Protein and Nitrogen Metabolism Flashcards
Protein Turnover
- Protein turnover- proteins are constantly degraded; 75% recycled in synthesis and 25% catabolized and used for gluconeogenesis
- Diff proteins have diff half-lives
Nitrogen Balance (+ Pos and Neg Balances)
- Nitrogen balance- nitrogen intake - nitrogen excretion
- Pos balance - more input
- Pregnancy, growth, lactation
- Recovering from recent illness (after)-metabolic stress, injury
- Neg Balance - more excretion
- Inadequate dietary protein
- During actual illness- metabolic stress, sepsis, trauma
- Deficiency of an essential AA
- Pos balance - more input
What is the minimum dietary protein requirement for adults?
- MIN dietary requirement - 56g/day for avg 70 kg adult
this is minimum needed to maintain nitrogen balance
Biological Value
- Biological Value - depends on whether or not that protein has essential AAs and digestibility
- Generally animal sources»_space; plant sources
Why is a mixed diet important for vegetarians?
Mixed diet ensures that you will get all essential AAs (whatever is missing from one ingredient will be in another)
Supplementation or complementation
Name the Essential AAs (+ 1 conditionally essential)
- PVT TIM HALL
- phenylalanine, valine, threonine
- tryptophan, isoleucine, methionine
- histidine, arginine**, leucine, lysine
- **Arginine is “conditionally essential” because only needed as an infant/child then can make it on own
What is the metabolic fate of dietary AAs? (6 options)
- NOT STORED
- Digested —> AA pool then…
replenish tissue proteins (turnover) make non-essential AAs, used immediately as energy, makes specialized products (nucleotides, heme, cell signal molecules, pigments) leaves as nitrogenous waste
How does protein malnutrition lead to edema?
- Dec protein intake —> dec albumin (hypoalbuminemia)
- Less albumin in blood means that fluid is not contained in blood vessels and leaks —> edema (swollen abdomen)
Proenzymes
Endopeptidases
Exopeptidases
- precursors of proteolytic enzymes (inactivate); activated once in GI tract via limited proteolysis
- proteolytic enzymes that hydrolyze/cleave protein in middle of polypeptide chain
- proteolytic enzymes that hydrolyze/cleave protein @ ends (either C or N end)
Peptidases in Stomach, Pancreas and SI
Stomach- pepsinogen- inactive precursor of pepsin; secreted by chief cells; autocatalytic activation if low enough pH; once active it cleaves C term of proteins
Pancreas- zymogens (trypsin, chymotrypsin, elastase, CPA an CPB)
SI-oligopeptidases
What are the 2 nonenzymatic components of gastric acid? What does each do?
HCl- secreted by parietal cells; dec pH to denature proteins and create low enough pH for activation of pepsinogen/ activity of pepsin
Gastrin- peptide hormone secreted by mucosal cells; gastrin works on parietal and chief cells to stimulate their release of above molecules
How are pancreatic proteases activated?
CASCADE
- Trysinogen is activated —> trypsin (via enteropeptidase)
- Trypsin then goes on to activate remaining zymogens (chymotrypsin, elastase, CPA and CPB)
What are the final products of protein digestion in the lumen of SI?
free AAs + di/tripeptides
How are AAs transported in and out of SI cells?
- Lumen side- depends on Na+ gradient (active transport)
- Brush border side- Na+ indep (passive transport) ; AAs leave cell to enter portal circulation by moving down natural concentration gradient
What 2 conditions are caused by long-term use of proton pump inhibitors?
- Proton Pump Inhibitors - used for acid reflux/heartburn
- Block parietal cells (HCl) which also release intrinsic factor which is needed to absorb B12)
- B12 deficiency —> anemia OR peripheral sensory/motor neuronal deficiencies
What 2 reactions remove amino groups from AAs?
- 1- deamination
- Prod free NH4+ directly
- 2- transamination
- transfer NH4+ to an acceptor
Glucogenic v Ketogenic AAs
- Gluconeogenic - carbons contribute to glucose production (gluconeogenesis)
- All but 2 AAs have this capacity
- Make pyruvate, OAA, alpha-KG, succinyl CoA, fumarate
- Ketogenic - carbons contribute to ketone formation
- Only pure keotgenic AAs are lysine and leucine
- Make acetyl CoA and acetoacetyl CoA
What are the 3 main alpha-keto acids that accept amino groups?
alanine + alpha-KG pyruvate + glutamate
aspartate + alpha-KG OAA + glutamate
glutamate + NADP+ —> alpha-KG + NADPH + NH4+
What 2 major reactions prevent accumulation of ammonia in peripheral tissues?
