripheral cells, only provide about 5 % of the necessary energy. Pentose phosphate cycle is also another important metabolic pathway in lens since it provides NADPH necessary for the maintenance of the redox status of the lens proteins.

 

The majority of the proteins in the lens are alpha, beta and gamma crystallines. They should maintain a transparent environment, so they should be in a native, non aggregate state. Some disturbances, as changes in the redox states of these proteins or changes in osmolarity in the lens can produce lost of the native state and aggregation of these proteins.

 

Cataracts results from changes in solubility and aggregation of the crystallin proteins.

The most frequent kinds of cataracts are those that appear as result of aging (senile cataracts) or as a result of Diabetes Mellitus (diabetic cataracts). Other conditions can also result in cataracts: cataracts that appear in galactosemia are very similar in the way of production to the cataracts that appear in Diabetes.

 

Aldose reductase is an enzyme that usually reduce aldehyde group of aldoses to a primary alcohol, so the aldose becomes a polyalcohol. The enzyme uses NADPH as hydrogen donor.

 

Typical reactions catalyzed by aldose reductase are the formation of sorbitol (glucitol) and the formation of dulcitol (galactitol):

 

Glucose +NADPH —à Glucitol (Sorbitol) + NADP+

 

Galactose +NADPH –à Galactitol (Dulcitol) + NADP+

Sorbitol (Glucitol)

 

 

Aldehyde reductase function is mainly in the conversion of glucose to fructose.

Dulcitol (Galactitol)

 

 

 

 

The sequence of reactions is:

 

1. – Reaction of Aldehyde reductase:

Consist in the reduction of the aldehyde group of glucose to a primary alcohol group, with the conversion of the aldohexose glucose to a polyalcohol.

 

Glucose + NADPH.H+ –à Sorbitol + NADP+

 

2. – Reaction of Sorbitol dehydrogenase (SORD):

      Consist in the oxidation of the secondary alcohol group of Carbon 2 of Sorbitol to

       a ketone group. It results in the conversion of Sorbitol in Fructose, a ketohexose.

  

      Sorbitol + NAD+ -à Fructose + NADH.H+

 

This sequence of reactions is particularly important in the formation of fructose in the seminal vesicles and the liver, and it has the advantage over the use of the sequence in glycolysis for obtaining fructose – Glucose 6 (P) to Fructose 6 (P) – that this polyalcohol pathway does not require the expending of ATP.

 

Lens contains aldehyde reductase and also a very low activity of sorbitol dehydrogenase, so some of the glucose that enter in the lens is converted in fructose. This quantity is usually very low since the enzyme aldehyde reductase has a very high Km for glucose.

 

In conditions of hyperglycemia, since the concentration of aldehyde reductase substrate (glucose) is high, this enzyme becomes very active, and a high quantity of sorbitol is formed. Unfortunately for diabetic patients, the activity of sorbitol dehydrogenase in lens is very low (enough for normal conditions, but not for this abnormal situation) and for complicating more the problem, sorbitol formed in the lens diffuse with difficulty out of it.

 

As a result, Sorbitol accumulates and increases the osmotic effects producing cell swelling and structural damage (this effect would explain also the neuropathy and vascular problems present in Diabetic patients).

 

In the lens, these changes in osmolarity will affect the native conformation of the crystalline proteins, in such a way that they aggregate and form structures that scatter the light: Cataracts are being formed.

 

In patients with galactosemia, a congenital disease in which the patient can not metabolize galactose and this sugar accumulates, the physiopathology of cataracts formation and nerve damage apparently is similar to the mechanism described for diabetes:

 

Through the reaction of Aldehyde reductase occurs the reduction of the aldehyde group of galactose to a primary alcohol group, with the conversion of the aldohexose galactose to its corresponding polyalcohol, galactitol:

 

Galactose + NADPH.H+ –à Galactitol + NADP+

 

Galactitol accumulates increasing the osmotic pressure with similar results to those found in sorbitol accumulation in Diabetes Mellitus.

