CFB 24: Citric Acid (TCA) Cycle

Reactions of the TCA cycle take place within the...

Mitochondrial matrix

What can be transported across inner membrane?

Pyruvate, citrate, α-ketoglutarate, malate, ATP/ADP/Pi

What does not cross the inner membrane?

Acyl CoA, oxaloacetate, NAD+/NADH

TCA cycle

Oxidation of Acetyl CoA to CO2 and hydrogen atoms

What is the coenzyme on E1 of pyruvate dehydrogenase?

Thiamine pyrophosphate

Thaimine deficiency leads to

Beriberi

Wernicke-Korsakoff Syndrome

A young adult is seen with symptoms of fatigue and muscle cramps. The patient is diagnosed to have a thiamine deficiency. Which of the following metabolic acids is the most likely to accumulate under these circumstances?

Pyruvate

3 multiple choice options

What is the active arm on E2 of pyruvate dehydeogenase?

Lipoate

What reoxidizes lipoate to regenerate active coenzyme?

FAD

What is FAD derived from?

Riboflavin (Vitamin B2)

What does FAD do?

Accepts electrons from lipoate onto flavin ring

Symptoms of Riboflavin (Vitamin B2) deficiency

Angular stomatitis

Cheilosis

Glossitis

What does pyruvate dehydrogenase do?

Converts pyruvate to acetyl-CoA

What inhibits PDH?

Acetyl CoA, NADH, ATP, CO2

Pyruvate dehydrogenase is inhibited by _______________.

Phosphorylation

Inhibitor PDH kinase is activated by ______ and inhibited by ___________.

Activated by product

Inhibited by substrates

Is PDH kinase activated/inhibited by low energy (ADP)?

Inhibited

Dephosphorylation _________ PDH.

Activates

What promotes dephosphorylation?

Insulin (well-fed) and Ca++ (exercise)

Summary of pyruvate dehydrogenase regulation

A 15-month-old child presents with sudden onset cough and hyperventilation along with evidence of metabolic acidosis. She has past history of recurrent vomiting, episodes of abnormal posturing, and has been showing regression of developmental milestones. Biochemical examination revealse persistently elevated serum lactate levels with a high lactate pyruvate ratio.

PDH deficiency

Greater than 50% of PDH complex deficiency arise from a defect in ______ pyruvate dehydrogenase from alpha 1.

PDHA1

Most common treatment for PDH deficiency

Cofactor supplementation (Thiamine B1)

Ketogenic diet

Carnitine supplementation

8 intermediates of the TCA cycle

Citrate

Isocitrate

α-ketoglutarate

SuccinylCoa

Succinate

Fumarate

Malate

Oxaloacetate

What is an aresnic target?

α-KGdH

NADH is at complex _ and gives _ ATP

FADH2 is at complex _ and gives _ ATP

NADH, I, 3

FADH2, II, 2

Energy from glutamate occurs especially in what cells?

Intestinal epithelial

A particularly important part of the TCA cycle is connecting _____ _____ to _______.

Amino acids to glucose

Anaplerotic pathways

Pathways that replenish/increase TCA cycle intermediates

What is the point of activating pyruvate carboxylase?

Make oxaloacetate as a way of providing more TCA cycle intermediate

What activates pyruvate carboxylase?

Acetyl-CoA

What two enzymatic steps in the TCA cycle result in production of CO2 through oxidative decarboxylation?

Isocitrate dehydrogenase and α-ketoglutarate dehydrogenase

3 multiple choice options

An increase in which of the following ratios causes an increase in the activity of regulated enzymes of the TCA cycle?

NAD+/NADH

3 multiple choice options

What does a high NAD+/NADH ratio mean?

Low NADH. NADH is usually an inhibitor, so when low on inhibitor, the cycle runs better!

Succinate dehydrogenase mutations are associated with what?

Phaeochromocytoma and paraganglioma

Fumarase mutations cause predisposition to what?

Cutaneous and uterine leiomyomas (fibroids) and kidney cancers

Isocitrate dehydrogenase mutations are associated with what?

Variety of tumors

Pyruvate is interconvertible with what?

Alanine and lactate

What produces acetylCoA for the TCA cycle?

Pyruvate dehydrogenase (PDH)

PDH exploits _ important coenzymes and exhibits many general principles of enzyme regulation.

5

What introduces actylCoA into the TCA cycle?

Citrate synthase

The 8 reactions of the TCA cycle connect what?

Carbohydrate, fatty acid, and amino acid metabolism

AcetylCoA is oxidized to CO2 as what are made?

NADH, FADH2, and GTP

Electron transport uses coenzymes to drive an electrochemical gradient that ___ ________ uses to make ATP.

ATP synthase (up to 12 ATP/AcetylCoA)

Cycle intermediates are produced from/used to make _____ _____. What is an energy source?

Amino acids; glutamate

The TCA cycle connects amino acids to _______ production

Glucose

What is the name of the intermediate concentrations that regulate the TCA cycle?

Anaplerotic reactions

What small molecules regulate the TCA cycle?

NADH, Ca++, ATP, ADP

The pyruvate dehydrogenase complex (PDC) is a three-enzyme complex that decarboxylates pyruvate to acetyl-CoA, linking glycolysis to the TCA cycle. All of the following vitamins/coenzymes are involved in the pyruvate decarboxylation reaction except:

Biotin

3 multiple choice options

Glycolysis takes glucose and converts it into pyruvate. Depending on the environmental conditions and the needs of the cell, pyruvate can then be most directly converted into all of the following except?

Serine

3 multiple choice options

Which compound inhibits pyruvate dehydrogenase and activates pyruvate carboxylase?

Acetyl CoA

3 multiple choice options

Which step in the TCA cycle produces a high energy nucleotide?

The conversion of α-ketoglutarate to succinyl CoA

3 multiple choice options

The citric acid cycle is a series of reactions used by all aerobic organisms to generate energy through the oxidation of acetate. Which of these claims about the citric acid cycle is incorrect?

The cycle is cytoplasmic

3 multiple choice options

Which of the following enzymes are the most important control points for the TCA cycle?

Isocitrate and α-ketoglutarate dehydrogenases

3 multiple choice options

What describes the process of electron transport. and ATP synthesis?

Protons are imported into the matrix driving ATP synthase

3 multiple choice options