L40 41 Urea Flashcards
Overall AA catabolism
catbon skeleton metabolized to TCA intermediates and glucose
alpha amino group NH3 excreted in urine at NH4
-primary detoxed by liver via urea cycle
significance of protien degredation
body pool of AA
-amino acids derived from dietary protien: stimulate just enough insulin to prevent muscle breakdown
-breakdown of body protein- fasting state-muscle major source of aa for carbon for gluconeogenesis (broken down via ubiquitination and proteosomal degradation) high during low IG ratio
what does the liver process
location of MOST aa catabolism
EXCEPTION-BCAA (LIV) - these are only done in peripheral tissues
during fasting AA are the main source of carbon for the liver..then for the kidney
Three types of amino acids
glucogenic
-aa that enter the TCA cycle (w/out consuming OAA) and lead to a net INCREASE in OAA which can be used for gluconeogenesis
Ketogenic: AAs that form AcCoA or Acetoacetate (cannot form glucose because two acetyl coA carbons are lost as CO2 in subsequent reaction) no net gain of C
BOTH: several AAs (ty, ile, phe, trp can be used for either glucose or Ketone body synthesis) depending where they enter the TCA cycle
Muscle Breakdown during fasting. What is happening in the Muscle Liver and Kidney
liver will catabolize MOST for gng, lacks enzymes to break down BCAA (liv)
Muscle: BCAA carbons: cat for energy: TCA/ETC
- BCAA nitrogens released into circ on alanine and glutamine
- nBCAAs- released into circ intact (for liver)
liver: takes up nbcaa for gng/kg
- alanine from muscle used for glucose production
- amino nitrogens are siposed of as urea or glutamine
Kidney: takes up glutamine for nitrogen disposal and glucose synthesis (PROLONGED FAST)
draw path of branch chained and non branch chained AAs to disposal or recycle
include BCAA- Glutamate- alanine, glutamine, liver/kidney S9
ala and gln concentrations in circulation and where it is going
incorporation of amino groups from BCAA into alanine and glutamine in muscle results in disproportionately large release of ALA/GLN in circ.
C skeletons of these AA are used for gluconeogenesis, or in case of glutamine, provide nutrients to tohter tissues.
Transamination reaction of Nitrogen disposal
- remove alpha Amino from AA (deaminate)
- 12 different aminotransferase enzymes for the 20 AA
- named for the donor of the amine group
- only muscle has aminotransferase enzymes that can act of BCAA
- either direction
- depends on tissue
Transamination reaction components
Amino acid-NH3 donates NH3, the leftover carbon skeleton used for gluglose energy as ALPHA KETO ACID.
-combined with alphaketoglutarate : substrate for every transamination reaction, which becomes GLUTAMATE with NH3 attached, sent to urea cycle
Pyridoxal phosphate temporarily holds Alpha amino group as its being transferred from the AA to Alpha ketogluterate
-pyridoxal 5 phosphate (PLP) is active coenzyme form of Vit B6 (present at catalytic site of all amino transferase enzymes)
(also holds amino group for aspartate when its becoming OAA)
DRAW
BCAA Aminotransferases
in muscle transfers amino group from BCAA (liv) to a-ketogluterate
-products: glutamate and a-keto acid
-glutamate then gives nitrogen to alanine or glutamine
- -liver can then use ALANINE
- Kidney can then use glutamine
the a-keto acid then needs to have carboxyl group removed in breakdown
- BCketoacid dehydrogenase complex (BCKD)
- rate limiting enzyme in conversion of aketoacid into ketone bodies of TCA cycle
- DEFECTIVE IN maple syrup urine disease
MSUD
maple syrup urine disease
-inherited defect of BCKD
symptoms a few days after birth
elevated BCAA in blood stream due to alpha keto acid accumulation and inhibition of forward BCAA reaction
-BCAA isoleusine gives maple syrup odor to urine
ketoacidosis (accumulation of aketoacids of the BCAA)
encephalopathy (brain dysfuntion) from buldup of BCAA and aketoacids. Leucine is most abundant BCAA that acuumulates. Starts to cross BBB and metabolized to glutamine and glutamate
