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Oxidative Decarboxylation and Krebs Cycle

– Color Index:

. Important.

. Extra Information.

. Doctors slides. 436 team Objectives: Of Oxidative Decarboxylation:

 Recognize the various fates of pyruvate

 Define the conversion of pyruvate to acetyl CoA

 Discuss the major regulatory mechanisms for PDH complex

 Recognize the clinical consequence of abnormal oxidative decarboxylation reactions Of Krebs Cycle:  Recognize the importance of Krebs cycle.

 Identify various reactions of Krebs cycle

 Define the regulatory mechanisms of Krebs cycle

 Assess the yield of PDH reaction and Krebs cycle’s reactions From Lippincott. (Extra slide  to understand general concept) 

Overview of Krebs cycle: The cycle – also known as the tricarboxylic acid (TCA) cycle or the Krebs cycle– is a series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of acetyl-CoA derived from , and into dioxide and chemical energy in the form of triphosphate. In addition, the cycle provides precursors of certain amino acids as well as the reducing agent NADH that is used in numerous other biochemical reactions. The name of this is derived from citric acid (a type of tricarboxylic acid) that is consumed and then regenerated by this sequence of reactions to complete the cycle. In addition, the cycle consumes acetate (in the form of acetyl-CoA) and , reduces NAD+ to NADH, and produces as a waste byproduct. The NADH generated by the TCA cycle is fed into the oxidative (electron transport) pathway. In eukaryotic cells, the occurs in the matrix of the . Fates of Pyruvates (Remember: Pyruvate is the end product of )

Lactate *in humans and some Acetyl CoA “anaerobic” *in Krebs cycle Synthesis of non-essential using pyruvate + glutamine Oxaloacetate "essential” *In Krebs cycle (it’s an *Done by Alanine 1-Glutamine ‘donating group ‘ will intermediate) give NH2 to pyruvate () * Activated by acetyl CoA “ALT” 2-Glutamine will transfer into alpha- *• PLP = pyridoxal keto , while pyruvate *Importance: will transfer into alanine 1. Replenishes intermediates Ethanol of the TCA cycle. 2. Provide substrates for *It occurs in yeast and some Bactria (including intestinal flora)(Anaerobic) * -dependent 3. An irreversible reaction pathway Understanding the complex (PDC). • It’s the process of making acetyl Co-A It is made of three “mainly” & oxaloacetate from pyruvates One of them is Pyruvate Dehydrogenase by the enzyme: pyruvate dehydrogenase The pyruvate dehydrogenase complex • Produces 2 NADH 6 ATP contributes to transforming pyruvate into • Regulated by of acetyl-CoA by a process called pyruvate Acetyl coA and NADH decarboxylation. Acetyl-CoA may then be used in the citric acid cycle to carry out cellular • Increased amount of Acetyl CoA and respiration, so pyruvate dehydrogenase NADH act as “Negative Feedback” contributes to linking the glycolysis to Krebs inhibitors of their respective reactions. cycle and releasing energy via NADH.

*More in the next slide*

NOTE How? = enzyme adds P They activate “Pyruvate group “phosphorylates” dehydrogenase kinase which phosphorylates = enzyme and inactivates that removes P group “Pyruvate dehydrogenase” Note: phosphorylation can either activate or inactivate, according to the enzyme. PDH Complex: Covalent Regulation Regluation of PDH kinase Regulation PDH phosphatase PDH : enzyme complex “3 enzymes joint together” that convert pyruvate into PDH phosphatase +Ca acetyl CoA . ↑ pyruvates remove *Pyruvate dehydrogenase complex (PHD) has two forms active and inactive. Regulated by co-enzymes. Inactivation of PDH kinase active complex *inactive form: regulated by PDH kinase Active form of pyruvate ↑ acetyl Co-A ,NADH ,CO *active form: regulated by PDH complex 2 phosphatase *Those two enzymes are controlled by many factors

↑ ATP, acetyl co-A, NADH Direct regulation of the complex

Activation of PDH kinase ↑ NADH, Co-2 and acetyl Co-A can directly inhibit the active form Inactive form of the complex PDH Reaction: Clinical application

PDH complex plays a important role in CNS Extra info: Thiamine: B1, a How? coenzyme in the of Brain cells are unable to produce sufficient ATP if the PDH complex is and amino acids. inactive ‘no production of acetyl coA thus, no krebs cycle thus, no ATP’ Niacin,: also known as vitamin B3 A precursor of coenzymes called NAD and NADP, which are *Thiamine and niacin are co-factors that helps PDH complex needed in many metabolic processes. *Deficiencies of them can cause serious CNS problems

congenital Wernicke-Korsakoff PDH complex deficiency is the most (encephalopathy- common biochemical cause. psychosis syndrome): due to thiamine ‘too many pyruvates leads to the use deficiency, may be seen of which make especially with lactate accumulate’ abuse. Tricarboxylic Acid Cycle: Krebs Cycle The tricarboxylic acid cycle (Krebs) shown as a part of the essential pathways of energy . *has 8 reactions. Properties of the cycle

