Protein Metabolism Flashcards
Protein Metabolism: Overview
• Dietary proteins are digested to amino acids.
• Amino acids are absorbed by the ____ cells.
• Amino acids are used to synthesize proteins and other
nitrogen-containing substances.
• Carbon skeletons can be ____ for energy.
• Excess nitrogen must be ____.
intestinal
oxidized
excreted
catabolism of AA branches into other branches of cellular metabolism: connection with ____, with ____ (formation of glucose), metabolism of ____
metabolism of AA is highly ____
the primary use of AA obtained from diet is to build new ____; but they are also used in other branches
important part of metabolism of AA > to get rid of the amino groups > formation of ____ (waste product)
TCA
gluconeogenesis
FA
integrated
proteins
urea
Initial Digestion of Dietary Proteins
• Sequential action of ____ and ____: free amino acids, di- and tripeptides.
• ____ epithelial cells absorb the amino acids and small peptides using specific transporters.
• Amino acids are exported to the ____.
proteases
peptidases
intestine
circulation
digestive system provides a series of proteases (cut ____) and peptidases (cleave fragments of proteins that are ____) into 2,3 or free single monomer AA
at the level of ____ > absorption of AA takes place
digestion of protein begins at ____, but the bulk of proteases is produced by the ____
big proteins
<20 AA
small intestine
stomach
pancrease
Protection of the Pancreas from Self-Digestion
• The pancreas stores ____ in secretory granules: – All zymogens are inactive until they are cleaved by ____.
• The pancreas also synthesizes and stores trypsin inhibitor:
– Protein that binds to and inhibits ____
• Trypsin inhibitor inhibits any trypsin accidentally activated (by ____ from trypsinogen) inside the pancreas.
zymogens
trypsin
trypsin
self-cleavage
Protection of the Pancreas from Self-Digestion
Conversion of trypsinogen to trypsin is the removal of the ____ (green). The pink residue is the new N-terminus, which becomes ____ in the native structure.

N-terminal 6 AA
buried
Activation of the Gastric and Pancreatic Zymogens
• Pepsinogen self-cleavages by the ____ of the stomach.
• Trypsinogen is cleaved by ____ in the ____ to form the active protease trypsin
- Trypsin then catalyzes the cleavage and activation of the other ____
low pH
enteropeptidase
intestine
pancreatic zymogens
digestion of proteins begins in ____
only protease in stomach = ____; environment here is very acidic, so this zymogen is activated by low pH
____ houses the rest of the proteases (rest of digestion of food); they are all secreted by the ____
they do not become active until trypsinogen finds ____ (produced by epi of SI) > cascade now begins
enteropep cleaves the greeen AA > activates trypsin > now cleaves the rest of the ____
stomach
pepsin
small intestine
pancrease
enteropeptidase
pancreatic enzymes
Action of the Digestive Proteases
• Act in ____: no single protease can fully digest a protein.
• Serine endoproteases:
– Release small polypeptides by cleaving ____ sites of a protein chain.
– Trypsin is most specific (only cleaves after ____ or ____).
• Exoproteases:
– Release single amino acids from the ____ of a polypeptide.
– Aminopeptidases remove the amino acid at the ____. – Carboxypeptidases remove the amino acid at the ____.
concert
internal
Arg
Lys
end
N-terminus
C-terminus
Action of Digestive Proteases
____ are more specific than ____ (____ can cleave any AA)
action of these enzymes: the protein products reaching the enterocytes in the epi are mono, di, or tri peptides (at the ____ will cleave 2/3 peptides)
carboxypeptidase
aminopeptidases
aminopeptidases
surface of the cell
Absorption of Amino Acids
• Amino acids are absorbed from the lumen of the intestine via ____- specific ____.
• The co-transport of Na+ and the amino acid from the outside of the apical membrane to the inside of the cell is driven by the low intracellular ____:
– The low intracellular [Na+] is maintained by the ____.