neonatal screening, some states a routine test for heel blood of high leucine levels
infants must be fed synthetic formula with limited amounts of BCAA, and lifelong dietary mods to avoid BCAA
glucose alanine cycle
glutamate can donate N to pyruvate creating alanine and a-kg via muscle ALT (alanine aminotransferase), alanine goes into circulation, and used via liver ALT to generate BACK to pyruvate (a-keto acid) for gluconeogenesis in liver
GLUCOSE ALANINE CYCLE
alanine and glutamine in muslce
bcaa breakdown leades to glutamate which can be changed to glutamine or alanine. both non toxic amino acid carriers
alanine: in muscle, formed by the amination of pyruvate (by glutamate), then it is sent to liver and deaminated in the liver for glucose synthesis for gluconeogenesis (glucose alanine cycle) ONLY OCCURS IN FASTING STATE
glutamine: both muscle and liver- glutamate canundergo additional amination to form glutamine. ATP dependant and catalized by GLUTAMINE SYTHASE
- this is the mop up role that was not metabolized by urea cycle
breaking down non branch chain amino acids in liver
nBCAA (diet or muscle) taken up by liver and deaminated by 1 of 12 different aminotransferases
in liver, the nBCAA transfer aa to a-kg creating GLUTAMATE and a-ketoacid.
-glutamate will serve as N carrier for urea cycle OR become glutamine
glutamate in Urea cycle has two possible ways to donate NH3, urea is one way to dispose nitrogen
- oxidative deamination: produces free NH3
- GLUTAMATE DEHYDROGENASE uses NAD to oxidize the carbone skeleton of glutamate, making a-ketogluterate and free NH3 (can be protonated to NH4)
- NH4 will be in first step of urea cycle: synthesis of cabomoyl phosphate via the RL enzyme CARBAMOYL PHOSPHATE SYNTETASE I (CPSI) (2 ATP reguired) (in mito) - second transamination reaction creating aspartate
- aspartate transaminase ransferes the amino group from glutamate to OAA forming aspartate
- aspartate reacts with citrilline in the urea cycle to bring in a second nitrogen and creates arginosuccinate
CPSI
high Km for ammonia (NH4), as AA catabolism increases so does CPSI (mito) activity
if CPSI activity is impared, NH4+ level in the hepatocyte will increase
OTC(uses ornithine to create Citrulline using Carbamoyl phosphase (citrulline then leaves the cell when ornithine enters)
- OTC deficiency is most common inherted urea defect
- leads to dcreased urea cycle activity, accumulate NH3, hyperammonemia
CPSII
cytoplasmic enzyme
-first step in pyrimidine synthesis. Nitrogen source: glutamine
second way to dispose nitrogen: glutamine synthase
glutamate generated during the breakdown of nBCAA in liver
any NH3 that escapes being converted into urea is used to make glutamine and sent out of cells into circulation at glutamine
glutamate plus ATP plus NH3 is glutamine via GLUTAMINE synthase
-lower Km for ammonia than CPSI and anatomically located downstream of urea cycle in perivenous hepatocytes
requires less ATP than urea cycl
what happens to glutamine in circulation
metabolized by kidney, intestine, and WBCs
kidney: its converted to glutamate and or a-ketogluterate (TCA and gluconeogenesis) and NH3
- amino groups from glutamate and glutamine are used in RENAL AMMONIAGENESIS to create HH3, NH# binds to protons from the blood to produce NH4, filtered to urine (maintain pH) (carbon skeleton for gluconeogenesis)
intestines: catabolize to a-ketogluterate and relases NH3 to GI tract, ammonia lots in stool or back into si to liver
lymphocytes and macrophages cat glutamine for energy, important during infection
5 key points of nitrogen disposal
- during fasting, Box provides atp neede for both the urea cycle and gluconeogenesis
- since AA are both processed by both the urea cycle and gluconeogenesis (fasting phase) tehse increase/decrease at the same time
- N is always transported through circulation in non tox form: NBAA including Alanine and glutamine ; urea)
- ammonia is realeased from the body as UREA or NH4
