Unformatted text preview: Interested in Graduate School?
Dr. Christopher Pennell of the Dr. University of Minnesota Biomedical Studies Graduate Program will be here for a recruiting talk today: here Time: Wed Mar 30 at 6:30 pm Time: Place: Morey 321.
1 11.3.30 Recall: β -Oxidation of Palmitate, a C16 Fatty Acid Fig. 17-8b The overall equation is: Palmitoyl-CoA + 7 CoA + 7O2 + Palmitoyl-CoA 28 Pi + 28 ADP 8 Acetyl-CoA + 28 ATP + 7H2O Acetyl-CoA X
2 TCA cycle Through the TCA cycle, the 8 Through acetyl-CoAs generate another 80 ATPs + 16H2O + 16CO2 80 X Energy! 11.3.30 What Are Ketone Bodies?
They aren’t “bodies”, and Only two are ketones They build up if there is They ongoing β -oxidation in the -oxidation liver, but liver, The ability of acetyl-CoA The to enter the TCA cycle is limited limited How could this come How about? about? 3
ox rb ca de n io t la y a id ox n io t 11.3.30 β -oxidation of FAs Where Do These “Bodies” Come From? β -HBdH 4 Fig. 17-18 These 3 perfectly soluble small These molecules are molecules Produced in side reactions from Produced acetoacetyl-CoA at the penultimate step in β -oxidation, When there is excess acetyl-CoA for When the TCA cycle the This also produces free CoA Thus allowing continued oxidation of Thus fatty acids in the liver fatty Acetone, the most volatile, is exhaled Note reduction by β -HBdH and the Note -HBdH regeneration of S-CoA… regeneration 11.3.30 Ketone Bodies from Liver Can Be Used as Fuel Elsewhere The ketone bodies are then The exported to other tissues Renal cortex, skeletal and Renal heart muscle, and brain (but it prefers glucose) can oxidize ketone bodies for use in their TCA cycles TCA This occurs particularly during This starvation, when gluconeostarvation, genesis is depleting intermediates of the TCA cycle, mediates thereby slowing it thereby
5 Fig. 17-19 β -HBdH From TCA To TCA 11.3.30 Ketone Bodies Get Exported for Energy When gluconeogenesis is When needed needed The TCA cycle slows Acetyl-CoA builds up Ketone bodies are made Providing fuel for other Providing tissues (which?) tissues Allowing continued β Allowing oxidation (how?) Appreciate the interplay among pathways… among
Fig. 17-20 6 11.3.30 Overview: The Three Stages of Cellular Respiration
Stage 1: Acetyl-CoA Stage Production from Production Glucose Wh To at ha Fatty Acids t he pp ni t ens Amino Acids rog en? Stage 2: Acetyl-CoA Stage Oxidation (TCA cycle) Oxidation Stage 3: Electron Transfer Stage & Oxidative Phosphorylation Phosphorylation 11.3.30 7 Fig. 16-1 Outline – Amino Acid Metabolism Sources and destinations of amino acids Dietary and endogenous turnover Non-essential amino acids – biosynthesis Managing the ammonia – nitrogen metabolism From tissues to liver Reactions in the liver – the urea cycle Integrative pathways – TCA, urea, and gluconeogenesis Extracting the energy – carbon catabolism Carbon entry into the TCA cycle One-carbon transfer reactions Glucogenic and ketogenic amino acids 11.3.30 8 Where Do Amino Acids Come From? Where Do They Go? Sources of amino acids
Dietary – ~100 gm/day (“good” vs. “bad” protein) Endogenous protein turnover 300-600 gm/day Use it or lose it – amino acids can’t be stored
Protein biosynthesis Conversion to essential metabolites Oxidation for energy Excretion 9 11.3.30 Alternative Fates of Amino Acids Many amino acids are absorbed and used Many directly - as is - in tissue protein synthesis. For humans, 10 of the 20 natural amino acids are “essential”, and must be obtained from the must diet (“Pvt. Tim Hall”). diet Excess amino acids cannot be stored, but can Excess be oxidized for energy – carnivores derive up to 90% of their energy needs from amino acid oxidation (for people it’s 10-15%) oxidation
11.3.30 10 The Major Amino Acid Source: Protein in the Diet The hormone gastrin, secreted in secreted response to stomach protein, stimulates HCl release (lowering stomach pH) and pepsinogen secretion, denaturing proteins pepsinogen and activating pepsin from its zymogen and The hormone secretin (stimulated by low The secretin pH) causes the pancreas to release bicarbonate, to bring pH up to 7 bicarbonate, The hormone cholecystokinin (responding The cholecystokinin to amino acids) stimulates pancreatic release of protease zymogens, including trypsinogen and chymotrypsinogen trypsinogen chymotrypsinogen These and other proteases are activated These and digest most proteins to free amino acids (some fibrous proteins like keratin are resistant) are 11 Fig. 18-3 On to the liver… 11.3.30 Nitrogen and Carbon Go Separate Ways 12 Fig. 18-1 11.3.30 An Overview: Where Does Nitrogen Go and How Does It Get There?