 

 

For more information, please visit the following links:

 

About Cataracts

 

Sorbitol: A hazard to diabetes

 

About side effects of Sorbitol

 

About Polyol pathway and arterioral dysfunction in hyperglicemia

 

About galactitol and cataracts formation in galactosemia

 

 

Energetic Balance of the oxidation of 1 mol of Pyruvate up to CO2 and water

Answers to Questions about Carbohydrate Metabolism CM-07 and CM-08

 

Answer to (CM-07 )

 

Which is the energetic balance (expressed in moles of ATP) of the total oxidation of 1 mol of Glucose up to CO2 and water, assuming that the Glycolytic Pathway and the Glycerol Phosphate Shuttle have been used.

 

Answer: 36 moles of ATP

 

Note: We are assuming also, as described in the question, that each mol of NADH.H+ oxidized in the respiratory chain yields 3 moles of ATP and each “mol” of FADH2 oxidized in the respiratory chain yields 2 moles of ATP)

 

Details here.

 

 

Answer to CM-08

 

Which is the energetic balance of the oxidation of 1 mol of pyruvate up to CO2 and water?

 

Answer: 15 moles of ATP

 

Note: We are assuming also, as described in the question, that each mol of NADH.H+ oxidized in the respiratory chain yields 3 moles of ATP and each “mol” of FADH2 oxidized in the respiratory chain yields 2 moles of ATP)

 

Details:

 

Oxidative Decarboxylation of Pyruvate occurs in mitochondria, in a reaction catalyzed by the Pyruvate Dehydrogenase Complex.

 

The global reaction is:

Pyruvate +  NAD⁺ + CoA –à  Acetyl CoA +  CO2 +  NADH.H⁺ 

 

The resultant Acetyl Co A is oxidized  in the Krebs Cycle yielding:

 

Isocitrate to  alfaketoglutarate  (+CO2)                              1 NADH.H⁺

alfaketoglutarate to  Succinyl CoA (+CO2)                         1 NADH.H⁺

Succinyl CoA to Succinate (SLP)                                           1 GTP

Succinate to Fumarate                                                              1 FADH₂

Malate to Oxalacetate                                                                1 NADH.H⁺

 

Observe that at the end of the Kreb Cycle the 3 carbons of pyruvate have been oxidized to 3 CO₂

 

At the same time, from the energetic point of view:

1 ATP have been obtained in the Krebs Cycle as GTP through Substrate level Phosphorylation (1 GTP is equivalent, from the energetic point of view,  to 1 ATP).

1 FADH₂ have been obtained from the Krebs Cycle

4 NADH.H⁺ have been obtained inside the mitochondria (1 from Oxidative decarboxylation of Pyruvate and 3 from the Krebs cycle)

 

If we use the convention that each NADHH+ produce approximately 3 ATP in the Respiratory chain, and each FADH2 produce 2 ATP, the total ATP production is:

 

1 Substrate Level Phosphorylation (SLP)                           01 ATP

4 NADH.H⁺  x 3 ATP/NADH.H                                                12 ATP

1 FADH₂  x  2 ATP/FADH2                                                        02 ATP

 Total                                                                                               15 ATP

 

 

 

 

 

 

 

Energetic Balance of the total oxidation of one mol of glucose up to CO2 and H2O: Understanding the contradictions.

 

For understanding the process of total oxidation of glucose, it is necessary to consider the different steps and metabolic pathways involved as well as the cellular location of these processes:

 

   

STEPS	M. PATHWAY	CELLULAR LOCATION
Glucose to  Pyruvate	Aerobic Glycolysis	Cytosol
Pyruvate to Acetyl CoA	Oxidative Decarboxylation of Pyruvate	Mitochondria
Acetyl Co A to CO2	Krebs Cycle	Mitochondria

  

 

Aerobic Glycolysis describes the oxidation of one mol of Glucose (6 carbons)  up to the formation of two moles of pyruvate (3 carbons each).

