protein, nitrogen, b vitamins Flashcards
1
Q
- Describe how nitrogen balance differs in health and disease
A
● Nitrogen balance is a comparison between the intake of nitrogen (mostly in the form of protein) and the excretion of nitrogen (mostly in the form of urea or urine)
● Normal is nitrogen equilibrium (nitrogen balance); nitrogen losses = nitrogen intake
● A positive nitrogen balance (input exceeds output) occurs during growth, pregnancy, lactation, recovery from metabolic stress or injury (or surgery/trauma)
● A negative nitrogen balance (output exceeds input) occurs with low dietary protein, deficiency of essential AA and metabolic stress, sepsis, or trauma
2
Q
dietary protein requirement for healthy adults
A
● A 70 kg adult needs about 56 g protein/day
3
Q
essential a.a.
A
arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine
4
Q
Protein Excess consequenses
A
loss of calcium -> osteoporosis, hyperfiltration in kidneys
5
Q
why does hepatomegaly result from reduced protein intake
A
no vldl proteins to transport fat
6
Q
- Name a protease that is produced by the (a) stomach, (b) pancreas and (c) small intestine
A
● Stomach = pepsin
● Pancreas = trypsin and chymotrypsin
● Small intestine (no inactive precursors) = peptidases
7
Q
- Define proenzymes, endopeptidases and exopeptidases
A
● Proenzymes (aka zymogens) = inactive state; enzymes that need to be cleaved at one or more sites in order to become active, functioning enzymes
● Endopeptidases = cleave proteins by hydrolyzing peptide bonds within the polypeptide chain
● Exopeptidases = cleave AA from either N or C terminal ends of peptides or proteins
8
Q
- Name 2 nonenzymatic components of gastric juice and describe how each contributes to protein digestion
A
● Gastrin – secreted in response to vagal stimulation as the chyme enters the stomach; acts on the parietal cells to secrete HCl and the chief cells to secrete pepsinogen
● HCl – decreases pH to denature protein and provide proper environment for pepsin; initiates limited proteolysis to create active pepsin – at pH over 2, complex is inactive
9
Q
- Describe the cascade mechanism by which the pancreatic proteases are activated in the GI tract
A
● Secretin and CCK secreted in response to chyme moving through duodenum
● Secretin stimulates release of pancreatic juice (bicarbonate is basic to neutralize acidic contents from stomach)
● CCK stimulates release of pancreatic zymogens intot he lumer of the small intestine(inactive precursors to enzymes) and release of bile from the gall bladder
● Enteropeptidase (from brush border intestine) catalyzes convesion of trypsinogen to trypsin; trypsin then catalyzes limited proteolysis of the other zymogens to produce active chymotrypsin, elastase, CPA and CPB
10
Q
- List the products of protein digestion within the lumen of the small intestine
A
● About 35% neutral and basic AA
● About 65% oligopeptides
11
Q
- Describe the mechanisms by which AA are transported from the lumen of the small intestine to the portal blood
A
at brush border (oligopeptides->peptides and a.a.) Na pump transports them in and basolateral transporters facilitate diffusion into portal blood
12
Q
- Explain why inherited defects in intestinal absorption of specific AA correlate with increased renal excretion of those AA
A
● The system of reabsorption of AA in the kidney is the same as the system for absorption in the intestines, so if it can’t be absorbed in the intestine, it can’t be absorbed in the kidney and is excreted (which exacerbates the effect since intake and retention are both affected)
13
Q
- Name two disorders that can result from long-term use of proton pump inhibitors
A
vitamin B12 deficiency -> anemia and peripheral motor problems
14
Q
- Describe the two major types of reactions for removing amino groups from AA
A
deam, transam
15
Q
- Name three glucogenic compounds from aa catabolism
A
pyruvate, oxaloacetate, α-ketoglutarate, succinyl-CoA or fumarate (important AA = alanine, aspartate, glutamate)
16
Q
two ketogenic compounds from aa catabolism
A
acetyl-CoA or acetoacetyl-CoA (important AA = branched chain and aromatic AA)
17
Q
