Carbohydrates 2 Flashcards
How is pyruvate transported into the mitochondria? Driving force? (5)
- monocarboxylate transporter (antiporter)
- electrochemical potential is the driving force
- pyruvate goes in and OH comes out
What is the overall pyruvate dehydrogenase reaction? (6)
- a pyruvate molecule from glycolysis is oxidatively decarboxylated to an acetyl CoA for the TCA cycle and CO2
- both reducing equivalents are produced and CO2
What are the five enzymes and cofactors used in the PDH complex?
Enzymes:
- E1- pyruvate dehydrogenase
- E2- dihydrolipoyl transactylase
- E3- dihydrolipoyl dehydrogenase
- PDH Kinase- add phosphate
- PDH phosphatase- remove phosphate
Cofactors:
- Thiamine (Vit B1) pyrophosphate (TPP)
- Lipoic acid
- Coenzyme A (Vit B5)
- FAD (Vit B2)
- NAD+ (Vit B3)
- this is an example of structure and function relationship between enzymes and cofactors allowing them to convert pyruvate to acetyl CoA by passing intermediates from one enzyme/cofactor to the next
Steps of the PDH reaction catalyzed by E1, E2, E3? (8)
- E1 assists in attaching pyruvate to TPP at a reactive carbon
- rearrangements of electrons, and positively charged ring nitrogen of TPP, allows carboxylation - E2 transfers the remaining acetyl group from the ring of TPP to the ring of lipoamide (lipoid acid is directly attached to enzyme 2 via lysine side chain forming lipoamide)
- E2 moves the acetyl group (temporarily attached to itself via lipoamide) onto Coenzyme A to form Acetyl CoA
- the lipoamide is in reduced form and must be reoxidizes by an oxidation/reduction reaction catalyzed by E3, necessary for the process to continue
- the electrons are transiently transferred to FAD to form FADH2, then the electrons are transferred to NAD to form NADH
Summary of PDH complex? (9)
see chart
What is Coenzyme A (CoA)? Structure? Reactivity? (10)
- it is a complex molecule which contains a free sulfhydryl (SH) group
- nucleic acid: adenosine-3-monophosphate
- Pantothenic acid: Vit B5, an essential food factor
- the free sulhydryl group in CoA can react with a carboxyl group to form a thiol ester, used in transfer reactions involving acyl groups, acetyl, fatty acid
- Acetyl CoA- the acetyl group replaces the H on SH to form it
Why is Vit B5 an essential component of CoA? Deficiency?
- it is used for acyl (acetyl) transfer
- fatty acid synthesis
- deficiency:
- dermatitis
- enteritis
- alopecia (hair loss of baldness)
What is TPP? function?
- derivative of B1
- serves as a cofactor for:
- pyruvate dehydrogenase (glycolysis)
- alpha ketoglutarate dehydrogenase (TCA cycle)
- transketolase (PPP)
- branched chain AA dehydrogenase (AA catabolism)
What products inhibit PDH? (11)
-Acetyl CoA and NADH inhibit PDH in competitive fashion as feedback inhibitors
Regulation of PDH by covalent modification? (12)
- phosphorylation and dephosphorylation of E1
- PDH kinase phosphorylates and inactivates the complex
- PDH dehydrogenase dephosphorylates and activates the complex
Allosteric regulation of PDH? (14)
- NADH and Acetyl CoA inhibit PDH competitively, they also activate PDH kinase allosterically which leads to PDH phosphorylation (inactivation)
- Pyruvate, CoASH, NAD, ADP enhance PDH activity by inhibiting PDH kinase activity
- Mg and Ca enhance PDH activity by stimulating PDH phosphatase activity
Hormonal regulation of PDH?
