Test 4: Protein Digestion/Absorption & Metabolism, AA Metabolism

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amino acid metabolism

anabolism: proteins & N-containing compounds
• amino group → urea
⠀1. Excretion by kidney (majority)
⠀2. Excretion by intestinal tract (trace)
• carbon skeleton (α-keto acid)
⠀1. Energy + CO₂
⠀2. Glucose &/or ketone bodies
⠀3. Fatty acids

overlapping amino acid specificity

• There are many ATP using amino acid transporters with overlapping amino acid specificity → amino acids may compete for absorption.
• If an individual amino acid is consumed in great excess, this may inhibit the absorption of other amino acids.
• 76% of the AA we absorb are in SMALL PEPTIDE FORM.
• 34% are absorbed as FREE AA.

what are the two types of a.a. transporter dependency?

Na dependent (upper case letter)
Na independent (lower case letter)

peptide transport

1. Peptides are transported into the intestinal cell along with H+.
2. The H+ are pumped back into the intestinal lumen in exchange for Na+.
3. A NaK-ATPase pumps Na+ out of the cell in exchange for K+ across the basolateral membrane.
Peptides are cleaved to individual AA.
Absorbed AA are put into the blood by facilitated diffusion & active transport.
ATP dependent

where are most AA catabolized?

• liver
• uses carbon skeleton for ATP production
• gets approximately 50% of its energy from AA catabolism

why must the nitrogen group be removed from AA?

So AA can be used for functions other than protein or nitrogen containing compound synthesis

how is the nitrogen group removed?

by either deamination or transamination which yields the AA's carbon skeleton


• the removal of a nitrogen group. the nitrogen group IS transferred to another compound.
• alanine aminotransferase (ALT) & aspartate aminotransferase (AST), most active in the body
• aminotransferases use vitamin B6/PLP as a coenzyme
• alanine (PLP) ↔ pyruvate; α-ketoglutarate → [ALT] glutamate
• α-ketoglutarate (PLP) ↔ glumate; asparate (AST) ↔ oxaloacetate


the removal of a nitrogen group. the amino group is NOT transferred to another compound.
threonine dehydratase, loss of water
requires pyridoxal phosphate (PLP) = Vit. B6

what can the amino group be used for?

urea synthesis (urea cycle)

urea cycle

• in liver
• requires E from 4 ATP
• coupled w/ TCA & ETS
• tissues send nitrogen as glutamine or alanine to liver
• glucagon ↑ the mRNA for urea cycle enzymes, stimulate gluconeogenic things

urea & TCA

1. NH3 (from glu or gln) → carbamoyl-PO4; requires 2 ATP
2. Carbamoyl-PO4 → Citrulline
3. Citrulline → Arginino-succinate; requires 2 ATP & Asp
4.1 Arginino-succinate → Arg → (Arginase) Urea & Ornithine
4.2 Arginino-succinate → Fumarate (TCA) → Oxaloacetate → [transamination] Asp


histamine (allergy)


serotonin → melatonin




nitrogen carriers

what AAs make glutathione?

glutathione, a patent antioxidant
glutamate, glycine, cysteine


makes compounds that stimulate cells
1. polyamines
2. creatinine
3. nitric oxide
glutamate & proline

NO (nitric oxide)

free radical, produced by immune system.
1. smooth muscle cell relaxation (blood pressure)
2. kills bacteria/worms.


1. dopamine → norepinephrine →epinephrine
2. melanin
3. thyroid hormones

what must be true for an amino acid to be gluconeogenic?

the carbon skeleton must yield a TCA cycle intermediate

what are the gluconeogenic AA?

Ala, Gly, Cys
Ser, Trp, Thr, Asp, Asn, Phe, Tyr, Val, Met, Arg, His, Pro, Glu, Gln, Ile
No Leu or Lys

glucose-alanine cycle

1. Ala removes ammonia from the muscle
2. Ala sent to the liver
3. Ala → pyruvate
4. pyruvate → glucose
5. Glucose into blood & then into muscle as G6P
Ala helps deliver amino group to liver

intestinal cell amino acid metabolism

• SI has first access to AA. Glu is an energy component for SI, leaves glucose for RB, brain, peripheral tissues, & anaerobic conditions.
• yields oxaloacetate, citrulline (for N removal), ATP, Ala
• to portal blood: Ala, Pro, citrulline
Look at p. 170

liver AA metabolism
what are some plasma proteins?