Transamination 13 11.3.30 Glutamate and Glutamine are Important Nitrogen Carriers Most amino acid metabolism occurs in the Most liver, where liver, In the cytosol, α-ketoglutarate receives amino In -ketoglutarate groups transferred from other amino acids, to form glutamate(transamination) form Glutamate and glutamine then pass into liver Glutamate mitochondria for further metabolism mitochondria Glutamine is the common ammonia carrier Glutamine from other tissues by transamination of glutamate (except muscle, where alanine is used – why?) why
11.3.30 14 How Ammonia from Tissues is Transported by Glutamine in the Bloodstream The enzyme glutamine synthetase, The glutamine which plays a central metabolic role in all organisms, can transfer free ammonia to glutamate, producing glutamine, but costing an ATP glutamine, This two-step reaction (another one!) This requires an activated phosphorylated intermediate intermediate Glutamine thus carries two amino Glutamine two groups, is nontoxic and highly soluble, and is present in blood at higher levels than other amino acids higher Once in mitochondria, glutaminase Once glutaminase regenerates glutamate and ammonia regenerates 11.3.30 15 Fig. 18-8 NH4+ Is Toxic to Animals The brain needs to maintain [ATP] at very high levels. The very Why? Why? Getting rid of excess NH4+ from the brain costs ATP. This may also reduce α-ketoglutarate levels (because?) This and thus inhibit the TCA cycle. Moreover, glutamate and its cousin γ-amino-butyric Moreover, acid (GABA) are neurotransmitters. acid High levels of NH4+ are therefore toxic to the central nervous system - producing coma and death. nervous 11.3.30 16 Transamination is a Ping-Pong Reaction Fig. 6-13b The enzymes that catalyze it are called both transThe transaminases and aminotransferases aminotransferases In a ping-pong reaction, the first product must leave In the active site before the second substrate can bind In this case, the amino group is transiently (but In covalently) held by the prosthetic group pyridoxal phosphate (as pyridoxamine phosphate) phosphate
17 11.3.30 Aspartate Aminotransferase (AST) PLP (red) is in PLP the active site of one of the two subunits of this dimeric transtransaminase They are named They after the amino group donor group
18 Fig. 18-5 PDB: 1AJS 11.3.30 Transferring Amino Groups Requires Pyridoxal Phosphate (PLP), Derived from Vitamin B6 AST with PLP AST in aldimine linkage to Lys258 Lys 19 Fig. 18-4 Fig. 18-5b,d 11.3.30 Another Way to Get Ammonia to the Liver: Glucose-Alanine Cycle Anerobic muscle contraction yields Anerobic pyruvate (from glycolysis) and amino groups (from protein breakdown) groups Alanine aminotransferase makes alanine, which goes to the liver where it is reconverted to pyruvate (and then to glucose, via gluconeogenesis, to be exported back to muscle) exported This complements the Cori cycle with This lactate (see Box 15-1) And ammonia is excreted from the liver Fig. 18-9 20 Why does this make metabolic sense Why for liver and muscle function? for
11.3.30 Oxidative Deamination in Liver Mitochondria
Bad stuff! 4eFig. 18-7 As the recipient of amino groups from many sources, As glutamate now sheds it as ammonia for excretion, and the product α-ketoglutarate can recycle as a and -ketoglutarate nitrogen acceptor, enter the TCA cycle, or serve as a precursor in gluconeogenesis precursor