- ALT: alanine + alpha-KG pyruvate + glutamate
- AST: aspartate + alpha-KG OAA + glutamate
both reversible
What role does Vit B6 have in AA metabolism?
Vit B6 is precursor for pyridoxal-P (transaminase coenzmye) so required in order to transamination to work
Effects of isoniazid and penicillamine
- Isoniazid- used for tuberculosis; “suicide substrate” that covalently binds pyridoxal so that it cannot by phosphorylated by pyridoxal kinase
- Penicillamine- used to treat Willson’s disease; inactivates pyridoxal (amine of drug reacts w/ aldehyde of pyridoxal)
**Both treated w/ extra pyridoxine to normalize transaminase activity levels
What is the general route by which AA nitrogen in muscle gets incorporated into glutamate or aspartate in liver?
1-Nitrogen waste in muscle is converted to glutamate
2- Glutamate + pyruvate —> alanine (via ALT)
3- Alanine transferred to liver in blood (good because non-toxic unlike NH4+)
4- Alanine converted back to glutamate in liver (via ALT)
5- glutamate can also react w/ OAA —> alpha-KG + aspartate (via AST)
**both glutamate and aspartate can be converted to urea
Why do alanine and glutamine make up more than 50% of AAs released from muscle?
B/c these are by-products of branched chain AA transamination (which occurs primarily in muscle by BCAT)
What is the major metabolic fuel for the small intestine? What by-products are produced?
uses glutamine (from diet as well as muscle)
glutamine —> citrulline + alanine —> released into circulation
BCAA catabolism reactions and location
Leucine, isoleucine, valine
transaminated via BCAT (mainly in muscle mito)
BCAA –> alpha keto acid
carboxylated via BCKA DH (in muscle, kidney, liver and brain MITO)
alpha keto acid –> CoA derivative
BCKA DH v pyruvate and alpha KG DH
- All 3 unit enzymes w/ identical E3
- Like pyruvate DH regulated by own kinase/phosphatase that modify E1
- All in mito
- Similar structure and mechanism
- All require 5 coenzymes (thiamine pyrophosphate, lipoid acid, CoASH, NAD+, FAD)
How is propionyl CoA used?
- Propionyl CoA —> D-methylmalonyl CoA —> L-methymalonyl CoA —> succinyl CoA —> TCA Cycle
- Propionyl CoA carboxylase requires biotin
- Methymalonyl CoA mutase requires it B12
What are the symptoms of propionyl CoA utilization problems?
Back up of propionyl CoA or methylmalonyl CoA –> side reaction w/ carnitine –> acyl carnitine which is then excreted (SO CARNITINE DEF)
Hypoglycemia b/c that propionyl CoA is not being made into succinyl CoA –> TCA cycle for eventual gluconeogenesis
Hyperammonia b/c buildup –> dec ATP and metabolic stress on enzymes of urea cycle (ammonia not converted to urea as efficiently)
What defect causes maple syrup urine disease?
deficiency or defect in BCKA DH complex
How to treat propionic or methylmalonic acidemia?
Restricted protein intake (so no extra BCAA)
Some methylmalonic acidemia can be treated w/ Vit B12 supplements if it is an acquired acidemia from Vit B12 deficiency BUT not if it is a genetic defect w/ the enzyme itself
Regulation of BCAA Metabolism
- Allosterics - inhibited by acyl CoA and NADH
- Covalent - BCAK DH is deactivated when phosphorylated
- Even though this is a catabolic rxn (BCAA are being broken down), it contributes to the overall anabolic state (fed state) of the body b/c BCAAs are essential AAs so when the body is fed it can afford to break them down while when the body is fasted/starved it wants to conserve these essential AAs so it halts this step of metabolism
- So…fits w/ insulin —> more BCKA made —> inhibits the kinase —> no longer phosphrylates/deactivates BCKA DH
Name 3 Carriers of 1-Carbon Units + Oxidation States of Carbons They Carry
- Folic acid - active form is THF
- Oxidation states -
- Methyl (most reduced)
- Methylene
- Formyl
- Formimino
- Methenyl (most oxidized)
- Oxidation states -
- SAM- carries methyl groups (most reduced)
- Biotin - transfers CO2 groups
3 synthetic pathways that require 1-carbon units
- Making epinephrine from norepinephrine
- Synthesis of carnitine
- Synthesis of creatine (**greatest use of SAM)
What is the role of SAM in metabolism?