- Identify two major reactions that prevent accumulation of ammonia from amino acid metabolism in peripheral tissues
A
ALT, AST
18
Q
- Name the three principal α-keto acids that serve as acceptors of amino groups and discuss the role that each plays in AA metabolism
A
● α-ketoglutarate = nitrogen acceptor in most transaminase reactions; accepts amino groups from glutamate
● Oxaloacetate = in liver, OAA acts as acceptor for some amino groups from glutamate (AST, product is nitrogen donor for urea synthesis); accepts amino groups from aspartate
● Pyruvate = major acceptor of nitrogen in skeletal muscle (ALT); accepts amino groups from alanine
19
Q
- Identify the role of Vitamin B6 in AA metabolism
A
● Precursor for pyridoxal phosphate - coenzyme in transamination reactions and some deamination reactions
20
Q
- Describe the general route by which the AA nitrogen in muscle gets incorporated into glutamate or aspartate in liver
A
● N transferred to alanine by transaminase in the muscle
● Alanine transported to liver● In liver, N transferred from alanine to glutamate (ALT)
● N transferred from glutamate to aspartate (AST)
21
Q
- Explain why patients receiving either isoniazid or penicillamine therapy may require pyridoxine supplements
A
suicide substrates with b6
22
Q
- Provide an explanation for why alanine and glutamine make up more than 50% of the AA that are released from muscle
A
● The muscle metabolizes branched chain AA, resulting in excess release of alanine and glutamine by muscle
● BCAA are essential – you only get them from the diet! Liver can’t start the first step in BCAA breakdown (transamination – BCAT is not in the liver); now you have a nitrogen atom that you need to get rid of, and you’re either going to give it to pyruvate to get alanine or to α-KG to get glutamate. By the action of glutamine synthase (which is highest in the muscle), you will add ammonia to glutamate to get glutamine! This is a way of getting rid of waste nitrogen in the periphery and avoiding hyperammonemia.
23
Q
- Describe the significance of alanine synthesis in muscle, its metabolic fate in liver, and name the major enzymes involved in the interorgan metabolism of alanine
A
● Alanine synthesis in muscle (from pyruvate) allows removal of ammonia without it being released as free ammonia (catalyzed by transaminases)
● Alanine goes to liver, where its amino group is incorporated into urea and the carbon skeleton is used for gluconeogenesis
24
Q
- Name the major metabolic fuel for intestinal cells and the product generated from this fuel
A
● Glutamine is major metabolic fuel for intestine (from dietary protein or skeletal muscle)
● Glutamine metabolism produces alanine and citrulline, which are released into circulation
25
4. Name the three branched chain AA,
● Valine, Isoleucine and Leucine
26
describe the two common reactions that are involved in the catabolism of BCAA
transaminase and dehydrogenase
27
where are BCAAs transaminated to a-ketoacids
skeletal muscle
28
where are a-ketoacids dehydrogenated
liver
29
5. Compare and contrast BCKA DH with pyruvate DH and α-ketoglutarate DH
● The E3 subunit (dihydrolipoyl DH) is an identical gene product in all 3
● Both BCKA DH and pyruvate DH have a specific kinase and phosphatase associated with the complex to (de)phosphorylate the E1 subunit
● TPP, lipoic acid and FAD are prosthetic groups on BCKA DH (E1, E2 E3, respectively), but cofactors for pyruvate DH
● Both α-ketoglutarate DH and BCKA DH release NADH and CO2
30
6. Describe the reactions that are required to convert propionyl-CoA into succinyl-CoA and list the cofactors that are required for these interconversions
● Proprionyl-CoA carboxylase: Propionyl-CoA + CO2 + ATP D-methylmalonyl-CoA (biotin)
● Epimerase: D-methylmalonyl-CoA L-methylmalonyl-CoA
● Methylmalonyl-CoA mutase: L-methylmalonyl-CoA converted to Succinyl-CoA by (B12
31
7. Explain how a defect in propionyl-CoA utilization can lead to a carnitine deficiency, hypoglycemia and hyperammonemia
● Proprioynl-CoA becomes a substrate for CPT-I since it’s technically a fatty acyl-CoA, and you end up with the acyl-carnitine derivative, which is metabolically useless and gets excreted
