BCM 221 LECTURE BY OJEMEKELE O OUTLINE KETONE

BCM 221 LECTURE BY OJEMEKELE O.

OUTLINE • KETONE BODIES AND KETOSIS • PYRUVATE DEHYDROGENASE AND ALPHA KETOGLUTARATE DEHYDROGENASE COMPLEXES • TRICARBOXYLIC ACID CYCLE • INTERELATIONSHIP OF FAT AND CARBOHYDRATE METABOLISM

KETONE BODIES AND KETOSIS • Ketone bodies are water soluble, transportable equivalents of fatty acids • They are acetone, hydroxybutyrate. acetoacetate and β- • Acetone is a waste product and is usually exhaled or excreted from the body via urine. • The other two are used as source of energy by brain and heart tissues.

KETONE BODIES

BIOCHEMICAL BASIS FOR PRODUCTION OF KETONE BODIES • Ketone bodies are produced from acetyl Co. A in the liver mitochondria, by a process called ‘ketogenesis’. • They are produced during periods of starvation and condition of diabetes mellitus.

A. STARVATION: When there is scarcity of glucose, the body utilizes fats as energy source. Increased break down of fats yields high amounts of acetyl Co. A. TCA cycle cannot utilize all the acetyl Co. A, excess acetyl Co. A is used for production of ketone bodies. • B. DIABETES MELLITUS: In diabetes, insulin deficiency reduces glucose availability to cells. Fatty acid breakdown then becomes a major source of energy to most cells. Excess acetyl Co. A produced from oxidation of fatty acids is used for production of ketone bodies.

FIG: Ketogenesis pathway

UTILIZATION OF KETONE BODIES FOR ENERGY • β-hydroxy butyrate is converted to acetoacetate by β-hydroxy butyrate dehydrogenase for energy. • Acetoacetate is then converted to Acetoacetyl Co. A by “succinyl Co. Aacetoacetate Co. A transferase”. • This enzyme is absent in liver. Hence the liver cannot use ketone bodies for energy. Thus, ketone bodies formed in the liver are transported to peripheral tissues such as brain and heart for energy production. • Acetoacetyl Co. A is then cleaved by thiolase to produce two molecules of acetyl Co. A which can enter the TCA cycle for energy production • Note: the brain cannot use fat for energy, due to inabilty of fats to cross blood brain barrier(BBB). It relies on glucose for energy, in periods of glucose scacity, the brain gets a portion of energy from ketone bodies.

Succinyl Co. A – Acetoacetate

KETOSIS • When ketone bodies are produced in excess quantities, they accumulate in the body, this state is known as ketosis. Acetone can be perceived in breath of individual with ketosis. • When even larger amounts of ketone bodies accumulate such that the blood p. H is lowered to dangerously acidic levels, the state is called ketoacidosis. This is a life threatening condition because it impairs liver and kidney function.

• PYRUVATE DEHYDROGENASE COMPLEX Inside the mitochondria, pyruvate is oxidatively decarboxylated to acetyl Co. A by pyruvate dehydrogenase (PDH). • It is a multi-enzyme complex with 5 coenzymes and 3 apoenzymes. The coenzymes needed are: 1. Thiamine pyrophosphate (TPP) 2. Co-enzyme A (Co. A) 3. FAD 4. NAD+ 5. Lipoamide. (The lipoic acid). The Apoenzyme part of the PDH complex is made up of three component enzymes. 1. Pyruvate Dehydrogenase (Enzyme 1): It catalyses decarboxylation. TPP is required in this step. So, Thiamine, vitamin B 1 is essential for utilization of pyruvate. •

The two carbon unit remains attached to the enzyme, as hydroxyethyl thiamine pyrophosphate. 2. Dihydro Lipoyl Trans Acetylase (Enzyme 2): Then, hydroxyethyl group is oxidized to form an acetyl group and then transferred from TPP to lipoamide to form acetyl lipoamide. 3. Dihydro Lipoyl Dehydrogenase (Enzyme 3): The last step is the oxidation of lipoamide. At the end of the reaction the cofactors, namely TPP, Lipoamide and FAD are regenerated (Fig. 1)

• Importance of Pyruvate Dehydrogenase reaction is that it is through this reaction that acetyl Co. A is produced from glucose breakdown in order to enter TCA cycle. Thus this reaction links glycolysis to TCA cycle. • Another significance is that it is a completely irreversible process. Glucose through this step is converted to acetyl Co. A from which fatty acids can be synthesized. But the backward reaction is not possible, and so there is no net synthesis of glucose from fatty acids.