This conversion involves different reactions where ATP is produced or consumed:

Glucose to Glucose-6-(P)                                                                          - 1 ATP

 

Fructose-6-(P) to Fructose 1,6- bisphosphate                                 - 1 ATP

 

Since Fructose 1,6 bisphosphate becomes two trioses phosphate, the reactions after the aldolase reaction occurs twice (one for each triose that continues the glycolysis). In one reaction, NADH.H⁺  is produced; in other two reactions, a Substrate Level Phosphorylation (SLP) occur

 (2) Gliceraldehide 3 (P) to (2) 1,3 bisphosphoglycerate           +(2) NADH.H⁺

 

(2) 1,3 bisphosphoglycerate to (2) 3 phosphoglycerate (SLP*)     + 2 ATP

 

(2) Phosphoenol pyruvate to Pyruvate (SLP*)                                    + 2 ATP

 

(*Maybe it is necessary to recall now that there are two different ways of synthesizing ATP:  (a)SLP, in which ATP is synthesized using energy from some reactions of metabolism, like these reactions, and (b) Oxidative Phosphorylation, synthesis of ATP using the energy released in the Respiratory chain, from the oxidation of reduced cofactors)

 

Total energetic Balance from Glucose to (2) Pyruvate

(Aerobic Glycolysis): 

   ^_ 2 ATP  +  2 NADH.H⁺  + 4 ATP  = 2 ATP + 2 NADH.H⁺ in cytosol

 

 

An excellent animation of this process up to pyruvate can be found here.

 

Pyruvate will enter the mitochondria and will experiment Oxidative decarboxylation, in a reaction catalyzed by the pyruvate dehydrogenase complex.

The global reaction is:

2 Pyruvate + 2NAD⁺  +2 CoA –à 2 Acetyl CoA + 2 CO2 + 2 NADH.H⁺

This reaction occurs twice since each glucose (6 carbons) produce 2 pyruvates (3 carbons each), consequently these process produce

2 NADH.H⁺ in the mitochondria

 

Each Acetyl Co A is oxidized  in the Krebs Cycle yielding:

 

2 Isocitrate to 2 alfaketoglutarate            (+2CO2)                     2 NADH.H⁺

2 alfaketoglutarate to 2 Succinyl CoA (+2CO2)                          2 NADH.H⁺

2 Succinyl CoA to 2 Succinate (SLP)                                               2 GTP

2 Succinate to 2 Fumarate                                                                 2 FADH₂

2 malate to 2 oxalacetate                                                                   2 NADH.H⁺

 

An animation with these reactions can be found in this link. 

Observe that at the end of the Kreb Cycle the 6 carbons of glucose have been oxidized to 6 CO₂

 

At the same time, from the energetic point of view:

4 ATP have been obtained through Substrate level Phosphorylation (ATP synthesis without intervention of the energy of respiratory chain): 2 were obtained during the Aerobic Glycolysis and 2 were obtained in the Krebs Cycle as GTP (1 GTP is equivalent, from the energetic point of view,  to 1 ATP).

2 FADH₂ have been obtained from the Krebs Cycle

8 NADH.H⁺ have been obtained inside the mitochondria (2 from Oxidative decarboxylation of Pyruvate and 6 from the Krebs cycle)

2 NADH.H⁺have been obtained in the cytosol through the Aerobic Glycolysis.

 

It is important to do these distinctions about the cellular location of the NADH.H⁺. since those produced in the cytosol should enter the mitochondria to be oxidized. Since the internal mitochondria membrane is impermeable to these nucleotides, (the mitochondria has their own pool of NAD) the NADH.H⁺ produced in the cytosol should enter using one of the shuttles already described for transporting the reduction equivalents of cytosolic NADH.H⁺  through the internal membrane of the mitochondria:

 

a)     the malate aspartate shuttle.

b)     The glycerophosphate shuttle

 

As described in other post, the malate-aspartate shuttle regenerates NADH.H⁺  inside the mitochondria, the energy yielding of the cytoplasmatic NADH.H⁺  is the same as if it was generated directly in the mitochondria

