Amino acid Catabolism

Glucogenic vs Ketogenic

- Amino acids degraded to pyruvate, α-ketoglutarate, succinyl~CoA, fumarate, or oxaloacetate are called "glucogenic" because these intermediates can be used in gluconeogenesis - Amino acids degraded to acetyl~CoA or acetoacetate are called "ketogenic". Many fit both categories; however, leucine and lysine are exclusively ketogenic.

Summary of Amino Acid Oxidation - 1

- Amino group is removed by transamination to α-ketoglutarate or OAA - Glutamate dehydrogenase gives NH4+ (mito) NH4+ is converted to urea in its eponymous cycle - Which is restricted to the liver (other tissues lack arginase) - Urea cycle is interconnected with the TCA cycle - Urea cycle enzymes are regulated at the level of enzyme synthesis. - Flux through the urea cycle at the level of carbamoyl phosphate synthesis can be regulated allosterically by N-acetylglutamate.

Regulation of urea cycle

- Arg regulates urea cycle in liver via an intermediate N-acetylglu - This stimulates CPS-I - In the long term, cycle enzymes are induced on high protein diets or during starvation

Summary of Amino Acid Oxidation - 2

- Carbon skeletons end up in TCA where they can be ketogenic or glucogenic (or both) - Leu, Ile, and Val are degraded extrahepatically - Amino acids only undergo partial oxidation in the liver, but this can fulfill the "fed" liver ATP needs - For both making urea and for gluconeogenesis - Thus, gluconeogenesis and urea synthesis can be considered parts of the same pathway, integrated with the TCA cycle: high urea synthesis reflects a high need for glucose.

Overview of Carbon Flow: Glucogenic and Ketogenic Amino Acids

- Carbons from the pink amino acids can all participate in gluconeo-genesis - Carbons from the blue amino acids can all end up as acetoacetyl-CoA and, if in excess, can produce ketone bodies - Trp, Phe, Tyr, Ile are in both categories - Only Leu and Lys are solely ketogenic

Urea cycle

- Discovered by Hans Krebs (before the other one) - Converts neurotoxic ammonia to urea - Minimizes toxicity and water loss UREA SYNTHESIS - Ammonia used together with CO2 from respiration and ATP to form carbamoyl phosphate using carbamoyl phosphate synthetase I - This condenses with ornithine using ornithine transcarbamoylase to form citrulline which leaves the mitochondrion - Second amino group derived from asp formed by transamination from glu and OAA using appropriate transaminase - Asp condensing with citrulline via an activated intermediate (citrullyl-AMP) forms arginosuccinate using arginosuccinate synthetase - Arginosuccinate cleaved by arginosuccinase to form arginine and fumarate (joins TCA cycle) - reversible step - Arginine cleaved by arginase to ornithine and urea - Ornithine is then free to condense with next carbamoyl phosphate

Fates of amino acids

- Excess amino acids cannot be stored. - Surplus amino acids are used for fuel. = Carbon skeleton is converted to + Acetyl-CoA + Acetoacetyl-CoA + Pyruvate + Citric acid cycle intermediate = The amino group nitrogen is converted to urea and excreted. - Glucose and ketone bodies can be formed from amino acids.

Extracellularly ammonia is transported as glutamine

- Excess ammonia in tissues added to glutamate to form glutamine using glutamine synthetase - In liver mitochondria ammonia is released by glutaminase - Ammonia converted to urea Glutamate = further deaminated (to αKG) for fuel via TCA cycle = used for transaminations to provide amino groups for amino acid biosyntheses

Branched chain (Val, Leu, Ile) amino acid catabolism

- In muscle (adipose, kidney, brain) - Aminotransferase (absent in liver) to convert all 3 aa's to corresponding ketoacids - Branched chain α-keto acid dehydrogenase complex oxidizes all 3 aa's to the corresponding acyl Co A derivatives and CO2 - Same reaction as pyruvate dehydrogenase and α-ketoglutarate dehydrogenase, same subunits, cofactors (TPP, lipoate, CoA, FAD, NAD+) and mechanism

