Unformatted text preview: Once Safely in the Liver, How is Ammonia Converted to Urea for Excretion? 4eFig. 18-10 1 11.4.01 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 2 Fig. 18-11 11.4.01 The Urea Cycle 3 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
11.4.01 Here’s What CPS-I Looks Like (Bacterial) PDB: 1M6V ADP Glutamine enters at the Glutamine top, traversing the yellow “channel”, emerging as carbamoyl phosphate at the bottom the The large subunit (grey) The and small one (blue) cooperate; the channel between the 3 active sites is almost 100 A long is
4 Fig. 22-37 ADP 11.4.01 Overview: Ammonia Metabolism in and on Mitochondria 5 And it costs 3 ATPs… 4eFig. 18-10 11.4.01 Steps in the Urea Cycle Ornithine transcarbamoylase (OTCase), iin the n matrix, condenses CP with ornithine (an amino acid) to form (1) citrulline (also an amino acid) (1) Which transits to cytosol, where it is (2) condensed Which (2) with aspartate (from whence comes a second amino group) by argininosuccinate synthetase, argininosuccinate A two-step reaction using ATP, whose product is (3) two-step (3) cleaved by argininosuccinate lyase to yield free argininosuccinate arginine and fumarate arginine Arginine is (4) cleaved by arginase to give urea Arginine (4) arginase (which is carried to the kidneys and excreted) and ornithine (which re-enters mitochondria via ORNT1 antiporter, in exchange for citrulline export) antiporter,
6 11.4.01 Can You Now Answer These Questions? What transports nitrogen from amino groups to the liver? Why must ammonia be transported to the liver (and Why kidney) and how is it done? kidney) How do the Cori cycle and the glucose-alanine cycle How work together? work What happens to excess nitrogen once in the liver? How does the kidney counteract acidosis (remember How acidosis ketone bodies…)? ketone Do you know the α-ketoacid counterparts for alanine, Do -ketoacid glutamate, and aspartate? glutamate, What two reasons may account for the special toxicity of What ammonia in the brain? ammonia
7 11.4.01 Wise Words
“All biochemical pathways are somewhat artificial All in that where they begin and end are defined by us…” us…” - M. Watford (2003) We know where it ends, but where should the urea We cycle begin? cycle At the source of exogenous amino acids (dietary protein) At and endogenous amino acids (tissue and skeletal muscle proteolysis), when ammonia is first generated? proteolysis), Or, in the liver – which plays a central role in whole body Or, nitrogen and carbon homeostasis? nitrogen 8 11.4.01 Back to the Tissues – Glutamine Synthetase – A Homo-Dodecamer Side view Top view 9 Fig. 22-5 PDB: 2GLS 11.4.01 Regulation of Glutamine Synthetase
Fig. 22-6 The importance of this The enzyme is reflected in the complexity of its regulation (the E. coli enzyme is shown). And that’s all you need to know… to
10 11.4.01 Regulation of the Urea Cycle Arg
(except in mammals) 1. Increased glutamate in 1. the matrix, signalling excess nitrogen, will excess Drive the synthesis of Nacetylglutamate, acetylglutamate, Thus stimulating CPS-I 2. The synthesis levels of 2. CPS-I and the 4 urea cycle enzymes are up-regulated when the demand to excrete nitrogen is great
11.4.01 11 Fig. 18-13 The Urea Cycle Intersects the TCA Cycle The Aspartate-Argininosuccinate Shunt 12 4eFig. 18-10 11.4.01 “Krebs Bicycle” is only a principle
Connecting to the Connecting TCA cycle via the Aspartate-ArgininoAspartate-ArgininoSuccinate shunt Current thinking: Fig. 18-12 Excess matrix nitrogen (as glutamate) can be “dumped” as Excess can aspartate (by transamination – of what?) to the cytosol into of ?) the urea cycle the Cytosolic fumarate itself cannot re-enter the matrix, making Cytosolic cannot the “bicycle” impossible until fumarate is made into malate the The ORNT1 antiport transporter brings ornithine into the The matrix, exchanging citrulline to the cytosol matrix, 13 11.4.01 Summary of Amino Acid Oxidation – 1 14 Amino group is removed by transamination to αAmino αketoglutarate or OAA Glutamate dehydrogenase gives NH4+ (mito) (mito) NH4+ is converted to urea in its eponymous cycle is Which is restricted to the liver (other tissues lack Which arginase) arginase) Urea cycle is interconnected with the TCA cycle Urea cycle enzymes are regulated at the level of Urea enzyme synthesis. Flux through the urea cycle at the level of Flux carbamoyl phosphate synthesis can be regulated allosterically by N-acetylglutamate. allosterically 11.4.01 Overview: The Three Stages of Cellular Respiration
Stage 1: Acetyl-CoA Stage Production from Production Glucose Wh To at ha Fatty Acids th e c ppen arb s Amino Acids ons ? Stage 2: Acetyl-CoA Stage Oxidation (TCA cycle) Oxidation Stage 3: Electron Transfer Stage & Oxidative Phosphorylation Phosphorylation 11.4.01 15 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 16 11.4.01 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
11.4.01 17 Except for TV, what’s the difference? 18 3eTable 18-2 Pv t . T im H al l 11.4.01 The Ten Amino Acids We Can Make The rest, we The must ingest… must
Cys, Gly, Ser come from 3-phosphoglycerate 19 Fig. 18-15 11.4.01 What’s Wrong With This Child? Calories do not constitute nutrition… 20 11.4.01 Not All Proteins Are Created Equal (or) Why Rice and Beans?
Corn Lysine Methionine Threonine Tryptophan Legumes + + Rice + Soy + + “There are no good or bad foods, just good or bad diets…” 21 11.4.01 Sources of Stored Metabolic Energy
For a 70 kg male after overnight fasting… mass mass Glycogen – liver: 80 gm Glycogen – muscle: 150 gm Fat: 15,000 gm Protein: 6,000 gm
But, is the protein really “stored”?
22 11.4.01 % of cal of 0.2% 0.4% 85% 14.5% Survival Value of Stored Energy 23 11.4.01 ...
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- Spring '08
- Amino Acids