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Biochemistry > Chapter 18 > Flashcards

Chapter 18 Flashcards

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1
Q

What are the four amino acids that play key roles in the transport and distribution of amino acids

A

alanine, glutamate, glutamine, aspartate

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2
Q

Once proteins are broken down into amino acids what are their two potential fates

A
  • recycled into new proteins
  • oxidized for energy
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3
Q

______ is a hormone secreted within gastric mucosa when protein enters the stomach

A

Gastrin, stimulates HCl and pepsinogen

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4
Q

Under low pH what happens to pepsinogen ?

A

at low ph pepsinogen gets autocatalytically cleaved into pepson

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5
Q

What does pepsin do?

A

cuts proteins into peptides in the stomach

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6
Q

What hormone is secreted in the small intestine in response to low pH

A

secretin, it stimulates pancreas exocrine cells to secrete bicarbonate

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7
Q

What hormone is secreted in response to peptides entering the duodenum ?

A

cholecystokinin

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8
Q

The autocatalytic hydrolysis of pepsinogen is signaled by what

A

hormone gastrin and low pH

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9
Q

Pepsin cleaves after what amino acids

A

Leu, Phe, Trp, Tyr

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10
Q

Trypsin cleaves after what amino acids

A

the positively charged amino acids, Lys & Arg

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11
Q

Chymotrypsin cleaves after what amino acids

A

aromatic amino acids Phe, Trp, Tyr

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12
Q

How is chymotrypsin activated

A

by the hydrolysis of trypsin

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13
Q

What cleaves successive carboxyl-terminal residues

A

carboxypeptidase

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14
Q

What cleaves successive amino terminal residues

A

Aminopeptidases

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15
Q

Role of pancreatic trypsin inhibitor

A

protein inhibitor that protects the pancreas against self-destruction

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16
Q

Where are free amino acids absorbed

A

in the villi off the intestinal mucosa

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17
Q

where does oxidative deamination occur

A

within the mitochondrial matrix

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18
Q

Humans and great apes excrete both urea (from amino acids) and _______ (from purines)

A

we excrete urea and uric acid

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19
Q

Glutamate and glutamine can both be converted into _____ for use in the citric acid cycle

A

glutamate —-> alpha ketoglutarate
glutamine —–> alpha ketoglutarate

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20
Q

alanine can be converted into _____ for use in the citric acid cycle

A

alanine —> pyruvate

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21
Q

aspartate can be converted into ______ for use in the CAC

A

aspartate —-> oxaloacetate

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22
Q

Transaminations are catalyzed by _____?

A

aminotransferases which use the cofactor pyridoxal phosphate

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23
Q

oxidative deamination is catalyzed by what enzyme?

A

L-glutamate dehydrogenase

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24
Q

What is oxidative deamination positively regulated by?