Alanine cycle

- In muscle pyruvate available from glycolysis - AAT converts glu and pyr to αKG and ala - Ala reconverted to pyr and glu, amino group converted to ammonia and then urea - Pyruvate converted to glucose by gluconeogenesis - Supplements glutamine transport & Cori cycle

Catabolism of amino acids carbon "skeletons"

- Normally only 10 - 15% of energy production 20 amino acids converge on only 6 major products, all of which can enter the TCA cycle - Used for gluconeogenesis, ketogenesis or oxidized to CO2 & H2O = however this is where the liver gets its considerable energy requirements normally (rather than from glucose catabolism) as a by product of protein turnover

Amino acid catabolism

- Protein turnover is continuous - some amino acids are not required for further protein synthesis - Amino acids oxidized on protein rich diets when supply exceeds demand: proteins cannot be stored - During starvation (or uncontrolled diabetes) amino acids used as source of energy

Objectives

- Role of glutamine (alanine) in transport of amino groups - Role of pyridoxal phosphate cofactor in amino acid metabolism - Fate of carbon skeletons of amino acids - Role of C1 carriers OVERVIEW - Proteins are degraded into amino acids. - Protein turnover is tightly regulated. - First step in protein degradation is the removal of the nitrogen - Ammonium ion is converted to urea in most mammals. - Carbon atoms are converted to other major metabolic intermediates.

Amino Acid Catabolism

- The 20 common amino acids are degraded by 20 different pathways that converge to just 7 metabolic intermediates.

Cofactors Involved in One-Carbon Transfers

- Three major cofactors are employed for carrying one-carbon units: biotin, SAM (S- Adenosylmethionine), and THF (Tertahydrofolate) - Each is specific for particular oxidation states of carbon - Although THF is the most versatile, SAM is 1000x more reactive when methyl groups are being transferred

PLP Schiff's base formation in amino acid metabolism

- Transamination: PLP stores amino group, donating it back to another α keto acid by reversing this reaction - Racemization: D amino acids are reqd in bacteria e.g. for peptidoglycan synthesis - Decarboxylation: Used e.g. to make histamine from histidine

Transaminases/aminotransferases

- Transfer amino group from an amino acid to a keto acid, results in corresponding keto acid and amino acid - No loss of amino group, reaction at equilibrium - Function: to funnel amino groups from various amino acids to glutamate

Vitamin-Coenzymes in Amino Acid Metabolism

- Vitamin B-6 : pyridoxal phosphate - Enzymes that bind amino acids use PLP as coenzyme for binding: = Transaminases = Amino acid decarboxylases = Amino acid deaminases - Pyridoxal phosphate is bound at enzyme active sites as an aldimine (Schiff base) to a specific Lys residue

Overview of Amino Acid Catabolism: Interorgan Relationships

INTESTINE - Dietary amino acids absorbed - Utilizes glutamine and asparagine as energy sources = releases CO2, ammonium, alanine, citrulline as endproducts = utilizes glutamine during fasting for energy - Dietary amino acids and catabolites released to portal blood LIVER - Synthesis of liver and plasma proteins - Catabolism of amino acids = gluconeogenesis = ketogenesis = branched chain amino acids not catabolized = urea synthesis - Amino acids released into general circulation = enriched (% of total aa) in BCAA (2-3X) SKELETAL MUSCLE - Muscle protein synthesis - Catabolism of BCAA = amino groups transported away as alanine and glutamine (50% of AA released) + alanine to liver for gluconeogenesis + glutamine to kidneys KIDNEY - Glutamine metabolized to a-KG + NH4 = a-KG for gluconeogenesis = NH4 excreted or used for urea cycle (arginine synthesis) + important buffer preventing acidosis + [NH4+] : [NH3] = 100 : 1

THF

VERSATILITY OF THF - Both N5 and N10 can carry one-carbon groups - Or they can share 3 different oxidation states of carbon can be carried - Used as donors in one-carbon transfers THF CAN ADD or REMOVE SINGLE CARBONS - Although THF is used in both of these reactions, - Only in bacteria does the glycine a-carbon end up in the TCA cycle

Metabolism of amino groups in vertebrate liver

[Glutamine] in cells and blood much higher than other amino acids