Overview of Metabolism &

The Citric acid cycle

Dr.S.Chakravarty M.D.

Carbohydrates

Fats

36+-/

Proteins

recycling

3 Stages Of Metabolism

1

2

3

Definition of TCA cycle

The citric acid cycle (Krebs cycle, tricarboxylic acid cycle) is a sequence of reactions in mitochondria that oxidizes the acetyl moiety of acetyl-CoA and reduces coenzymes that are reoxidized through the electron transport chain, linked to the formation of ATP.

Biomedical Importance• The citric acid cycle is the final common pathway for the

oxidation of carbohydrate, lipid, and protein because glucose, fatty acids, and most amino acids are metabolized to acetyl-CoA or intermediates of the cycle.

• It is a source of reduced co-enzymes that provide the substrates for the respiratory chain.

• It is both catabolic and anabolic (amphibolic).

contd..

Biomedical Importance• It also has a central role in gluconeogenesis, lipogenesis, and

interconversion of amino acids. – So, components of the cycle have a direct or indirect controlling effects in key

enzymes of other pathways.

• Many of these processes occur in most tissues, but the liver is the only tissue in which all occur to a significant extent. – The repercussions are therefore profound when, for example, large numbers

of hepatic cells are damaged as in acute hepatitis or replaced by connective tissue (as in cirrhosis).

• Very few, if any, genetic abnormalities of citric acid cycle enzymes have been reported; such abnormalities would be incompatible with life or normal development.

STEP 1 PYRUVATE to Acetyl CoA

Enzyme :- Pyruvate Dehydrogenase Complex

ADVANTAGES OF A MULTI ENZYME COMPLEX :-

– RAPID TRANSFER OF INTERMEDIATES IN BETWEEN ACTIVE SITES OF INDIVIDUAL ENZYMES ENHANCING REACTION RATE

– MINIMIZES SIDE REACTIONS BY CHANNELING INTO A SINGLE PATHWAY AND DECREASING OUTSIDE REACTIONS .

– EFFECTIVE CONTROL AND COORDINATION OF THE REACTIONS BETTER METABOLIC CONTROL

Site :- Inner mitochondrial membrane

• E1 - Thiamine pyro phosphate (TPP) (B1) • E2 – Lipoic acid Co-enzyme-A – (Pantothenic acid)

• E3 – NAD – Niacin (B3) FAD – Riboflavin (B2)

The Enzyme subunits

Tender – Thiamine Loving - LipoamideCare -CoASHFor – (FAD)RiboflavinNancy- NAD (Niacin)

3 Enzymes:- 1)PDH,

2)α-KGDH(TCA cycle)

3)Branched keto acid dehydrogenase

How does Pyruvate enter mitochondria?

• Symport along with H+ ions

IMPORTANCE

• PDH is IRREVERSIBLE ( Fats cannot be converted to glucose.)

• COMMITTED STEP in oxidation of glucose.

• ENERGETICS :- 1 NADH IS GENERATED = 2.5 ATP

• REGULATION :- – End product as well as covalent modification– Phosphorylation of enzyme by a kinase decreases the activity and

dephosphorylation decreases the activity.

Regulation of PDH enzyme:

1. Regulation by end product inhibition (Allosteric)

2. Regulation by Covalent modification:

• PDH kinase – inactivation of enzyme

• PDH Phosphatase - activation

The final common pathway

Sites

• Tissues :- All tissues

• Subcellular site :- Mitochondrial Matrix

Enzyme bound

Enzyme bound

The Citric Acid Cycle

ATPSubstrate level

phosphorylation

NADH

NADH

FADH2

NADH

Thiamin, lipoate , FAD

Complete oxidation of Acetyl CoA

OAA is viewed as a catalyst , which enters into the cycle , causes complete oxidation of acetyl CoA , and is regenerated in the end without any loss.

Formation of ATPReaction catalyzed by Method of production ATP molecules formed

Isocitrate dehydrogenase Respiratory chain oxidation of NADH

2.5 (3)

-ketoglutarate dehydrogenase

Respiratory chain oxidation of NADH

2.5 (3)

Succinate thiokinase Substrate level phosphorylation

1

Succinate dehydrogenase Respiratory chain oxidation of FADH2

1.5(2)

Malate dehydrogenase Respiratory chain oxidation of NADH

2.5 (3)

Net –> 10 (12)

Significance of the cycle

1)Final common oxidative pathway

2)Fat is burnt on the wick of carbohydrates • Oxidation of fats need the help of Oxaloacetate

which enters into the cycle and is regenerated in the end .

• The major source of OAA is Pyruvate. (Carbohydrate)

3) Excess carbohydrates are converted to neutral fats via citrate and ATP-citrate lyase but not vice versa because Pyruvate dehydrogenase step is irreversible.

T

IRREVERSIBLE

P

4)Amphibolic ( Catabolic and Anabolic )

GABA

HEME

FATTY ACIDS,

STEROLSgluconeogenesis

5) TCA cycle plays an important role in Gluconeogenesis , Transmination and Deamination.

6) Anaplerotic ( filling – up) reactions -> As shown before ,TCA cycle acts as precursors of biosynthetic pathways , e.g Heme .

So, there is constant efflux of carbon units from the cycle .To counterbalance the loss , filling up reactions are necessary .

Eg. - Pyruvate to Oxaloacetate(PYRUVATE CARBOXYLASE) ( most important)

-- Phosphoenolpyruvate to Oxaloacetate (PEP CARBOXYLASE) --Pyruvate to Malate (Malic enzyme)

7) Metabolic traffic regulator -All metabolisms end in TCA. -Availability or lack of intermediates govern the directions of

pathways converging or going out of TCA.

Regulation of Iso-Citrate dehydrogenase:

Isocitrate Dehydrogenase

ATP

NADH

(-)

(-) Iso-citrate

Alpha ketoglutarate

(-)

Accumulation of Citrate

In well fed state:Inhibit Glycolysis

Enters fatty acid synthesis

In well fed state, increase in ATP and NADH will inhibit isocitrate dehydrogenase leading to accumulation of citrate. citrate will enter cytosol and inhibit Glycolysis and activates fatty acid synthesis.

Regulation of citric acid cycle

• Citrate synthase

• Iso-citrate dehydrogenase

• Alpha keto glutarate dehydrogenase

Inhibitors of TCA cycle

• Aconitase – is inhibited by fluoroacetate (non-competitive inhibition)

• -ketoglutarate dehydrogenase is inhibited by Arsenite (non-competitive inhibition)

• Succinate dehydrogenase is inhibited by Malonate (competitive inhibition)

Which of the following is required for cholesterol synthesis in hepatocytes?

• A. Citrate shuttle• B. Glycerphosphate shuttle• C. Malate-Aspartate shuttle• D. Carnitine shuttle• E. Adenine nucleotide shuttle

Thank you