- On the basal side, the amino acid is carried down its concentration gradient into the blood via a ____.
- Most subsequent amino acid metabolism takes place in the ____
semi
Na+-dependent co-transport
Na+ concentration
Na+/K+ ATPase active transporter
facilitated transporter
liver
Absorption and Metabolism of Amino Acids
• Intestine:
— Amino acids released by digestion of dietary proteins travel through the ____ to the liver.
• Liver:
— Amino acids are used for the synthesis of
proteins.
— Or converted to ____ or ____.
— Glucose is stored as ____ or released into the blood.
• Amino acids that pass through the liver are converted to proteins in cells of other tissues.
hepatic portal vein
glucose
TAG
glycogen
Biosynthesis of the Non-Essential Amino Acids
• The C-skeletons of ____ amino acids may be produced from glucose through intermediates of glycolysis or the TCA cycle.
• But not any of the ____ amino acids:
– Ile, Thr, Leu, Met, His, Val, Trp, Lys, Phe.
10
9 essential
Biosynthesis of the Non-Essential Amino Acids
the initial steps integrate the metabolism of AA w/ ____; the synthesis of AA themslves are very long themselves, but the ____ steps are highly integrated with the metabolism of carbs
some of the initial synthesis of AA (and final products) are quite simple (TA, GDH, TA) > VERY IMPORTANT STEPS > involved in the degradation and the production of ____ (TA = transamuriation, gluccodehydrogenaase activity = GDH)
carbohydrates
initial
urea
Amino Acid Metabolism: Basic Chemical Transformations
• For each amino acid there is a corresponding ____ or ____.
the only difference is that an amino group on an AA is switched with a ____ (on the keto acid/C-skeleton)
keto-acid
C-skeleton
ketone group
Basic Chemical Transformations : 1. Transamination
• ____: major process for removing N
from amino acids.
• The amino group from one amino acid is transferred to another:
– Pairs of amino acids and their corresponding α-keto acids are involved.
– Reactions are ____.
– ____ as a cofactor.
– The enzymes are called ____ or ____.
• Panel A: generalized reaction.
– An amino acid is converted to its corresponding α-keto acid.
transamination reversible pyridoxal phosphate (PLP; vitamin B6) transaminases aminotransferases
The (Simplified) Transamination Mechanism
Transamination reactions proceed through a ____* using pyridoxal phosphate (Vitamin B6) as a ____:
- = Ping-pong mechanism, also called a ____, is characterized by the change of the enzyme into an ____ when the first substrate-to-product reaction occurs.
transaminases > takes an amino group from an AA > amino group converts the ___ in the vitamin to an ____ group; then it transfer the amino to the ____
“ping pong” mechanism
cofactor
double-displacement reaction
intermediate
aldehyde group
amino
second AA
Basic Chemical Transformations : 1. Transamination
• Panel B: The aspartate transaminase reaction.
– N is transferred as an amino group from an amino acid to an α-keto acid to form a ____ amino acid.
• All amino acids except ____ and ____ can undergo transamination reactions.
aspartate > transaminase acts on aspartate and alpha ketglutarate (the keto carbon of the 2nd AA)
the enzyme transfers the amino to the keto skeleton, which then produces ____ (a different AA) > which you then produce the keto group for ____ (OAA)
different lysine threonine glutamate aspartate
Transaminations in AA Biosynthesis
Cmmon Keto Acids <> Selected Amino Acids
pyruvate <> ____
oxaloacetate <> ____
____ <> glutamate
alanine
aspartate
alpha-ketoglutarate
Basic Chemical Transformations : 2. Oxidative Deamination
• Glutamate can undergo ____ to alpha-ketoglutarate and NH4+:
– Catalyzed by ____.
– Freely ____.
• Glutamate can collect N from other amino acids (via ____) and then release ammonia through the glutamate dehydrogenase reaction.