● Excretion of carnitines leads to a deficiency, and long-chain FAs cannot be broken down
● You have reduced energy for gluconeogenesis, so you have hypoglycemia
● As you keep accumulating proprionyl-CoA, you’re tying up CoA molecules so that you cannot run the urea cycle properly (need CoA to make NAG) hyperammonemia
32
how is bcaa metabolism regulated
BCKA DH, allosterically by NADH/NAD, acylCoA/CoA, phosphoylation
33
10. Provide an explanation for why some cases of methylmalonic aciduria can be treated with vitamin B12 and others cannot
● The problem is a deficiency in methylmalonyl-CoA mutase, ● If the enzyme itself has a major defect or no functional enzyme can be synthesized, the addition of a cofactor will do nothing to help
34
11. Name the defect in maple syrup urine disease (MSUD)
● Deficiency in BCKA DH complex (problem with metabolism of BCAA, leads to all sorts of problems, including lack of myelin and reduced total lipids, major damage is to brain, but also other problems)
● Treatment = no BCAA in diet initially and then supplement to maintain normal concentrations, avoid breakdown of tissue protein (infection)
● The defect here can occur in any one of the subunits of BCKA DH, but a defect in the E3 subunit would also affect the PDH complex and α-ketoglutarate DH!
35
1. Name 3 carriers of one-carbon units and compare the oxidation state of the carbon carried by each
● Tetrahydrofolate (THF) from folic acid (more reduced /middle carbons) – the transfer potential is not sufficiently high for most biosynthetic reactions
● Formic Acid (more oxidized carbons)
● S-adenosyl methionine (SAM) carries methyl groups (most reduced)
● Biotin carries carboxylate group (most oxidized carbon)
36
2. List three biosynthetic pathways that require one-carbon units
● Synthesis of non-essential AA
● Synthesis of purines
● Synthesis of pyrimidines
37
3. Describe the role of S-adenosylmethionine (SAM) in metabolism
● SAM is the preferred “activated methyl” carrier in most methylation reactions in biosynthesis
● Involved in synthesis of creatine, carnitine, epinephrine and methylated DNA and RNA
38
4. Describe two types of reactions that are important in the synthesis of non-essential AA and give an example of each
● Transamination (of an amino group to an α-keto acid) example = alanine from pyruvate; aspartate from oxaloacetate; glutamate from α-KG; reversible!!
● Amidation (formation of amide bond, can either be addition of ammonia or transfer; requires ATP) example = glutamine from glutamate and ammonia; asparagine from aspartate and glutamine (enzymes are called ______ synthetase, i.e. glutamine synthetase)
39
5. Name the intermediates in glycolysis and the citric acid cycle that are precursors for non-essential amino acids
● 3-phosphoglycerate serine glycine
● Pyruvate alanine
● α-ketoglutarate glutamate (which then can form proline, arginine, and glutamine)
● oxaloacetate aspartate asparagine
40
6. Name two non-essential AA that are synthesized from essential AA
● Tyrosine (from phenylalanine – requires phenylalanine hydroxylase, THB, DHB reductase, and NADPH
● Cysteine (from from methionine – requires SAM, Vit B12, and folic acid!)
41
7. Compare and contrast classical PKU, atypical PKU and maternal PKU
● Classical PKU = deficiency in phenylalanine hydroxylase (can’t catalyze formation of tyrosine)
● Atypical PKU = deficiency in biopterin or in dihydrobiopterin reductase (can’t rereduce cofactor for phenylalanine hydroxylase reaction)
● Maternal PKU = damage to fetus by maternal phenylketones (phenylketones formed when tyrosine can’t be synthesized)
● All three have to do with not being able to form tyrosine from phenylalanine, but different parts of pathway deficient, so treatments may be different
42
8. Describe the consequences of a folic acid deficiency
● A folic acid deficiency would mean an inability to carry all one-carbon groups except methyl, which could mean free formaldehyde (which is toxic)
● The synthesis of compounds that require one-carbon units would also be impaired (synthesis of purine, methionine, thymidylate, etc.)
