Chapter 16, Problem 1P

(a)

Summary Introduction

To write: A balanced equation of the reactions catalyzed by each enzyme of citric acid cycle.

Concept introduction: Citric acid cycle is also known as the TCA cycle (Tricarboxylic acid cycle) or Krebs’s cycle. Pyruvate obtained from glycolysis is first converted into acetyl groups, these groups enter into citric acid cycle. In citric acid cycle the sugars, amino acids, and fatty acids are oxidized into CO₂ and H₂O.

Explanation of Solution

Explanation:

Enzymes used in TCA are citrate synthase, aconitase, isocitrate dehydrogenase, α- ketoglutarate dehydrogenase, succinyl CoA synthetase, succinate dehydrogenase, fumarase, and malate dehydrogenase.

Before citric acid cycle starts, pyruvates give acetyl-CoA which enters the citric acid cycle. Acetyl CoA reacts with oxaloacetate to produce citrate with the help of citrate synthase as shown in Fig.1.

Aconitase is responsible for conversion of citrate into isocitrate as shown in Fig.2.

Isocitrate dehydrogenase converts isocitrate into α- ketoglutarate as shown in Fig.3.

α- ketoglutarate dehydrogenase complex converts α- ketoglutarate into succinyl- CoA as shown in Fig.4.

Succinyl co A is converted into succinate, the succinyl CoA synthetase catalyses the reaction. Succinate is an intermediate and quickly gets converted into fumarate with the action of succinate dehydrogenase as shown in Fig.5.

In the next step of reaction hydrolysis of fumarate takes place and it gets convert into malate as shown in Fig.6.

Malate dehydrogenase catalyzes malate to get it convert into oxaloacetate as shown in Fig.7.

Pictorial representation: The reactions catalyzed by the enzymes in citric acid cycle is represented in the following figures.

Fig1: Enzyme citrate synthase activity.

Fig2: Enzyme aconitase activity.

Fig3: Enzyme isocitrate dehydrogenase activity.

Fig4: Enzyme α- ketoglutarate dehydrogenase complex activity.

Fig5: Enzyme succinyl CoA synthetase activity.

Fig6: Enzyme fumarase activity.

Fig7: Enzyme malate dehydrogenase activity.

(b)

Summary Introduction

To name: The cofactors which are required for each enzymatic reactions.

Concept introduction:

Citric acid cycle is also known as the TCA cycle (Tricarboxylic acid cycle) or Krebs’s cycle. Pyruvate obtained from glycolysis is first converted into acetyl groups, these groups enter into citric acid cycle. In citric acid cycle the sugars, amino acids, and fatty acids are oxidized into CO₂ and H₂O.

Explanation of Solution

Explanation:

Presence of cofactors in the reactions is mandatory for the functioning of enzymes. Enzymes can catalyze the reaction only in the presence of cofactors.

When isocitrate converts into α- ketoglutarate then isocitrate dehydrogenase enzyme gets involve. The cofactor which is required at this step is NADH.

In the next step when α- ketoglutarate gets convert into succinyl CoA α- ketoglutarate dehydrogenase complex enzyme takes part and NADH is found present to facilitate the enzyme functioning.

GTP is the required cofactor when succinyl CoA synthetase catalyzes the conversion of succinyl CoA into succinate.

Fumarase enzyme uses FADH₂ as its cofactor when fumarate is converted into malate.

NADH is present for the functioning of malate dehydrogenase when malate gets convert into oxaloacetate.

(c)

Summary Introduction

To determine: The types of reactions catalyzed, condensation, dehydration, hydration, decarboxylation, oxidation-reduction, substrate level phosphorylation and isomerization.

Concept introduction:

Citric acid cycle is also known as the TCA cycle (Tricarboxylic acid cycle) or Krebs’s cycle. Pyruvate obtained from glycolysis is first converted into acetyl groups, these groups enter into citric acid cycle. In citric acid cycle the sugars, amino acids, and fatty acids are oxidized into CO₂ and H₂O.

Explanation of Solution

Explanation:

Fig1 is showing the formation of citrate from acetyl coA, this process is Claisen condensation process because methyl group found on acetyl coA gets converted into methylene of citrate.

The conversion of citrate to isocitrate is isomerization reaction. Fig2 is showing the processof conversion of citrate into isocitrate with the formation of one intermediate cis- aconitate, in this process first dehydration and then rehydration takes place. Citrate undergoes dehydration and cis aconitate is obtained, then rehydration of cis aconitate takes place which produces isocitrate.

Fig 3 is showing the production of α- ketoglutarate from isocitrate via oxidative decarboxylation, hydroxyl group of isocitrate is oxidized into carbonyl.

Fig 4 is the reaction of conversion of α- ketoglutarate into succinyl coA which is a oxidative decarboxylation process.

Fig 5 is the step of conversion of succinyl coA into succinate via substrate level phosphorylation process.

The succinate is converted into fumarate via oxidation reaction. Fumarate is converted into L-Malate by hydration reaction. Finally by oxidation reaction  L-Malate is converted into oxaloacetate.

Fig 7 is the dehydrogenation of malate to get convert into oxaloacetate.

(d)

Summary Introduction

To write: Net balanced equation of the catabolic reaction of acetyl CoA to CO₂.

Concept introduction:

Citric acid cycle is also known as the TCA cycle (Tricarboxylic acid cycle) or Krebs’s cycle. Pyruvate obtained from glycolysis is first converted into acetyl groups, these groups enter into citric acid cycle. In citric acid cycle the sugars, amino acids, and fatty acids are oxidized into CO₂ and H₂O.

Explanation of Solution

Explanation:

Acetyl CoA is converted into CO₂ along with amount of energy. Net equation of reaction of acetyl CoA to CO₂ is:

$\begin{array}{l}\text{Acetyl}{\text{coA+3NAD}}^{\text{+}}{\text{+FAD+GDP+Pi+2H}}_{\text{2}}\text{O}\to \\ {\text{2CO}}_{\text{2}}{\text{+3NADH+FADH}}_{\text{2}}{\text{+GTP+2H}}^{\text{+}}\text{+CoA}\end{array}$