• Several other amino acids can be directly deaminated, but these reactions are ____:
– The main chains of serine and threonine by B6- dependent ____.
– The main chain of histidine by ____.
– The side chains of arginine and glutamine by ____ and ____.
oxidative deamination
glutamate dehydrogenase
reversible
transamination
irreversible serine dehydratase histidine deaminase arginine glutamine deaminases
Biosynthesis of the Non-Essential AA
- The C-skeletons of 10 AA may be produced from glucose through intermediates of glycolysis or the TCA cycle
- But not any of the 9 essential AA: Ile, Thr, Leu, Met, His, Val, Trp, Lys, Phe
- Tyr is considered ____, but it can only be formed by the hydroxylation of ____, which is ____
- Cys is considered ____, but requires ____, which is ____, for its S atom
non-essential Phe essential non-essential Met essential
Nutrition: Essential AA
• About half the amino acids are ____:
– Humans cannot synthesize them so they must be
consumed in the diet.
• The essential amino acids are: Lys, Thr, Leu, Met, Ile, His, Phe, Val, Trp
– ____
• Animal proteins are termed ____ because they contain all the essential amino acids.
• Plant proteins are considered ____ because they lack one or more of the essential amino acids (??).
• A proper combination of non-animal protein sources is needed for a ____ vegetarian diet.
essential
ingesting these little molecules has value when knowledge fails
complete
incomplete
Nutrition: Essential AA
Deficiency diseases associated with protein malnutrition include:
____: edema, distended abdomen, enlarged liver, dermatosis, tooth loss
____: emaciation, tissue and muscle wasting, loss of adipose tissue
kwashiorkor
marasmus
Breakdown of Non-Dietary Proteins
• All cellular proteins have a half-life:
– Time after which half of the protein will have broken down. – Varies from minutes to days/weeks/years.
• Proteins with a short lifespan undergo extensive synthesis and degradation:
– ____ protein can be degraded during fasting.
– Digestive enzymes degrade both themselves and the lining of the GI tract.
– Every day 3 x 1011 red blood cells die and are phagocytosed, releasing their hemoglobin.
- Only ____ of protein that enters the digestive tract is excreted as solid waste; the rest is recycled.
- Proteins are also ____ within cells.
collagen, elastin has ____ half-lives
TF produced/degraded all the time (____ half-life)
muscle
6%
recycled
long
short
Changes in Amino Acid Metabolism with Diet and Physiological State: Fasting
• Hormonal regulation of liver amino acid metabolism:
• + = glucagon-mediated activation of enzymes or
pathways.
• ↑ = induction of pathways by glucagon and glucocorticoids.
• Induction of urea cycle enzymes occurs both during ____ and after a ____ meal.
• In a ____, levels of urea cycle enzymes do not fluctuate appreciably.
fasting
high-protein
chronic high-protein
Catabolism of Amino Acids
• Fate of amino acid carbons and nitrogen:
– Energy ____ (glycogen, fatty acids).
– For energy ____.
– Amino acid N used for secretion as ____:
—- For synthesis of other molecules (e.g. ____, ____).
—- Excess: excreted as ____.
–——- One nitrogen in urea comes from ____, the other from ____.
storage production urea purines pyrimidines urea NH4+ aspartate
Sources of NH4+ for the Urea Cycle
• Sources of N: – \_\_\_\_ catabolism. – \_\_\_\_ nucleotide catabolism. – \_\_\_\_ catabolism. • All of the reactions shown in the figure are \_\_\_\_ except that of glutamate dehydrogenase (GDH).
• The reactions that are not shown occurring in the muscle or the gut can all occur in the ____, where the NH4+ generated can be converted to ____.
• N must be transported from peripheral tissues to liver:
— ____ cycle.
— ____ cycle.