43
9. Describe the two reactions in humans that require Vitamin B12 and explain how a deficiency in Vitamin B12 can result in a secondary folate deficiency
● Conversion of L-methylmalonyl-CoA to succinyl-CoA in BCAA metabolism (Vitamin B12 deficiency results in anemia because succinyl-CoA is required for heme synthesis)
● Remethylation of SAM requires Vitamin B12 as a cofactor for Methionine synthase to convert homocysteine to methionine
● B12 is required for conversion of 5-Methyl-THF to THF (the active carrier form of folate), so 5-Methyl-THF builds up and you don’t have any usable folate – this is part of the methyl transfer reaction that converts homocysteine to methionine
44
10. Describe the pathway for endogenous synthesis of arginine
● Synthesized by liver as an intermediate in urea cycle, but no net synthesis since it is cleaved to urea and ornithine almost immediately
● Citrulline from small intestine (from glutamine) transported to kidney, where it is added to aspartate to make arginine (little arginase activity in kidney, so it is intact)
● Defects in any of 4 urea cycle enzymes will also affect arginine synthesis
45
11. Name the amino acid whose plasma concentration is an indicator of functional small intestinal mass
citrulline
46
why does pee turn black
alkaptonuria - defect in homogentisic acid oxidase - part of tyrosine catabolism
47
1. List three metabolic functions of glutamine and indicate organs where they are most important
● Precursor form of nitrogen for purine and pyrimidine biosynthesis, precursor for other AA
● Nontoxic transporter of ammonium ion from extrahepatic tissue to liver, also helps maintain acid-base balance in kidney (especially important for metabolic acidosis because it provides proton sink)
● Major fuel for the enterocyte, macrophages and lymphocytes
● In the brain, the synthesis of glutamine converts ammonia into a non-toxic form that can be used as a source of amino groups for biosynthesis
● Excess glutamine is degraded in the liver to α-KG and NH4+ where urea the ammonia is converted to urea
48
2. Describe the reaction by which glutamine is synthesized, where does it occur and what is the physiologic significance of this reaction
● Glutamate converted to glutamine by glutamine synthetase using ATP
● Occurs in cytosol of all tissues, but especially the brain (ammonia is super toxic to CNS) and muscle (high turnover of muscle protein, and therefore high ammonia production)
● Important for detoxification of ammonia and for synthesis of Glutamine, which is used in biosynthesis and as a fuel in some tissues
49
3. Describe how the kinetic properties and hepatic localization of CPS-1 contributes to efficient detoxification of ammonia by the liver
● CPS-I is in mitochondria of periportal hepatocytes (near portal vein) and begins ammonia transformation into urea (enriched in glutaminase and enzymes of urea synthesis), ridding the body of excess nitrogen, and removing most of the ammonia from the blood.