- insulin
- glucagon
- catecholamines
- dont know mechanism
What are symptoms of PDH deficiency? treatment? (16)
symptoms:
- increased serum levels of pyruvate, lactate, alanine
- chronic acidosis (low pH)
- severe neurological defects, which could lead to death
- treatment:
- diet reduced carbohydrates
- dicholoacetate, inhibitor of PDH kinase
Alpha ketoglutarate dehydrogenase? similarities with PDH? (17)
- similar to PDH, shares same cofactors, but only uses enzymes E1, E2, E3
- does not have its own kinase and phosphatase so it catalyzes a similar reaction but regulation is different - reduce 5C down to 4C to release CO2, in PDH 3C is reduced to 2C to release CO2
- alpha keto acid dehydrogenase complex uses same cofactors as PDH and alpha ketoglutarate
- deficiency of cofactors will affect activity of both kinds of enzymes
What is Vit B1 a cofactor for?
- pyruvate dehydrogenase (glycolysis)
- alpha ketoglutarate dehydrogenase (TCA cycle)
- transketolase (PPP)
- branched chain AA dehydrogenase (AA catabolism)
What are the 3 names for the PPP?
- Pentose phosphate pathway
- phosphogluconate pathway
- hexose monophosphate shunt
What are the important enzymes and products in the oxidative phase of PPP? (20)
enzymes:
- G6P dehydrogenase (G6PDH)
- 6 phosphogluconate dehydrogenase
products:
- NADPH (anabolism)
- 5C sugar (Ribulose 5 Phosphate) for DNA/RNA synthesis
Describe the oxidative phase of PPP? (20)
- G6P is oxidized and decarboxylated to pentose phosphate, catalyzed by 2 enzymes:
- G6P dehydrogenase and 6 phosphogluconate dehydrogenase - NADPH is generated in this path is important for fatty acid synthesis, glutathione reduction, and other anabolisms
- Ribulose 5 phosphate can be converted into Ribose 5 phosphate and used for nucleotide biosynthesis and other important factors such as CoA, NAD, FAD
- irreversible phase
Deficiency in G6P dehydrogenase? (20)
- results in imbalance between NADPH and NADP+
- lower NADPH levels will in turn affect glutathione status
- due to dependency on PPP for generation of NADPH, erythrocytes are more sensitive to G6P dehydrogenase deficiency
- mutations of this enzyme could result in hemolytic anemia
Describe the non oxidative phase of PPP? (20)
- various C3, C4, C5, C6, C7 monosaccharides are generated and used for synthesis of glycoproteins (more AA than carb), glycolipids and proteoglycans (more carb than AA)
- transketolase (transfers C2 unit and uses Vit B1 as cofactor) and transaldolase (transfer C3 unit) are involved in these reactions
- PPP eventually loops back to glycolysis path through either F6P or glyceraldehyde-3-phosphate
- reversible reaction
- links other sugars to glycolysis
Regulation of PPP? (23)
- NADPH is a strong inhibitor of glucose 6 phosphate dehydrogenase
- lower NADPH levels will enhance PPP and NADPH generation
- higher NADPH levels will inhibit PPP and NADPH generation
- liver G6PD is an inducible enzyme - the synthesis of liver glucose 6 phosphate dehydrogenase is induced by the increased insulin/glucagon ratios after a high carb meal, favors PPP
- higher carb meals will lead to higher insulin secretion, efficient glucose transport, biosynthesis, and overall growth
G6PDH deficiency?
- the most common human enzyme defect
- x linked recessive disorder
- hemolysis induced by oxidative stress (RBC vulnerable)
- cannot produce enough NADPH
Summary of everything? (26)
see chart
Glucokinase nuclear translocation is enhanced by
A. Glucose
B. Glucose 6-phosphate
C. Fructose-6-phosphate
D. Pyruvate
E. Acetyl CoA
C
The Order of the Affinities of Glucose Transporters is
A. Glut1<glut2></glut2>B. Glut1<glut4></glut4>C. Glut2<glut1></glut1>D. Glut2<glut4></glut4>E. Glut4<glut1>
</glut1>
D