AA
purine
heme
irreversible
liver
urea
glucose/alanine
glutamine/glutamate
Nitrogen Transport in the Blood: Glucose/Alanine Cycle
• The glucose/alanine cycle:
- • Muscle:
- –• Typical of exercise: muscle uses ____ glucose.
- –• Pyruvate is ____ by glutamate to alanine.
- –• Requires amino acid degradation: transfer of N to α-KG and pyruvate.
- –• Alanine travels to the ____.
- • Liver:
- –• Alanine is transaminated back to ____: gluconeogenesis.
- –• N from alanine: ____ and secretion.
blood-borne
transaminated
liver
pyruvate
urea biosynthesis
Nitrogen Transport in the Blood: Glucose/Alanine Cycle
glucose/alanine cycle > typical of muscle being ____ > it uses glucose as a fuel (muscles can store glucose in glycogen, but tiny amount, so use internal stores of glucose quickly) > switch in fuels from internal stores of glucose into glucose that is produced by the liver
from liver stores of glycogen, or gluconeogenesis
exercise gets too long > glycogen stores in the liver gets used up
muscle starts degrading its own ____ > undergoes TA reaction > the ultimate acceptor of amino group is ____ to convert into alanine (bc glucose is producing a lot of pyruvate)
alanine then exported into BS; and reabsorbed by the ____ > liver splits alanine into carbon skeleton (____) and realeases the nitrogen > the molecules of pyruvate produces glucose in the ____ pathway
the extra nitrogen that arises is transported in blood via ____; c-skeleton of alanine is converted into glucose to be used in the ____
very intensive pathway (energy comes from degradation from ____ [AA])
exercised
protein pyruvate liver pyruvate gluconeogenesis
alanine
glycolytic pathway
muscle protein
Nitrogen Transport in the Blood: Glutamine/Glutamate Cycle
• Peripheral Tissues:
-• Exercise or fasting: rapid amino acid degradation.
-• Increased ____ in tissues.
-• α-KG accepts ____ NH4+ to form Gln:
–1. ____ to Glu by GDH.
–2. ____ by ATP-dependent ____ of Glu to Gln.
-• Gln is transported to the ____.
• Liver:
- • ____ deaminates Gln to Glu and NH3.
- • GDH ____ Glu to α-KG and NH3.
[NH4+] two amination nitrogen fixation glutamine synthetase liver
glutaminase
oxidatively deaminates
Nitrogen Transport in the Blood: Glutamine/Glutamate Cycle
not every AA can be transaminated; can only be ____
NH4+ cannot cross membranes; must use the ____
muscle that has been working hard; increase in NH4+; one way to export NH4+ is to couple to molecule that can carry more than one amino group
using GDH working in ____; can couple one ammonia ion to alpha carbon of alphaketo > glutamate > glutamine synthease, using ATP, it couples a second amino group to the delta part of glutamate to convert into ____
glutamine carrying ____ amino groups; non-____; reabsorbed by the liver
in liver: two enzymes, ____ (converting glutamine into glutamate, releasing an ammonium); using ____ in reverse to release ammonium
both ammonias used in synthesis of ____
deaminated
glutamine/glutamate cycle
reverse
glutamine
two
toxic
glutaminase
GDH
urea
Necessity of the Urea Cycle
• Nitrogen is released from amino acids as ____ (NH3) which in solution ionizes to ____ (NH4+).
• NH4+ is in equilibrium with uncharged NH3, with [NH4+] almost ____ higher than the [NH3] at physiological pH:
• NH4+ is highly ____: alters pH balance, osmotic pressure.
• Aquatic animals: secrete NH3 by dilution with ____ at the gills.
• Terrestrial vertebrates:
– Minimize toxicity and water loss.
– ____: urea.
– ____: uric acid*.
- = uric acid is the final product of the degradation of ____. In mammals it is converted to ____ before excretion.