HIGH Km. Liver siphons a lot of NH4 to keep it running at max rate
50
Describe how the kinetic properties and hepatic localization of Glutamine synthetase contributes to efficient detoxification of ammonia by the liver
● Glutamine synthetase is located in the cytosol of perivenous hepatocytes (near central vein), and takes any trace amounts of ammonia that remain in the blood to form glutamine so that the free ammonia in the blood going to extrahepatic tissues is very low
LOW Km. get NH4 out of blood before it returns to extrahepatic circulation
51
4. Identify the immediate precursors for the nitrogen, carbon and oxygen atoms in urea
● The nitrogen (in the form of amino groups) comes from aspartate and ammonium ion
● The carbon and oxygen (in the form of a carbonyl group) comes from a bicarbonate ion
52
5. List the properties of urea that make it a good physiologic choice as a molecule for disposal of waste nitrogen
● Nontoxic
● Polar (dissolves readily in water for disposal in urine)
● Simple molecule, easy to synthesize (energy cost is not too high)
53
6. Understand the relationship between ureagenesis and gluconeogenesis
● Both happen in liver
● Degradation of amino acids produces carbon skeletons for gluconeogenesis via propionyl-CoA and the TCA cycle
● Alanine from skeletal muscle (from pyruvate and ammonia) is used to transport ammonia for ureagenesis to liver, and then is used to regenerate pyruvate in liver (which is used for gluconeogenesis, which goes to the blood and may return to skeletal muscle) – alanine cycle
54
7. Understand the energy cost for synthesizing each molecule of urea
● The equivalent of 4 high energy phosphate bonds per urea (from 3 ATP – 2 become ADP, one is hyrdrolyzed to AMP)
55
8. Compare and contrast the mechanisms for short and long term regulation of the urea cycle
● The short term regulation is exerted at the level of carbamoyl phosphate synthetase I (CPS-I) which catalyzes the initial, committed step in urea synthesis; allosterically regulated by NAG which is synthesized inside the mito from glutamate and acetyl-CoA
● The long-term regulation depends on changes in the levels of urea cycle enzymes, primarily by changes in the transcription of the corresponding genes (response to dietary intake of protein)
● Since NAG synthase is activated by arginine (an AA), both pathways are somewhat regulated by AA levels
● Urea excretion based on nutritional state: normal (normal); high protein diet (increased); protein-free diet (decreased); starvation (increased)
56
What is NAG and why is it important
allosteric activator for CPS-1, requires acetylcoA and glutamate
57
● Hyperammonemia Type I
defect in CPS-I (low plasma citrulline, low urinary orotate)
58
● Hyperammonemia Type II
defect in OTCase (low plasma citrulline, high urinary orotate)
59
● Citrullinemia
defect in argininosuccinate synthase - ASS (high plasma citrulline)
60
● Argininosuccinic aciduris
defect in argininosuccinate lyase - ASL (medium plasma citrulline, argininosuccinate and anhydrides in plasma)
61
● Hyperargininemia
defect in arginase (no hyperammonemia)
62
why use benzoate and phenylacetate to treat urea cycle disorders
both promote alternative pathways of nitrogen excretion. phenylacetate gives off two nitrogen, benzoate gives off one
63
1. Name the four major end products of nitrogen metabolism and describe the pathways that give rise to these compounds
● Urea (urea cycle)
● Creatinine (spontaneous cyclization of creatine or creatine phosphate, non-enzymatic)
● Ammonium Ion (successive removal of amide group from the side chain of glutamine and then the α-amino group of glutamate; in the kidney)
● Uric Acid (catabolism of purines – nucleotide to nucleoside to purine base to Xanthine to Uric acid)
64
2. Describe two enzymatic reactions required for renal ammoniagenesis
● Conversion of glutamine to glutamate by glutaminase (releasing NH3)● Conversion of Glutamate to α-KG by glutamate DH (generating NADH and releasing NH3)
● Carbon skeleton of α-KG can then be utilized by renal gluconeogenesis
65
3. Relate ammoniagenesis to acidosis and proton excretion and predict two abnormalities of metabolism that would result in increased ammoniagenesis
● Renal glutaminase is induced in response to acidosis
● Ammoniagenesis facilitates proton secretion by tubular cells because the protons in the final reaction come from H2CO3 or excess H+ in filtrate