Final common Major source for Exclusively in pathway for ATP mitochondria oxidation. (24 ATP)

Synthetic reactions (anabolic features): Mainly catabolic with 1- from amino acids some anabolic features 2-Nonessential amino acids 3-Fatty acids 4- CoA = coenzymeA. Krebs Cycle Reactions (1) (1st  3rd )

1st step: Acetyl Co-A + Oxaloacetate citrate (6C) Joined by condensation : 1 H2O in CoA out

2nd step: Citrate iso-citrate 6C Aconitase

2 3rd step: Iso-citrate α-Ketoglutarate (5C) Oxidized by IsoCitrate It is coenzyme Dehydrogenase: regulated NAD+ NAD+ is reduced “can be Co2 is out inhibited or activated” Understanding the molecules  (+)ADP, Ca2+ NAD(H) = dinucleotide (-)ATP, NADH α-Ketoglutarate: It is the keto acid derivative of 3 glutamate, and is an intermediate. 5C Krebs Cycle Reactions (2) (4th  7th )

4th Step *oxidation and decarboxylation* 4 α-Ketoglutarate oxidized by co-enzyme NAD+ 5C

α-Ketoglutarate Dehydrogenase complex • NAD+ is reduced to NADH + H+ It is regulated • CO2 is out • CoA is in It is activated by: Ca2+ Inhibited by: NADH Succyinyl CoA and Succinyl CoA

5th Step *oxidation and decarboxylation* Succinyl Co-A High energy 4C Succinate Thiokinase • Co-A is out Succinate Thiokinase • GPD+Pi  GTP The ONLY Substrate-Level Phosphorylation 5 Understanding the molecules  in Krebs GDP = diphosphate. 4C 6th step *oxidation of succinate to fumarate* Succinate Fumarate (4C)

Oxidized by Succinate co-enzyme Dehydrogenase: FAD • FAD is reduced

6 7th step *hydration of fumarate to L-malate*

Fumarate L-Malate (4C) Fumarase: • H2O is in

7

Understanding the molecules 

FAD(H2) = flavin adenine dinucleotide. Malate: is an with the molecular formula C4H6O5. It is a dicarboxylic acid that is made by all living organisms, contributes to the pleasantly sour taste of fruits, and is used as a . The malate anion is an intermediate in the citric acid cycle. Krebs Cycle Reactions (3)

8th step: 8 L-Malate Oxalo-acetate (4C) Oxidized by Malate co-enzyme dehydrogenase: NAD+ • NAD+ is reduced

Formation (regeneration) of oxaloacetate from malate. Krebs Cycle: Energy Yield

Number of ATP molecules produced from the oxidation of one molecule of acetyl (CoA) using both substrate-level and oxidative phosphorylation. Krebs energy We get 3 NADH from: Isocitrate → α-Ketoglutarate outcome α-Ketoglutarate → Succinyl CoA Malate → Oxaloacetate So, we get 24 ATP from 2 Acetyl CoA We get 1 FADH from: Succinate → Fumarate Other outcome Succinyl CoA “high energy compound” breaks down NADH = 3 ATP FADH = 2 ATP We get 2 CO2 from: which leads to a substrate level phosphorylation of Isocitrate  α-Ketoglutarate GDP to GTP, which means 1 ATP. GTP = 1 ATP α-Ketoglutarate  Succinyl CoA Regulation of Oxidative Decarboxylation and Krebs Cycle

PDH complex and the TCA cycle are both up-regulated in response to a decrease in the ratio of TCA CYCLE PDH complex & TCA: make ATP • ATP : ADP & NADH IN LOW ENERGY • NADH : NAD+ CONDITIONS PDH: The Pyruvate Dehydrogenase TCA: Tricarboxylic Acid Activators Inhibitors

ADP ATP

Ca2+ NADH videos  Krebs cycle made simple حلقة كربس  Krebs cycle MCQs

1\ Allosteric regulation in oxidative decarboxylation of 4\ ADP AND Ca 2+ are: pyruvate is done by: A- TCA inhibitors A- Acetyl CoA B- TCA activators B- NADH 5\ net ATP production by oxidative C- ATP decarboxylation is: D- A & B A- 8 ATP 2\ PDH kinase is inhibited by: B- 24 ATP A- Acetyl CoA C- 6 ATP B- Pyruvate D- 38 ATP C- ATP 6\ net ATP production by complete glucose

D- ADP

oxidation is:

A -

3\ deficiencies of thiamine or niacin can cause serious A- 38 ATP 6

C - problems in: B- 24 ATP 5

B - A- C- 6 ATP 4

C - B- kidney D- 8 ATP 3

C- CNS B -

D- GIT 2 D - 1  Girls team members:

1- هيفاء الوعيل. 2- روان الوادعي. 3- زينة الكاف. 4- نجود العنزي. 5- نورة الشبيب.

-Contact us: -Team leaders: [email protected] نوره السهلي. عبدهللا المانع. twitter.com/436biochemteam