Nitrogen waste excretion ____ in different animals
ammonium
ammonium ion
100x
toxic
water
ureotelic
uricotelic
purines
urea
varies
Necessity of the Urea Cycle
Fish: water floods away the ammonia by the gills; requires huge amount of water which is not a problem
mammals: couple ____ amino groups into urea (ureotelic); excretion requires a lot less water than lushing from gills; urea can precipitate and clog the renal system (urine contains water)
birds (uricotelic) > efficient at retaining water; excrete uric acid; very ____; excreted as final stage of metabolism of nt bases (resembles a ____)
two
solid
nitrogen base
Secretion of Nitrogenous Waste Products
most nitrogen excreted in ____; uric acid also excreted from ____ (somewhat)
urea
nitrogen bases
The Urea Cycle: Overview
• Conversion of amino acid N to urea: _____.
• Urea cycle occurs mainly in the _____.
• Urea is formed from NH4+, bicarbonate and an amino group from _____:
• The cycle is controlled by _____:
– When amino acid degradation is occurring the rate of the cycle is _____.
urea cycle liver aspartate feed-forward regulation increased
beginning of urea cycle: takes place within _____ (ornithine to citruline)
rest of the urea cycle takes place in _____
mitochondria
cytoplasm
The Urea Cycle: 1. Synthesis of Carbamoyl Phosphate
• The urea cycle begins in the _____.
• NH4+, _____ (CO2 in water), and ATP react to form _____ (Rx. 1).
– _____ molecules of ATP are required.
• Carbamoyl phosphate synthetase I (CPSI) for metabolic carbamoyl phosphate synthesis is located in the _____ of the liver and intestine.
– Note: the enzyme Carbamoyl phosphate synthetase II (CPSII) located in the _____ produces carbamoyl phosphate for its use in _____ and is _____ involved in the urea cycle.
mitochondria bicarbonate carbamoyl phosphate two mitochondria cytosol pyrimidine biosynthesis not
The Urea Cycle: 2. Production of Arginine (1/3)
• Carbamoyl phosphate reacts with ornithine to form _____ (Rx. 2):
– Reaction energy provided by the hydrolysis of the _____ of carbamoyl phosphate.
– Mediated by _____.
– _____ is transported across the mitochondrial membranes to the cytoplasm in exchange for ornithine.
• Note: Citrulline, ornithine and argininosuccinate are amino acids not _____ present in proteins.
citrulline phosphate bond ornithine transcarbamoylase citrulline naturally
The Urea Cycle: 2. Production of Arginine (2/3)
• Citrulline reacts with aspartate to produce _____ (Rx. 3):
– Catalyzed by _____.
– Energy provided by the hydrolysis of ATP to AMP + pyrophosphate (PPi; _____ ATP equivalents).
–Aspartate derives from transamination of _____ (see slide 37).
• Argininosuccinate contains:
– 1 N from NH4+.
– 1 N from Aspartate.
argininosuccinate
argininosuccinate synthetase
2
oxaloacetate
The Urea Cycle: 2. Production of Arginine (3/3)
• Argininosuccinate is cleaved to form _____ and _____ (Rx.4):
– Catalyzed by _____.
– Carbons for fumarate derive from _____.
– Arginine carries _____ amino groups (in the guanidinium group) with different origins.
• The carbons of fumarate can be recycled to _____ (see slide 37):
– Fumarate is converted to _____.
–Malate is used either for the synthesis of glucose by the _____ or for the regeneration of _____ (TCA).
fumarate arginine argininosuccinate lyase aspartate two asparate malate gluconeogenic pathway oxaloacetate
The Urea Cycle: 3. Cleavage of Arginine to Produce Urea
• Hydrolysis of arginine regenerates _____ and releases _____ (Rx. 5):
– The urea comes from the _____ side chain of arginine.
– The portion of arginine originally derived from citrulline is converted back to _____.
• Ornithine transported back into the mitochondria in exchange for _____:
– In the mitochondria, ornithine reacts with _____ to initiate another round of the cycle.