● Increased ammoniagenesis would result from acidosis (increased production of lactic acid) because there is a limit to how much pumps can create a proton gradient (but ammonium ions can be a buffer for excreted protons)
● Problems with the TCA cycle or conversion of pyruvate to Acetyl-CoA (leading to increased lactic acid generation and lactic acidosis) would lead to increased ammoniagenesis
● Diabetic ketoacidosis (or starvation, causing ketoacidosis) would cause increased ammoniagenesis as well
66
4. Explain how increases in renal ammoniagenesis can result in increases in gluconeogenesis
● The process of ammoniagenesis generates α-ketoglutarate, which can be utilized for renal gluconeogenesis via the TCA cycle and renal PEPCK
● Since α-ketoglutarate would build up with increased ammoniagenesis, it is logical that gluconeogenesis would proceed based on the equilibrium (an excess of substrate generally increases activation in pathways that use that substrate)
67
5. Compare and contrast the reactions that give rise to creatine and creatinine
● Creatinine is generated from a non-enzymatic spontaneous cyclization of creatine or creatine phosphate requiring arginine, glycine, and SAM
● Creatine requires an aminotransferase and a transmethylase, as well as SAM as a methyl carrier
● Creatine is built through addition of carbons, whereas creatinine is really more of a change of formation via hydrolysis
68
6. Name the end product of purine catabolism and describe the general strategy used by mammalian cells to degrade purines
● Uric Acid
● Catabolism of purines – nucleotide to nucleoside to purine base to Xanthine to Uric acid (Xanthine oxidase and oxygen used in final step, which generates hydrogen peroxide)
● The strategy is to remove substituents from the purine ring to make it more soluble by oxidation of the carbon atoms
● Remember that humans have no enzymes that can open up the purine ring system and degrade it to smaller fragments for excretion!
69
7. Name three enzyme defects that result in the overproduction of uric acid
● Partial deficiency in HGPRT (Lesch-Nyhan Syndrome – characterized by elevated uric acid, uncontrolled muscular movements and other CNS defects, and compulsive self-mutilation)
● Abnormally High Activity of PRPP Synthetase
● Glucose-6-phosphatase deficiency
● Overproduction or decreased renal clearance of uric acid causes gout
● Increased tissue turnover (from psoriasis and some cancers) can be secondary causes of hyperuricemia
70
8. Provide a biochemical rationale for the use of allopurinol to treat gout
● Allopurinol is an analog of hypoxanthine that acts as a suicide inhibitor of xanthine oxidase, which prevents the formation of uric acid
71
digestion products of polynucleotides
purines, pyrimidines, ribose, deoxyribose, PO4
72
where is ribose made
pentose phosphate
73
how are purines synthesized
Ribose turned into PRPP by regulated enzyme PPRP synthase, PPRP turned into inosine and then either adenine or guanine
74
how are pyrimidines synthesized
form carbamoyl phosphate from GLN CO2 2ATP with regulated enzyme CPS2. Carbamoyl PO4 turned into oroate and then OMP then UMP. U is interchangable with C using glutamine
75
primary reg mech controlling nucleotide metabolism
allostery
76
what property of cells sensitizes them to inhibitors of nucleotide metabolism
their proliferation rate
77
enzyme and co factors for conversion of ribo to deoxyribo
ribonucleotide reductase, thioredoxin
78
enzymes targeted by cancer drugs
thymidylate synthase (5-fluoruracil), azidothymidine (reverse transcriptase), viral thymidine kinase (acyclovir), dihydrofolate reductase (regenerates THF, drug = methotrexate), bacterial dihydropteroate synthetase (THF regeneration, sulfonamides), monophosphate dehydrogenase (mycophenolic acid)
79
B1 Thiamine
Dehydrogenases (Pyru, a-KG, BCKA) & Transketolase (HMP Shunt)
80
B2 – Riboflavin
FAD
81
B3 – Niacin
NAD/NADPH
82
B5 – Pantothenic Acid
Coenzyme-A
83
B6 – Pyridoxal
Transaminases (ALT, AST, etc.) & Glycogen Phosphorylase
84
B7 – Biotin
Carboxylases (Pyru, acetyl-CoA)
85
B9 – Folate
1-C transfers & Purine Synthesis
86
B12 – Cobalamin
Mutase used to convert Prop-CoA to Succ-CoA
87
no citrulline in blood and low oroate in urine =
CPS1 problem
88
no citrulline in blood and high oroate in urine =
OTCase deficiency
89
argininosuccinate in plasma =
ASL problem
90
massive amount of citrulline in blood
ASS problem