A

ADP, low levels of glucose

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25
What is oxidative deamination negatively regulated by?
GTP, high levels of alpha ketoglutarate
26
ammonia is processed into _____ for excretion
urea so it can be excreted through the urea cycle
27
Alpha-ketoglutarate can enter the citric acid cycle or be used for _______
gluconeogenesis
28
What molecule serves as an ammonia transporter
Glutamine
29
What does alanine aminotransferase create in the muscle and what in the liver
in muscle: creates alanine in liver: creates glutamate and pyruvate
30
What makes free ammonia so toxic to the brain
Ammonia (NH4+) competes with K+ for transport into astrocytes via Na+K+ATPase , causing an elevated level of extracellular K+
31
The pathway by which ammonia brought to the mitochondria of hepatocytes (epithelial liver cells) is converted to urea
Urea cycle
32
1st nitrogen acquiring reaction of the urea cycle?
carbamoyl phosphate synthesis - catalyzed by carbamoyl phosphate synthetase 1
33
What pathways link urea and citric acid cycles?
aspartate-argininosuccinate shunt
34
What shuttle converts aspartate to malate via oxaloacetate
the malate-aspartate shuttle
35
The presence of peptides in the duodenum causes the release of the blood hormone ______ which stimulates the secretion of several pancreatic proteases
cholecystokinin
36
Alpha-ketoglutarate can enter the citric acid cycle or be used for _______
gluconeogenesis
37
what enzyme catalyzes the conversion of inactive trypsinogen to active trypsin
enteropeptidase (a proteolytic enzyme secreted by intestinal cells)
38
Why are digestive enzymes synthesized as inactive precursors
to protect exocrine cells from destructive proteolytic attack
39
What does bicarbonate do in the small intestine
neutralizes gastric HCl, returning the pH to 7
40
All transaminases have ______ as a cofactor
PLP (pyridoxal phosphate)
41
In the mitochondria, glutamate undergoes oxidative deamination catalyzed by _______ to produce ________ and ________
catalyzed by L-glutamate dehydrogenase to produce alpha-ketoglutarate and NH4+
42
Free ammonia is combined with glutamate to yield _______ via glutamine synthetase
glutamine
43
Excess glutamine goes to the intestine, liver & kidneys where it is converted into _______ and _______ by glutaminase
glutamine ---> glutamate + NH4+
44
______ can replace glutamine in the transport of amino groups from muscle ---> liver in a non-toxic form
Alanine
45
Pyruvate and alanine are interconverted via transamination with glutamate via the enzyme __________
alanine transaminase
46
What is the goal of the urea cycle?
to eliminate ammonia from the body by converting it into urea
47
When is the expression of urea cycle enzymes increased ?
- high protein diet - starvation, when protein is being broken down for energy
48
Ketogenic Amino Acids
amino acids that can be converted into ketone bodies through ketogenesis - converted into Acetyl-CoA
49
Glucogenic Amino Acids
amino acids that can be converted into glucose through gluconeogenesis (and therefore pyruvate) - all of the amino acids except Lysine, Leucine
50
What amino acids are exclusively ketogenic
Leucine & Lysine
51
What amino acids are both ketogenic and glucogenic
Phe, Try, Tyr, Isoleucine, Lysine
52
Carbamoyl phosphate is synthesized by carbamoyl phosphate synthetase 1 from ______ & ______
Carbamoyl phosphate is synthesized from NH4+ and HCO3- (requires 2 ATP)
53
What is the second nitrogen-acquiring reaction in the urea cycle ?
aspartate entry into urea cycle, performed by arginosuccinate synthetase
54
How is the energetic cost of the urea cycle reduced?
by cycle interconnections (ex: fumarate to malate, leading to NADH generation in the mitochondria)
55
What are the cofactors for the catabolism of amino acids that involve one-carbon transfers
- biotin - tetrahydrofolate (THF) - S-adenosylmethionine - Pyridoxal phosphate (PLP)
56
What is the preferred cofactor for methyl transfer in biological reactions?
S-Adenosylmethionine
57
What enzyme catalyzes the conversion of serine to pyruvate
serine dehydratase, PLP dependent
58
Cholecystokinin stimulates the release of what pancreatic proteases
Trypsinogen Chymotrypsin Procarboxypeptidases A&B (the zymogens of carboxypeptidases A&B)
59
This cofactor functions as an intermediate carrier of amino groups at the active site of transaminases
PLP, the coenzyme form of pyridoxine (B6)
60
What activates carbamoyl phosphate synthase I
N-acetylglutamate which is formed by N-acetylglutamate when glutamate and acetyl CoA concentrations are high - activated by Arginine
61
What two amino acids can produce oxaloacetate
Asparagine and Aspartate
62
Where are the branched amino acids (Leucine, Isoleucine, Valine) degraded?
NOT in the liver - degraded in the extrahepatic tissues of the muscle, adipose, kidney, brain
63
What enzymes catalyze the breakdown of branched-chain amino acids into acetyl-CoA derivatives
- branched chain aminotransferase - branched chain alpha keto acid dehydrogenase complex
64
Under what 3 circumstances do amino acids undergo oxidative degradation?
1. Amino acids released during normal protein turnover are not needed for new protein synthesis. 2. Dietary amino acids exceed the body’s needs for protein synthesis. 3. Cellular proteins are used as fuel because carbohydrates are either unavailable or not properly utilized due to starvation or uncontrolled diabetes mellitus.
65
In transaminations, what accepts the amino group
alpha-ketoglutarate
66
The first step in the catabolism of most l-amino acids, once they have reached the liver, is ?
The removal of the alpha amino groups by aminotransferases
67
PLP is covalently bound to an enzymes active site through a _______ linkage
ALDIMINE (Schiff base) linkage to the amino group of Lysine
68
The first step in most PLP-catalyzed reactions is aldimine displacement. Explain
Aldimine Displacement - the aldimine linkage to Lys is replaced by the amino group of an amino acid
69
What is the role of PLP in reactions at the alpha carbon such as transaminations, decarboxylation, racemizations
PLP provides resonance stabilization to the carbanion intermediates - its highly conjugated structure allows for delocalization of the negative charge
70
Oxidative Deamination can use _____ or _____ as an electron acceptor
NAD+ or NADP+
71
What is the second major source of ammonia in hepatocyte mitochondria
Glutamine
72
What are the two nitrogen-acquiring reactions of the urea cycle?
1. Carbamoyl phosphate synthesis 2. Aspartate entry into the urea cycle by arginosuccinate synthetase

Biochemistry (10 decks)

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