• _____ is secreted to the bloodstream.
ornithine urea guanidinium ornithine citrulline carbamoyl phosphate urea
The Krebs Bi-cycle
• Common steps between the TCA and urea cycles.
I. _____ enters the TCA cycle to regenerate oxaloacetate.
II. Oxaloacetate is _____ to aspartate.
III. Aspartate supplies one of the amino groups in the _____.
fumarate
transaminated
urea cycle
_____ and _____ cycles is highly integrated
krebs
urea
Regulation of the Urea Cycle
• Generally regulated by _____ (“feed forward”):
– Feed forward activation is a general characteristic of _____.
–Requirement: _____ ATP to synthesize carbamoyl phosphate + _____ ATP equivalents to synthesize argininosuccinate → proceeds only when energy is _____.
– High rates NH3 production → _____ rates of urea production.
• Induction/repression of the synthesis of urea cycle enzymes:
– _____ in response to conditions that require increased protein metabolism:
—• Glucagon and glucocorticoids.
• Major regulatory point: _____.
induction/repression at the level of _____
substrate availability disposal pathways 2 2 available high
induction
Carbomyl phosphate synthetase I
mRNA production
Allosteric regulation of CPSI
•Only urea cycle enzyme regulated _____.
• _____ step.
• _____ allosterically stimulates the synthesis of N-Acetyl-glutamate (NAG) from Glu and
Acetyl CoA.
• NAG (the only known fxn for this protein in mammals) allosterically stimulates the activity of _____: higher rate of carbamoyl phosphate synthesis.
(formation of N-acetyl glutamate is allosterically activated by arginine (one of last steps of _____))
allosterically
irreversible
arginine
CPSI
urea cycle
The Urea Cycle During Fasting
• Urinary excretion of urea _____ during initial fasting.
• As fasting progresses , the brain uses _____, so
less muscle protein is cleaved for _____.
• _____: major amino acid substrate for gluconeogenesis:
– Two Ala molecules generate one molecule of _____.
– N from the two Ala molecules converted to one molecule of _____.
•Conversion of alanine to glucose and urea:
- _____ of Ala to Pyruvate, which is converted to Glucose (gluconeogenesis).
- N in Glutamate can be released as NH4+ and enters the _____ (as carbamoyl phosphate); or
- Transferred to _____ to form Aspartate, which also enters the urea cycle.
high
ketone bodies
gluconeogenesis
alanine
glucose
urea
transamination
urea cycle
OAA
Changes in AA Metabolism with Diet and PHysiological State: Fasting
in liver: glucagon _____ AA degradation; _____ rate of gluconeogenesis (and production of glucose)
there is now an _____ of urea cycle; and urea formation
in response to glucagon > _____ of urea cycle enzymes during fasting and after high-protein meal (body may not need all that protein > immediately converts xs AA into _____ > increasing the intraliver concentration of _____ > increase urea production
increases
increases
increase
induction
carbon skeletons/keto acid
ammonia
What you should know about the urea cycle:
• Localization of the urea cycle: organs, cellular compartments.
- Origin of the N atoms in urea.
- Regulatory points: types of regulation and reactions involved.
- Energy requirements.
Review
So, what happens to the C-skeletons?
C-skeletons can be converted in _____, glucose…
not every AA is used to produce glucose…
the alpha keto acids are integrated with the rest of _____
pyruvate
So, what happens to the C-skeletons?
• Glucogenic C-skeletons: metabolized to _____ or intermediates of the _____.
• Ketogenic C-skeletons: metabolized to _____ and cannot be used for _____:
– They can be converted to _____.
• Many amino acids have multiple breakdown pathways allowing them to be _____ glucogenic or ketogenic:
– _____ and _____ are strictly ketogenic (yield ketone bodies in the liver).
pyruvate
TCA cycle
acetyl-CoA
glucose formation
FA
either
leucine
lysine