 

With the glycerophosphate shuttle, the reduction equivalents of the cytosolic NADH.H⁺  are transferred to FAD in the inner membrane. It means that the cofactor that will be oxidized in the respiratory chain when this shuttle is used, is FADH₂ (see the explanation in this related post)

 

In conclusion, when the malate aspartate shuttle is used for transporting the reduction equivalents from the cytosolic NAD+ inside the mitochondria, we can consider that the t oxidation of glucose has produced:

4 ATP

10 NADH.H⁺  to be oxidized in the Respiratory chain

2 FADH₂ to be oxidized in the Respiratory Chain.

 

If we use the convention that each NADHH+ produce approximately 3 ATP in the Respiratory chain, and each FADH2 produce 2 ATP, the total ATP production is:

 

Substrate Level Phosphorylation (SLP)                           04 ATP

10 NADH.H⁺  x 3 ATP/NADH.H                                          30 ATP

02 FADH₂  x  2 ATP/FADH2                                                04 ATP

 Total                                                                                           38 ATP

 

Using the same convention (each NADHH+ produce approximately 3 ATP in the Respiratory chain, and each FADH2 produce 2 ATP,), but now assuming that the shuttle use is the glycerophosphate shuttle:

 

Substrate Level Phosphorylation (SLP)                         04 ATP

08 NADH.H⁺  x 3 ATP/NADH.H                                        24 ATP

04* FADH₂  x  2 ATP/FADH2                                            04 ATP

 Total                                                                                         36 ATP

(*02 from the use of the glycerophosphate shuttle and 02 from the Krebs Cycle)

 

It explains that some textbooks say that the energetic Balance of the Total Oxidation a a mol of Glucose is 36-38 moles of ATP (since it depends on the shuttle that is used for entering the reduction equivalents of the NADH.H+ produced in the cytosol through glycolysis).

 

Other possible results:

 

As described in other post, some books use the convention that each mol of NADH.H+, when oxidized in the respiratory chain, produce approximately 2.5 moles of ATP, while each mol of FADH2 produce 1.5 moles of ATP. Using this convention:

When the malate-aspartate shuttle is used:

Substrate Level Phosphorylation (SLP)                         04 ATP

10 NADH.H⁺  x 2.5 ATP/NADH.H                                     25 ATP

02 FADH₂  x  1.5 ATP/FADH2                                           03 ATP

 Total                                                                                         32 ATP

 

When the glycerophosphate shuttle is used:

Substrate Level Phosphorylation (SLP)                         04 ATP

08 NADH.H⁺  x 2.5 ATP/NADH.H                                    20 ATP

04* FADH₂  x  1.5 ATP/FADH2                                        06 ATP

 Total                                                                                        30 ATP

(*02 from the use of the glycerophosphate shuttle and 02 from the Krebs Cycle)

 

It explains that some textbooks say that the energetic Balance of the Total Oxidation a a mol of Glucose is 30-32 moles of ATP

 

An advice: When solving a problem of this kind is absolutely necessary to know the conventions used for the yielding of the reduced cofactors (2.5 or 3 ATP/ NADH.H⁺ ? 1.5 or 2 ATP/ FADH₂?) and the kind of shuttle that has been used for entering the reduction equivalents from the cytosol to the mitochondria.  A fair question will have both information. If not information is provided in the question, use the conventions followed by your professor during the lectures.

For students preparing for USMLE exams, the most used review books, like Harvey and Champe, in the “Lippincott Illustrated Reviews” of Biochemistry, Dawn B. Marks in “Biochemistry, Board Review Series”, Kaplan Biochemistry Lecture Notes for USMLE and “First Aid for the USMLE Step I”, agree in using the equivalence of  ‘approximately” 3 ATP for each NADH.H⁺  that is oxidized and ‘approximately” 2 ATP per FADH₂. 

 

Related post:

To understand the mechanism of the shuttles and their difference in yielding ATP, I strongly recommend to read this post.