Carb metabolism: special pathways week 2 Flashcards
What is the cellular localization of the pentose phosphate pathway (PPP)?
What tissues utilize the PPP?
What are the functions of this pathway?
What are the 2 phases of this pathway?
The pentose phosphate pathway is also known as the hexose monophosphate shunt. It produces reducing power in the form of NADPH, and pentose intermediates.
- Located in the cytosol of liver, mammary gland, skin, adipocytes and red blood cells.
- NADPH is needed for fat synthesis and to protect against free radicals (glutathione).
- Also converts hexoses into pentoses, needed for RNA and DNA synthesis.
- Also interconverts C3, C4, C6 and C7 sugars for further use or to redirect them to the glycolysis pathway.
The Pentose Phosphate Pathway Has Two Phases
- In the first stage, a hexose-P is decarboxylated to a pentose-P, including two oxidation steps that lead to formation of NADPH.
- Then 6 molecules of pentose-P undergo rearrangements to yield 5 molecules of hexose-P.
List and explain the steps and enzymes involved in the first phase of the PPP. What is produced in this phase?
What is the committed step in this phase? What enzyme is regulated and what is it regulated by?
In this phase, 2 molecules of NADPH, 1 molecule of ribose-5-phosphate and 1 carbon dioxide is produced.
- G6P is dehydrogenated at the C1 to form 6 phosphoglucono-delta-lactone and NADPH by glucose-6-phosphate dehydrogenase. This is the regulatory step and the committed step. NADPH/NAD regulates the enzyme, which is strongly inhibited by NADPH and fatty acyl-CoA, the end-product of fatty acid synthesis.
- In the next step the lactone is converted into 6 phosphogluconate.
- In the next step, 6-phosphogluconate is converted to ribulose 5-P (another dehydrogenation and a decarboxylation) by 6-phosphogluconate dehydrogenase with conversion of NADP+ to NADPH + H+.
- In the final step, ribulose 5-P is isomerized to ribose 5-P by ribose 5-phosphate isomerase.
What are the uses of NADPH?
What are the uses of ribose-5-phosphate?
What blood disese can glucose-6-phosphate dehydrogenase deficiency cause?
What drugs, if given to pts with G-6-P-DH deficiency, cause this disease?
What is the mechanism of this disease?
Antimalarials, antipyretics or sulfa antibiotics given to some patients can cause acute hemolytic disease within 48-96 hrs. This is usually due to a genetic deficiency of glucose 6-phosphate dehydrogenase. When the demand for NADPH is high, the enzyme-deficient cells cannot keep up with the demand. Also, the cells will not reduce enough NADP to maintain glutathione in the reduced state and thus erythrocyte membranes will be damaged. Young red blood cells may have a more active variant. After an episode of hemolysis, young red blood cells will predominate and thus diagnosis may not be possible until the cells age. There are more than 300 variants of G-6-P-DH.
What is the overall goal of phase 2 of the PPP?
In this phase, sugar phosphates of different carbon number will be produced that will be converted to glycolytic intermediates.
- Certain cells have a greater need for NADPH for reductive biosynthesis than for ribose 5-P.
- A sugar rearrangement forms triose, tetrose, hexose and heptose sugars from the pentoses thus creating a means of disposal and providing a link between the pentose phosphate pathway and glycolysis.
There are multiple outcomes of phase 2 of the PPP. One involves scavening ribose-5-P from the PPP or from degradation of nucleic acids.
Generally, what classes of enzymes are needed for this?
Specifically, what 2 enzymes are key in this? What cofactors are required?
What 2 glycolytic intermediates are formed?
- For interconversions, several enzymes should work in concert. These are: epimerases, isomerases, transaldolases and transketolases.
- Transketolase, which requires TPP and Mg2+ transfers a C2 unit, while transaldolase transfers a C3 unit to form sugar phosphates of different carbon number.
- Glyceraldehyde-3-phosphate and fructose-6 phosphate are formed.
- Thus, excess ribose 5-P whether from initial oxidation of G6P or from the degradation of nucleic acids, is scavenged for glycolysis.
A second possible outcome of the second phase of the PPP is that glucose-6-phosphate can be completely oxidized to CO2.