21 Note the intersection of pathways! Note 11.3.30 Urea Destroys Vitalism!
1773 – urea isolated from urine by Rouelle 1820 – obtained in pure form by Proust 1828 – synthesized by Wohler From ammonia and lead cyanate, Thus proving that a recognized “organic” Thus compound could be derived chemically from inorganic precursors… from 22 11.3.30 The Urea Cycle – a Biochemical First (100 years later) 1932 – Krebs and Henseleit postulated 1932 the first cyclic pathway, because the
In incubated liver slices, ornithine could In stimulate the synthesis of urea manyfold, but but Ornithine was not consumed in the reaction! Ornithine It thus had to be either It Solely catalytic, or Solely or Regenerated as part of a cyclic process… 23 11.3.30 How to Get Rid of Toxic Ammonia?
Amino acids Amino
(ingested protein) (ingested Only urea output is proportional to protein in the diet Glutamine
(muscle and other tissues) other Alanine
(muscle) 4eFig. 18-2b
Ammonium ion Urea Excrete it, as:
Uric acid Creatinine Mmol-N/day (gm/day) (gm/day) 24 40 (0.7) 800 (25) 15 (0.8) 40 (1.5) 11.3.30 What’s So Great About Urea? Urea synthesis is essential to humans, because We require protein in our diet to provide amino acids Which can’t be stored, unlike fats and carbohydrates The resultant excess nitrogen after conversion to ammonia The must be excreted as urea, or must It will circulate in the bloodstream and be toxic if it rises It above certain levels above 2 of its 8 atoms are nitrogen (what % by weight?) it is very water soluble while being nontoxic Urea is compact (and symmetric), Urea is not so great for birds and terrestrial reptiles Urea not
excretion requires large water loss, so these “uricoteles” excretion eliminate crystalline uric acid eliminate 25 11.3.30 Amino Acids vs. Urea vs. Ammonia “Normal” plasma levels in N-equivalents
micromolar micromola L-Glutamic Acid L-Glutamine L-Alanine Urea NH 3 103 554 436 9,200 9,200 22 (10-50) These values demonstrate the efficiency of the conversion of ammonia to urea. Luckily for us!
26 11.3.30 Once Safely in the Liver, How is Ammonia Converted to Urea for Excretion? 4eFig. 18-10 27 11.3.30 Note: The CPS-I Reaction Has Two Activation Steps
In the mitochondrial matrix, the In enzyme carbamoyl phosphate synthetase I condenses synthetase bicarbonate, ammonia, and a phosphoryl group phosphoryl To form carbamoyl phosphate Which enters the urea cycle Which by condensing with ornithine (an amino acid!) (an 28 Fig. 18-11 11.3.30 The Urea Cycle 29 Scavenges nitrogen from Scavenges both mt matrix (from bicarbonate and ammonia) and the cytosol (from aspartate) (from The 5 enzymes involved The are clustered, and employ substrate channeling (interesting architecture?) (interesting Only urea is released into Only urea the cytosolic pool of metathe bolites
11.3.30 Next time… Friday: Friday: read further in the text about the urea cycle (p. 682-687) the SKIM (!) carbon metabolism (section 18.3) (section appreciate Fig. 18-15, Table 18-2, Box 18-2, and Summary 18.3 Box
11.3.30 30 ...
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- Amino Acids