• the liver obtains hydrophilic substances
plasma proteins:
1. albumin
2. retinal-binding protein
3. blood clotting proteins
4. globulins
5. acute phase proteins
6. heat shock proteins


transports fatty acids (which are hydrophobic)
some vitamins
some minerals (bind to prevent free radical production caused by free divalent cations)

retinal-binding protein

• specific cell receptor, so can control which tissues & amount taken
• transports vitamin A


• lipoproteins for HDL, VLDL, LDL
• for immunoglobulin synthesis
• for transferrin synthesis for iron transport (can be regulated

acute phase proteins

• regulate systemic inflammation
• C-reactive protein (CRP): inflammation marker, binds bac for immune cell target & destruction
• fibronectin: clot formation, wound healing
• metallothionin: binds minerals like Fe to decrease bacterial growth bc bac use Fe for growth

heat shock proteins

released under stress, function not clear

nitrogen containing nonproteins

derived from amino acids
1. glutathione
2. carnitine
3. carnosine
4. choline
5. purine/pyrimidine bases
6. creatine


liver aa transport
Cys, Gly, Glu


FA transport into mitochondria
Lys, Met


muscle energy-P
a good muscle maker
Arg, Gly, Met


phospholipid →
1. phosphotidylcholine
2. sphingomyelin
3. acetylcholine


functions as a reducing agent in two reactions.
In both reactions for carnitine synthesis, the vitamin is needed to reduce the iron atom that has been oxidized (Fe3+) in the reaction back to its reduced (Fe2+) state

purines & pyrimidines

pyrimdine: Glu, Asp
purine: Glu, Asp, Gly

phenylalanine/tyrosine metabolism

1. melanin
2. thyroid hormones
3. acetyl-CoA
4. dopamine → norepinephrine → * epinephrine
* Methylation: methionine → S-adenosyl Met (SAM) → [methyltransferase] SAH
Tyr is non-essential

how are epinephrine, norepinephrine, & dopamine related?

Epinephrine: CH₃
Norepi: No CH₃
Dopamine: No OH or CH₃

tryptophan metabolism

serotonin → melatonin
NAD → * NAD+
* Glutamine → Glutamate
* ATP + H₂O → [NAD Synthase] AMP + PPi
NAD+ → * NADP+
* NAD Kinase & ATP → ADP

methionine metabolism

SAM → * homocysteine
* acceptor of methyl group → [methyl transferase] methylated acceptor
methylcobalamin → [methionine synthetase, CH3 transfer] cobalamin
5-methyl tetrahydrofolate (THF) → [CH3 transfer] THF
products: cystenine, taurine, glutamate

arginine metabolism

arginine →
1. + glycine → [transamidinase, kidney] ⠀guanidoacetate → * creatine
⠀⠀* Met → SAM → [methyltransferase, liver] SAH
2. [H₂O → Urea / Arginase] Ornithine → glutamate

Histidine → [decarboxylase, - CO₂] histamine

branch chain amino acid metabolism

leucine → [BCAA aminotransferase (PLP)]
Glutamine (NH₃ removal) [glutamine synthetase] ↔ [glutaminase]
Alanine (NH₃ removal)
products: ammonia or ammonium, α-ketoglutarate
1. important E source for muscle
2. stimulate PRO & glycogen synthesis (muscle recovery)
BCAA = Ile, Val

what are the breakdown components of BCAA?

• α-ketoglutarate → [BCAA transaminase] glutamate → α-ketoglutarate
• α-ketoglutarate → [BCAA transaminase] glutamate → * glutamine
⠀* ATP → [glutamine synthetase] ADP + Pi
• Asp → fumarate → TCA & ETS cycle

what are areas of BCAA/AA metabolism?

Brain: Trp → serotonin, Tyr → dopamine → norepi
Muscle: BCAA (Ile & Val)
Kidney: Urea; AA catabolism puts stress on kidney; Gln & Ala excrete amino group here

ubiquitination & protein degradation

ubqituin conjugation: marks PRO for degradation w/ ubiquitin-activating enzyme & ubiquitin-ligase complex
protein degradation: tagged goes inside a PROTEASOME, peptides & AA go into general AA pool →
1. Energy
2. PRO synthesis
3. further metabolism (e.g. neurotransmitters)


in liver:
• ketogenesis: a way to deal w/ high FA (from adipose) oxidation to meet E needs
• glycogen is depleted within 1st day
adipose also sends FA to muscle


sepsis = systemic infection
Glutamine important E source for immune cells, will stimulate their function.

last page?

burns, sepsis, surgery, trauma → stimulation of the CNS →
1. antidiuretic hormone → water retention
2. catecholamines →
⠀• lipolysis
⠀• (-) insulin → hyperglycemia
⠀• (+) glucagon release → hyperglycemia & proteolysis
3. adrenocorticotropin hormne (ACTH) → (+) glucocorticoid release → gluconeogenesis → hyperglycemia


pepsinogen → [HCl or pepsin] pepsin


trypsinogen → [enteropeptidase or trypsin] trypsin

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