Degradation of amino acids Amino acid breakdown can yield: –Acetyl-CoA –  -KG –Succinyl-CoA –OAA –fumarate

 -KG is generated from five amino acids Proline Glutamate Glutamine Arginine Histidine

Not a surprise Proline, glutamate, arginine, and glutamate are synthesized from  -KG, here use distinct enzymes for breakdown But, histidine is not; A completely different pathway for histidine catabolism than for anabolism, In this case, incoming amino acid (glutamate) binds, donates its amino group to pyridoxal phosphate, and leaves as an  - keto acid (  -KG). Then, an incoming a- deto acid binds and accepts the amino group and leaves as an amino acid

Four amino acids are converted to Succinyl-CoA Methionine –Converted to homocysteine through methyl group transfer, generates cysteine as converted to  - ketobutyrate Isoleucine –Transamination, oxidative decarboxylation to acetyl- CoA and propionyl CoA Valine –Transamination, decarboxylation to propionyl CoA Threonine –  -ketobutyrate generated and converted to propionyl CoA

Propionyl-CoA is a common intermediate for amino acids  succinyl-CoA

Branched-chain  -keto acid dehydrogenase complex In certain body tissues, this enzyme catalyzes the oxidative decarboxylation of valine, isoleucine, and leucine yielding CO 2, and acyl-CoA derivatives. Shares ancestry with pyruvate dehydrogenase complex,  -KG dehydrogenase complex – another example of gene duplication

Branched-chain …complex

Asparagine and aspartate are degraded to OAA

Fate of metabolites derived from amino acids In addition to feeding the citric acid cycle, amino acids can result in ketone bodies, while others are gluconeogenic

Ketone bodies The six amino acids that are degraded to acetoacetyl-CoA and/or acetyl-CoA (in blue on previous slide) can be converted to acetoacetate and  -hydroxybutyrate

Gluconeogenic amino acids Amino acids that are degraded to pyruvate,  -KG, succinyl-CoA fumarate, and/or OAA can be converted to glucose

Tempting to take a dietary perspective on carbohydrate and protein metabolism, but…

We’ll just re-emphasize ammonia metabolism

You seen this many, many times All aminotransferases have PLP

PLP enzymes Generally found in enzyme active site covalently bound to amino group of lysine

PLP-mediated transformation

Aminotransferases exhibit Ping- Pong kinetics Ping-pong – no ternary complex is formed between substrates and enzyme; first substrate binds, reacts, then that products leaves before second substrate binds

Ammonia from amino acid catabolism During amino acid breakdown, amino is generally transferred to glutamate (serves as nitrogen source and sink) From there, amino group can be released as ammonia

Transdeamination The combined action of aminotransferase and glutamate dehydrogenase is called transdeamination Glutamate dehydrogenase operates at an important intersection of carbon and nitrogen metabolism – as a result, highly regulated

Linkage of TCA with aa catabolism by allostery ADP is a positive effector of glutamate dehydrogenase, while GTP is a inhibitor

Ammonia is toxic, so cells need to get rid of it… Fix ammonia onto glutamate to form glutamine and use as a transport mechanism Transport ammonia by glucose-alanine cycle Excrete nitrogenous waste through urea cycle

Glucose-alanine cycle

Ammonia transport using alanine Alanine aminotransferase transfers the  - amino group from glutamate to pyruvate, forming alanine This shuttle funnels ammonia out of tissues that have high glycolytic flux, to the liver, which can remove ammonia via urea cycle

Dumping ammonia as urea The glutamine, glutamate, and alanine feed the urea cycle The urea cycle generates urea, which can be deposited as waste The urea cycle spans both the cytosol and mitochondria and four intermediates – you are responsible for the urea cycle

Entering the cycle Ammonia derived from glutamate or glutamine is immediately linked to bicarbonate – catalyzed by carbamoyl phosphate synthetase I (mitochondrial)

Starting the cycle Carbamoyl phosphate reacts with ornithine to form citrulline with relase of inorganic phosphate (similar to OAA and acetyl-CoA) – catalyzed by ornithine transcarbamoylase Citrulline is passed to cytosol, where a second amino group is introduced via aspartate to form argininosuccinate – catalyzed by argininosuccinate synthetase with an ATP requirement

Producing urea Argininosuccinate lyase generates fumarate and arginine from argininosuccinate Cytosolic enzyme arginase cleaves arginine to yield urea and ornithine Ornithine is transported back to the mitochondria to start another round of the urea cycle Substrate channeling!! – except urea

Urea cycle and citric acid cycle can be linked Fumarate generated by the urea cycle can be converted to OAA in cytosol, and transported to mitochondria for use in citric acid cycle Conversely, transamination of OAA in mitochondria yields aspartate, which is used in the cytosolic urea cycle

“Kreb’s bicycle”

Urea cycle regulation More protein metabolism, more urea production Carbamoyl phosphate synthetase I is allosterically activated by N-acetyl- glutamate

The urea cycle is cost effective Looking at the urea cycle you observe three ATP spent for every turn, BUT generation of OAA from fumarate yields an NADH which can be used to generate 2.5 to 3 ATP via oxidative phosphorylation