Figure 1. PDH COMPLEX CATALYZED REACTION

TRICARBOXYLIC ACID CYCLE SIGNIFICANCE OF TCA CYCLE • Citric acid cycle, a series of reactions also known as the tricarboxylic acid (TCA) cycle or the Krebs cycle is the final common pathway for the oxidation of acetyl Co. A to CO 2. Acetyl Co. A is derived from pyruvate, ketogenic amino acids and beta oxidation of fatty acids. • Citric acid cycle is the source of reduce coenzymes (form NADH and FADH 2) that are oxidized via electron transport chain and ATP is produced in the process.

• The overall pattern of the citric acid cycle is shown in Figure 2. 1. A four- carbon compound (oxaloacetate) condenses with a two-carbon acetyl unit to yield a six-carbon tricarboxylic acid (citrate). 2. 3. Aconitase then converts citrate to an isomer of citrate (isocitrate) NADH is produced in this step. 4. The resulting five-carbon compound (alpha-ketoglutarate) also is oxidatively decarboxylated to yield a four carbon compound succinyl Co. A. The Second NADH is produced in this step. 5. Succinyl Co. A is the converted to Succinate by Succinyl Co. A synthetase. Note that GTP is produced in this step 6. Succinate is dehydrogenated (oxidized) to fumarate by Succinate dehydrogenase, note that FADH 2 is produced in this step 7. Fumarate undergoes hydration and is converted to malate 8. Malate is dehydrogenated to oxaloacetate. Oxaloacetate is regenerated in this step. Note that this reaction catalyzed by Malate dehydrogenase also produces NADH NOTE: Two carbon atoms enter the cycle as an acetyl unit and the two carbon atoms leave the cycle in the form of two molecules of carbon dioxide in steps 3 and 4).

FIG. 2 TCA CYCLE

10 ATP MOLECULES ARE PRODUCED FROM ONE ROUND OF TCA CYLCE • A total of 10 ATP molecules are generated during one cycle. • NADH produces 2. 5 ATP molecules and FADH produces 1. 5 ATP molecules, when oxidized via electron transport chain. • 1 GTP produced by Succinyl Co. A synthetase reaction is equivalent to 1 ATP. 3 NADH (from steps catalyzed by ISD, alpha KGD and MDH) = 3 X 2. 5 ATP = 7. 5 ATP 1 FADH 2 (from step catalyzed by SDH )= 1 X 1. 5= 1. 5 ATP 1 GTP= 1 ATP TOTAL= 7. 5+ 1. 5 + 1= 10 ATP molecules

ALPHA KETOGLUTARATE DEHYDROGENASE COMPLEX • Alpha ketoglutarate is the enzyme that catalyzes oxidative decarboxylation of alpha ketoglutarate to form succinyl Co. A. • It is a multi enzyme complex and is similar to pyruvate dehydrogenase (PDH) complex. It has three enzymes and five coenzymes, just like PDH complex (figure 3)

Alpha ketoglutarate catalyzed reaction

INTERRELATIONSHIP BETWEEN CARBOHYDRATE AND FAT METABOLISM • Carbohydrate can be converted to fat. Glucose is a precursor for both the glycerol and the fatty acid components of triacylglycerols (fat). • The glycerol portion of TAG which comes in as Glycerol 3 -phosphate can be formed from dihydroxyacetone phosphate (which is derived from glucose). This is shown in fig 4 (in note)

• It should be noted that although carbohydrate (glucose) can be converted into both the glycerol and the fatty acids components of fat, only the glycerol portion of fat can be converted to carbohydrate • This is because the pyruvate dehydrogenase reaction is not reversible (i. e Acetyl Co. A derived from fatty acids cannot be converted to pyruvate or glucose)