Synthesis of creatine
Coordinated between kidney enzyme (AGAT) and liver enzyme (GAMT)
What 2 reactions are important in making nonessential AAs? Examples?
- Transamination -transfer amino group to alpha-keto acid
- Ex) alanine + alpha-KG —> glutamate + pyruvate
- Amidation -formation of amide bond
- Ex) Glutamate + NH4+ + ATP —> glutamine + ADP + P
- Ex) Aspartate + glutamine (N source) + ATP —> asparagine + glutamate + ADP + P
Which glycolysis and TCA molecule are precursors for non-essential AAs?
- Phosphoglycerate —> serine —> glycine
- Pyruvate —> alanine
- Alpha-ketogluterate —> glutamate —> proline, arginine, glutamine
- OAA —> aspartate —> asparagine
Which 2 non-essential AAs are made from essential AAs?
- Phenylalanine —> tyrosine
- Methionine —> cysteine
Classic PKU
Atypical PKU
Maternal PKU
- Classic- deficiency in phenylalanine hydroxylase (auto recessive)
- Atypical - deficiency in enzymes needed for biopterin biosynthesis or regeneration of BH4 (source of reducing power)
- Maternal PKU - damage to fetus from mom’s phenylketones
What are the consequences of folic acid deficiency?
in pregnancy can lead to spina bifida
Which 2 reactions require Vit B12?
- B12 deficiency —> cannot recycle methionine —> folate trapped in 5-CH3-THF form —> secondary folate deficiency (cannot be used in this form)
- B12 also required for methylmaolonyl CoA —> succinyl CoA
Endogenous Arginine Synthesis
- First 2 enzymes of urea cycle in SI - make citrulline —> blood —> kidney
- Next 2 enzymes of urea cycle in kidney - make arginine
***So citruline levels in plasma are indicative of function SI mass
Deficiency in which enzyme leads to black urine?
- *Alkaptonuria**
- deficiency in homogentisic acid oxidase (catalyzes third step in tyrosine catabolism)
- Urine has high levels of homogentisic acid which readily oxidizes and turns urine black
Metabolic Functions of Glutamine (in which tissues?)
- Precursor form of nitrogen (esp for purine and pyrimidine synthesis) - All tissues
- Non-toxic transporter of ammonium ions from extrahepatic tissues to liver where it is converted to urea
- Major fuel for enterocytes and macrophages and lymphocytes - Gut and immune system
- Maintains acid-base balance - can be made into bicarb during metabolic acidosis - In kidney
How, where and why is glutamine synthesized?
- Made via detox in extrahepatic tissues
- Glutamate + NH4+ + ATP –> Glutamine + ADP
- Glutamine synthetase (GS)
- Glutamate + NH4+ + ATP –> Glutamine + ADP
- To get rid of NH4+ (toxic)
- Cytosol of all tissues but esp brain (NH4+ is toxic) AND muscle (high protein turnover)
How do different liver cells lead to efficient detox?
- Periportal Hepatocytes - enriched in glutaminase (glutamine –> NH4+) and CPS-1 (NH4+ –> carbamoyl phosphate)
- Glutaminase inc the conc of ammonium in this cell in order for CPS-1 to work (has a high Km - so need high conc of NH4+)
- Perivenous Hepatocytes - enriched in glutamine synthestase (NH4+ –> glutamine)
- Has low Km for ammonium so that the ammonia ions that are not used in urea cycle are easily converted to glutamine and trapped in perivenous cells
Where does urea get its carbon, oxygen and nitrogen?