- NH4 released into the uring is product of the deamination of glutamate or glutamine within renal cells. Nh4 does not cituclate in large quantities unless there is hepatic or renal malfunction
N-Acetyl Glutamate
positive allosteric effector of CPSI
mito reactions
increased AA met leads to increased glutamate
increased bOx for atp results in increased AcCoA
increase in both glutamate and acCoA results in increased N-acetylglutamate by NACETYL GLUTAMATE SYNTHASE, a positive allosteric effector of CPSI (that’s its sole activity)
Regualtion of urea cycle by gene expression in either fed state or fasting state
Fed state: high protein dietL enzmes of the urea cycle increase 20 fold
low protein dietL urea cycle enzyme levels decline
fasting state: during a fast, AA cat and activity of the urea cycle are directly ties to gluconeogenesis rate
GLUCAGON signalling in hepatocytes increases synthesis of urea cycle enzymes
LOOK AT CHART ON slide 41
substrate availability (high dietary levels protein increase urea cycle activity)
NEGATIVE:
protons are negative allowsteric effectors of CPSI (low pH bad)
decreased protein consumption
decreased bOX (need ATP generation)
Hyperammonemia
hereditary hyperammonemia: genetic defects in any one of the five urea cycle enzymes, two transporters, or n acetyl glutamate synthase
NH3 cannot be adequately metabolized by glutamine synthetase (mop up), NH3 increase in blood stream
May be the result of end stage liver disease
- hepatic cirrhosis, hep replaces with fibroblasts that lack urea cycle and GS
- portal hypertension (secondary to cirrhosis) portal flow to liver is restricted, leading to shunting of ammonia rich portal blood away from the liver and into systemic circulation
why does this lead to toxicity:
at neutral pH, NH4/NH3 100/1 ratio
-ammonia can diffuse across membranes
once inside can be trapped as NH4 (when this happens in brain :
NH3 cross BBB
-NH3 becomes NH4 in cells, which results in increased glutamate and glutamine which are also trapped. Glutamine draws in water, swelling cerebral edema. increased glutamate: abnormal signaling
tremors, slurred speech, blurred vision
what are the normal sources of NH3 in blood
slide 44
high protein low carb meal
increase both insulin and glucagon (primarily glucagon)
enough insulin to inhibit muscle breakdown
muscle can use dietary AA (BCAA and some nBCAA) for E releasing N into blood at glutamine and alanine
glucagon drives gluconeogenesis, glucose to brain and blood
right after eating: substrate availability drives AA degredation. cat enzymes have high Km
box and FA will help supply energy…need 6 atps for glucose 4 atps for urea
What happens to AA cat when we fast
decreased insulin and increase in cortisol result in muscle proteolysisL alanine transported to liver
glucagon and cortisol stim hepatic uptake of AA
-induce synth of enzymes needed for AA degredation
- conc of aa arise from protein degredation falls below high km of cat enzymes.
- driving force for aa cat in fast state is PHOSPHORYLATION dependent increases in catabolic enzyme activity which occur as the result of glucagon binding to receptor on hep.
as ketone body prod increses…need for aa cat is decreased slightly
cortisol
chol derived glucocorticoid, response to stress, synth by adrenal cortex
binds to cytosolic receptors within cells that then bind to DNA to alter GE
increases levels of BG:
liver: increase expression of gluconeogenic enz
musleL increase muscle prot catabolism
adipo: increase lipolysis for increased circulating FFA to feul gluconeogenesis
also inhibnits GLUT4
hypercatabolic state
EXTREME STRESS
-negative nitrogen balance
increased circulating catabolic hormones (cortisol, glucagon, epi)
relase of inflame cytokines
insulin resisitance
muscle wasting
increased gluconeogenesis and increase BG
increased circ of glutamine and other AA from muslc supports met and cell division of immune cells for would healing
mucle AA support synth of acute phase protiens (blood clot/uimmune) by liver
