Question (B-15): Flavoproteins in Krebs Cycle


In this representation of Krebs Cycle, each reaction, is marked with a number.

Select the two reactions that require the participation of Flavoproteins.


a)     1 and 3


b)     2 and 4


c)      3 and 5


d)     4 and 6


e)     5 and 7


f)       6 and 8


Q: About the production of NADH in the Krebs Cycle


Cycle_de_krebs numerado

In this representation of Krebs Cycle, each reaction is marked with a number.  The reactions where NAD+ is reduced to NADH.H+ are those marked with the numbers:


a)     1,3 and 4


b)     3,4 and 5


c)      3,4 and 6


d)     3,4 and 8


e)     5,6 and 7


f)       4,5 and 6

There is Biochemistry also in “Twilight”!

“And Edward was staring at me curiously, that same, familiar edge of frustration even more distinct now in his black eyes.

I stared back, surprised, expecting him to look quickly away. But instead he continued to gaze with probing intensity into my eyes. There was no question of me looking away. My hands started to shake.

“Mr. Cullen?” the teacher called, seeking the answer to a question that I hadn’t heard.

The Krebs Cycle,” Edward answered, seeming reluctant as he turned to look at Mr. Banner.”



Related posts:


Porphyrias at the movies


Q: About Metabolic Cycles


Glyoxylate Cycle

Glyoxylate Cycle


(I-07) A metabolic cycle can be defined as a metabolic pathway in which one of the final products is at the same time, one of the initial metabolites of the pathway, e.g. one initial metabolite is regenerated through the reactions of the metabolic pathway.  Observe the following metabolites. All of them have in common that they participate in metabolic cycles.


a)     acetyl CoA

b)     arginine

c)      alpha-ketoglutarate

d)     citrate

e)     citruline

f)       fumarate

g)     isocitrate

h)    malate

i)       ornithine

j)       oxalacetate

k)     succinate

l)       succinyl CoA

m)  urea



For each of the following metabolic cycles, which of the metabolites that appears above is the one that initiates and “close” the metabolic pathway, and so, allow us  to consider the pathway as a cycle?


1.- For the Krebs Cycle




2.- For the Urea cycle





Polygamy, Endogamy and Fumarase deficiency.




Original Question


Q ( B-08 ) ¨ The deficit of enzymes of the TCA cycle is rare, indicating the importance of this pathway for survival. Several cases, however, are on record in which there is a severe deficiency of the enzyme that catalyzes the interconversion between fumarate and malate.  The patient is characterized by severe neurological impairment, encephalomyopathy, and dystonia developing soon after birth. Urine contains abnormal amounts of fumarate and other metabolites of Krebs Cycle. Which of the above enzymes would be deficient?



A ( B-08 ): (d) fumarase (aka fumarate hydratase deficiency)



The twin border communities of Hildale, Utah, and Colorado City, Arizona, have the highest prevalence of known fumarase deficiency cases. And around half the world population of known fumarase deficiency patients have been found in Arizona (Birth Defect is plaguing children in FLDS towns)


Hildale, Utah:




Colorado City, Arizona:







Because of the existence of two closed communities in this area, founded by fundamental polygamist whose families are very inbreeded. In fact, it should be noted that it was not the polygamy which produces this high prevalence of the fumarase deficiency, as could be understood from some press reports, but the fact that this polygamy occurred in a closed population and with a high endogamic behavior. Apparently the founders had a recessive gen for the fumarate deficiency and the marriage among relatives of these two families, or among relatives in the same family, produced the high prevalence of this genetic  disease.


For more information about the history of these communities and the prevalence of fumarase deficiency in them, follow these links:


Polygamist community faces rare genetic disorder



Tracing the Polygamist’s Family Tree






More information about Fumarase deficiency:


Kerrigan, J.F. et al: Fumaric aciduria: clinical and imaging features.


Genetics Home reference: Fumarase deficiency



Composition of Alpha keto Acid Dehydrogenase Multienzymatic Complexes


Original Question


Q (B-07): This is the enzyme of the Krebs cycle that is most similar to pyruvate dehydrogenase.


A (B-07): (b) Alpha-keto glutarate dehydrogenase


It is a multifunctional enzyme complex formed by the association of molecules of three different enzymes that participate directly in the reaction, and require five cofactors. 


Pyruvate Dehydrogenase Complex:


– E1: Pyruvate dehydrogenase (aka Pyruvate decarboxylase)

          Cofactor: TPP

– E2: Dihidrolipoyl transacetylase

          Cofactors: Lipoic acid; Co A

– E3: Dihydrolipoyl dehydrogenase

          Cofactors: FAD, NAD


Alphaketoglutarate Dehydrogenase complex:


-E1: Ketoglutarate dehydrogenase

        Cofactor: TPP

-E2: Dihydrolipoyl succinyl transferase

        Cofactors: Lipoic acid; Co A

-E3: Dihydrolipoyl dehydrogenase

        Cofactors: FAD, NAD


A very similar complex is the Branched chain alpha keto acid dehydrogenase complex. It results of association of molecules of the following enzymes and cofactors:  


-E1: Branched chain alphaketoacid dehydrogenase (aka branched chain alpha keto acid decarboxylase)

        Cofactor: TPP

-E2: Dihydrolipoyl transacylase (aka dihydrolipoyl acyltransferase)

       Cofactors: Lipoic acid; Co A

-E3: Dihydrolipoyl dehydrogenase

         Cofactors: FAD, NAD


Because of the presence of Lipoic acid in these complexes, they are affected by arsenic intoxication (see related posts: Question B-02 and Answer B-02)



Mutations in E1 or E2 of the Branched chain ketoacid dehydrogenase complex produces the Maple Syrup Urine Disease (Branched chain ketoaciduria), characterized by vomiting, convulsions, and early death or mental retardation in survivors (See related posts: Question P-01 and Answer P-01)



About Substrate Level Phosphorylation (SLP) in the Krebs Cycle.

Original Question.


Q (B-06): This enzyme catalyzes the SLP reaction of the Krebs Cycle in which GDP is phosphorylated to GTP.







A (B-06): (h) Succinyl Co A synthase (aka Succinyl CoA synthetase, aka Succinyl thiokinase)





In the succinyl CoA synthetase reaction, the thioester bond between HS-CoA and the succinyl group is hydrolyzed. 


Since it is a rich in energy bond, the energy released is enough for synthesizing GTP from GDP + (P). 


This GTP is equivalent, from the energetic point of view, to ATP. In fact, GTP can transfer the (P) group to ADP to form ATP:


GTP + ADP ————–à GDP + ATP


Since ATP can be produced from this reaction, without participation of the respiratory chain, this process is called Substrate Level Phosphorylation (SLP) in contrast to the Oxidative Phosphorylation (ATP synthesis using the energy released in the Electron Transport Chain).


A few other reactions in metabolism are also coupled with ATP synthesis without participation of the respiratory chain. They are considered also SLP reactions.