- Nitrogen- ammonium ions (from blood and action of glutaminase and glutamate DH) and aspartate (AA pool or transamination of OAA)
- Carbon and oxygen- bicarbonate (reacts w/ NH3 to make the carbomyl phosphate)
Relationship b/n ureagenesis and gluconeogenesis
Urea cycle produces fumarate which then enters TCA cycle and adds net carbons to it –> produce more malate–> OAA –> gluconeogenesis
What is the energy cost to make 1 urea molecule?
uses 3 ATP but one ATP is further broken down to AMP so uses equivalent of 4 high energy phosphate bonds
Short term and long term regulation of urea cycle
- Short-term
- CPS-1 is regulated step
- Allosterically activated by NAG (high acetyl Coa-high energy + glutamate –> NAG in mito)
- Long-term
- Level of urea cycle enzyme transcription changes w/ level of dietary protein intake
- Inc in high protein diet or starvation (because protein broken down then)
- Dec in low protein diet
How would you detect a CPS-I deficiency?
low citrulline but no high urinary orotate b/c carbamoyl phosphate not made
How would you detect a OTCase deficiency?
high AAs, low citrulline, high urinary orotate (side reaction of carbomyl phosphate–> pyrimidine synthesis)
How would you detect a ASS deficiency?
build up of citrulline and no argininosuccinate in blood
How would you detect a ASL deficiency?
build up of arginosuccinate in blood
Why are urea cycle disorders treated w/ benzoate and phenylacetate?
- Sodium benzoate- makes benzyl CoA which reacts w/ glycine (N) –> hippuric acid –> excreted in urine
- Sodium phenylacetate- makes phenylacetate Coa which reacts w/ glutamine (2N) –> phenylacetylglutamine –> excreted in urine
**phenyl more effective b/c gets rid of 2N
4 major end products of nitrogen metabolism
- Urea -urea cycle in liver
- Creatinine -derived from spontaneous cyclization of creatine or creatine phosphate
- Ammonium Ion -from glutamine –> glutamate –> alpha-ketogluterate
- Uric Acid - from purine catabolism
What 2 reactions are required for renal ammoniagenesis?
- Glutamine –> glutamate (via glutaminase)
- Glutamate –> alpha-KG (via glutamate DH)
**prod 2 NH3
How is ammoniagenesis related to H+ excretion?
Gluatamine –> glutamate also produces 2 molecules of bicarb (acts as buffer)
What metabolic abnormality would lead to increase in renal ammoniagenesis?
If acidosis …you do not want to use that many cations so inc ammoniagenesis so that you now use more NH3/NH4+ buffer instead
How does increased renal ammoniagenesis lead to increase in renal gluconeogenesis?
b/c alpha-KG produced in ammoniagenesis–> TCA cycle –> inc OAA –> gluconeogenesis
Creatine synthesis v creatinine synthesis
- Creatine - made by enzymatic reaction (using SAM); also requires ATP
- Creatinine- made by spontaneous, non-enzymatic reaction (creatine or creatine phosphate spontaneously cyclizes)
How are purines catabolized? What is the end product?
- Remove phosphate –> nucleoside
- Remove ribose –> purine base alone
- Oxidize w/ xanthine oxidase –> xanthine
- Oxidize again w/ xanthine oxidase –> uric acid (end product)
What 3 enzyme defects can lead to inc uric acid production?
- Def in HGPRT (cannot recycle purines)
- Abnormally high activity of PRPP Synthetase (inc purine synthesis)
- Gluc-6-P def
Why is allopurinol used to treat gout?
- inhibits xanthione oxidase so less uric acid produced and build up of hypoxanthine which will be recycled back to purines via HGPRT
What are the digestion products of polynucleotides?
purines, pyrimidines, ribose, deoxyribose, phosphate
Purine synthesis v pyrimidine synthesis
- Purines - (PRPP then make ring on ribose molecule)
- Pyrimdines - (make ring then add to PRPP- make ring before on ribose molecule)
What metabolites give nitrogen and carbon to purines?
- Carbon from… glutamine, glycine, THF and CO2
- Nitrogen from glutamine, aspartate
What metabolites give nitrogen and carbon to pyrimidines?
- Carbon- aspartate and CO2
- Nitrogen- glutamine, aspartate
What is the primary regulatory mechanism to control nucleotide metabolism?
- Allosterics
- Act by availability of substrates
- Inhib by end products
What makes certain cells more sensitive to inhibition of nucleotide metabolism?
rapidly proliferating cells more sensitive (autoimmune diseases, cancers, ectopic pregnancy, immunosuppression in transplant, anti-tumor)
Name 5 enzymes in nucleotide metabolism that are targeted by therapeutics
- PPAT (PRPP –> IMP)
- Enzymes that convert IMP –> ATP and GTP
- HIV reverse transcriptase
- Viral DNA polymerases
- Thymidylate synthase that converts dUMP –> dTMP
