Enzyme Question No.10 (E-10)
The measuring of the Km in an isomerase that catalyzes the transformation of different D-carbohydrates in the corresponding L- isomers, show different values, depending on the carbohydrate that is transformed. Given the following carbohydrates with the corresponding values of Km, mark the one for which the enzyme show less affinity:
a) D-glucose Km = 2000 uM
b) D-galactose Km = 4500 uM
c) D-mannose Km = 8000 uM
d) D-Ribose Km = 10000 uM
e) D-fructose Km = 9000 uM
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)
(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.
Once dietetic lipids have been hydrolyzed and absorbed, the fatty acids are reesterified to other lipid products in the intestinal cells, to form new neutral fats, new cholesterol esters, new phospholipids and other lipids.
These new molecules, formed inside the intestinal cell, should be transported to different tissues through the polar environment of blood, so it is necessary to assamble, inside the intestinal cells, these lipidic molecules with amphipatic apolipoproteins, to form Chylomicrons.
Chylomicron remnants will be taken up by liver cells.
The hepatic uptake of chylomicrons remnants is based on a receptor-mediated endocytosis, that involves the recognition of chylomicron remnant ApoE by receptors on the surface of liver cells. It should be noted that throughout this process, chylomicrons have acted as a means of transportating the component of lipids in diet, from the intestinal cells to peripheral tissues as adipose and muscular tissues, including the heart, whose activity depends heavily (up to 80 % in certain physiological conditions) on the oxidation of fatty acids.
Chylomicron metabolic process allows the understanding of the causes and characteristics of Hiperlipoproteinemia Tipe I, familial hyperchylomicronemia or familial lipoprotein lipase deficiency, whose main feature is an increase of chylomicron concentration and a very slow plasma clearence of them. The genetically identified causes of this disease are the deficit of Liporptein Lipase, the production of an abnormal LPL, or the deficit of Apo C-II (in that case, LPL would not be activated. Clinical features are abdominal pain afer the ingestion of fat-rich meals, xanthomas, acute pancreatitis, hepatic steatosis, as result of an excess of fatty deposit in various tissues.
Related links
http://en.wikipedia.org/wiki/Chylomicrons
http://themedicalbiochemistrypage.org/lipoproteins.html#uptake
http://themedicalbiochemistrypage.org/lipoproteins.html#chylomicrons
Related Posts:
Structure and Classification of Lipoproteins
Metabolism of VLDL and Formation of IDL
The Biochemistry Questions Site 