What tissue type is able to perform this?
What enzymes are involved?
How many cycles of this is needed to completely oxidize G6P to CO2?
How many NADPHs are produced?
- Certain tissues, like the lactating mammary gland, have a pathway for complete oxidation of G6P to CO2, with concomitant reduction of NADP+ to NADPH.
- Ribulose 5-P made by the pentose phosphate pathway is recycled into G6P by transketolase, transaldolase and certain gluconeogenic enzymes.
- Hexose continually enters the system. Six G6P are oxidized to 6 ribulose 5-P and 6 CO2.
6 cycles of this shunt are required to get rid of all 6 carbons of G6P in the form of CO2. This would produce 12 NADPH total. Note that this is a theoretical calculation.
A third possible outcome of phase 2 of the PPP is to produce fructose-6-phosphate and glyceraldehyde-3-P for a purpose other than glycolysis.
Why does this occur? What enzymes are involved?
Pentose Phosphate Pathway Produces Ribose-5 Phosphate
- When more ribose 5-P than NADPH is needed, G6P is converted to fructose 6-P and glyceraldehyde 3-P by glycolysis.
- Two fructose 6-P and 1 Glyceraldehyde 3-P are converted into 3 ribose 5-P by reversal of the transaldolase and transketolase reactions.
Note that this is theoretical. Mostly likely, when the PPP shunt is working, it is used for more than one purpose: interconverting sugars, NADPH production, ribose-5-phosphate production.
Explain the uses of the PPP in the following tissues:
erythrocytes
liver
mammary gland
testes
adrenal cortex
striated muscle
- The pentose phosphate pathway is used in erythrocytes for the production of NADPH to reduce glutathione that fights oxidative attack of the membrane.
- It also works in liver, mammary gland, testis, adrenal cortex; sites of fatty acid or steroid synthesis, all of which require NADPH.
- Liver also uses the NADPH in the cytochrome P450 system in detoxification reactions.
- In contrast, in mammalian striated muscle, which has little fatty acid or steroid synthesis, all catabolism proceeds via glycolysis and the TCA cycle and no direct oxidation of glucose 6-P occurs through the PPP.
What types of rxns are need to interconvert carbs? What enzymes are required for these interconversions?
What process needs to happen to sugars for them to enter biochemical rxns? What is the purpose of these rxns?
Isomerization and phosphorylation are common reactions for interconverting carbohydrates:
- Hexoses, taken up by food, can be interconverted to each other.
- Phosphorylation is needed to enter the interconversion, which is catalyzed by different kinases. Hexokinase for glucose and mannose, galactokinase for galactose and fructokinase for fructose.
- Isomerases and mutases convert sugar phosphates into each other.
Sugars need to be activated to enter biochemical reactions:
- Nucleotide-linked sugars are produced to activate sugars. These are used in the interconversion and in producing complex carbohydrates.
- Sialic acid (N-acetylneuraminic acid), xylulose and modified sugars (N-acetyl glucosamine, N-acetyl galactosamine and N-acetyl mannosamine) are synthesized in these pathways to be used to form special oligosaccharides and glycosaminoglycans. These can modify proteins and lipids to form glycoproteins, glycopilids and proteoglycans.
Explain the enzymes and intermediates involved in the conversion of fructose to glucose.
In the liver, fructose is phosphorylated by a special ATP linked kinase (fructokinase) yielding fructose 1-P. A special aldolase (F1P aldolase, aldolase B) then cleaves F-1-P to make one dihydroxyacetone phosphate (DHAP) and one glyceraldehyde. The latter is reduced to glycerol. Two DHAP then forms glucose.
Fructose
↓
F-1-P
↓
DHAP + Glyceraldehyde
↓
Glycerol
↓
DHAP <–G-3-P
↓
Glucose
What disease results when fructokinase is deficient? What are signs of this disease?
Essential Fructosuria: Deficiency of Fructokinase
Fructose may account for 30-60% of the total CHO intake of mammals and is metabolized in a special pathway. Fructokinase, the first enzyme in the pathway, is deficient in essential fructosuria. Following intake of fructose, blood levels and urinary fructose are unusually high, however 90% is eventually metabolized.
What disease results from aldolase-B deficiency? In what organ is this enzyme deficient? What are the consequences of this disease?
Patients with hereditary fructose intolerance are deficient in the liver aldolase B responsible for splitting fructose 1-phosphate into dihydroxyacetone phosphate and glyceraldehyde. It is characterized by severe hypoglycemia after ingestion of fructose since F1P accumulates intracellularly and upregulates the release of glucokinase from nuclear storage. Prolonged ingestion of fructose may lead to death in young children. Consumption of fructose by these patients results in the accumulation of fructose 1 phosphate and depletion of Pi and ATP in the liver. Tying up Pi in the form of fructose 1-phosphate makes it impossible for liver mitochondria to generate ATP by oxidative phosphorylation. The ATP levels fall precipitously, making it also impossible for the liver to carry out its normal work functions. Damage results to the cells in large part because they are unable to maintain normal ion gradients by means of the ATP-dependent cation pumps. The cells swell and eventually lose their internal contents by osmotic lysis.
Why do humans have a limited capacity to handle fructose? What enzyme is involved?
Although patients with fructose intolerance are particularly sensitive to fructose, humans in general have a limited capacity to handle this sugar. The capacity of the normal liver to phosphorylate fructose greatly exceeds its capacity to split fructose 1-phosphate (fructokinase is fast, aldolase B is slow). This means that fructose use by the liver is poorly controlled and that excessive fructose could deplete the liver of Pi and ATP (through F1P accumulation due to fructokinase activity. also, F1P signals glucokinase to be released from the nucleus, further depleting ATP stores). Fructose was actually tried briefly in hospitals as a substitute for glucose in patients being maintained by parenteral nutrition. The rationale was that fructose would be a better source of calories than glucose because fructose utilization is relatively independent of the insulin status of a patient. Delivery of large amounts of fructose by intravenous feeding was soon found to result in severe liver damage. Similar attempts have been made to substitute sorbitol and xylitol for glucose. These sugars also tend to deplete the liver of ATP and, like fructose, should not be used for parenteral nutrition.
What pathway do spermatozoa use to produce fructose for energy utilization?
The major energy source for spermatozoa is fructose. It is formed from glucose by NADP dependent reduction of glucose to sorbitol, followed by an NAD+ dependent oxidation of sorbitol to fructose. Fructose is secreted from seminal vesicles in a fluid that becomes part of the semen. Tissues that come in contact with semen utilize fructose poorly, allowing fructose to be conserved for the sperm. Since spermatozoa contain mitochondria, they can metabolize fructose.
What disease processes can galactosemia lead to?
The absence of which enzyme involved in galactose metabolims causes more severe disease?
What products of galactose metabolism lead to disease processes in galactosemia?
In galactosemia, patients are unable to metabolize galactose from lactose (milk sugar) to glucose metabolites. They often have cataract formation, growth failure, mental retardation, or eventual death from liver damage. It is caused by cellular deficiency of either galactokinase, which causes a relatively mild disorder of cataract formation, or of galactose 1-P uridylyltransferase, resulting in severe disease. Galactose is reduced to galactitol in a reaction similar to reduction of glucose to sorbitol. Galactitol is the initiator of cataract formation in the lens and may play a role in CNS damage. Accumulation of galactose 1-P is responsible for liver failure.
Explain the formation of glucuronic acid.
What are 3 uses of glucuronic acid?
Glucuronic acid is synthesized from glucose through activation of glucose to UDP-glucose and reducing it by UDP-glucose dehydrogenase. This enzyme produces NADH in liver cytosol, which needs to be shuttled to the mitochondria.
Uses of glucuronic acid
- Glucuronic acid is a precursor in complex carbohydrate synthesis (glycosaminoglycan polysaccharides).
- Glucuronic acid is crucial in modification of certain drugs and bilirubin to help their excretion.
- Glucuronic acid is also a precursor of L-ascorbic acid. Humans, other primates and guinea pig lack the enzyme that converts L-gulonolactone to L-ascorbic acid and thus ascorbic acid must be ingested as vitamin C.
