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Gastrointestinal > Protein Digestion > Flashcards

Protein Digestion Flashcards

1
Q

Describe nitrogen balance

A
  • Body protein state of lux all the time (continually turned over)
  • Protein broken down, Amino acids enter pool so available for other purposes (re-conversion, energy as metabolised to glucose and ketone bodies)
  • Ammonia- urea= excretory
  • Dietary protein= replenishes pool
  • Nitrogen balance= (Total N ingested) - (Total N excreted)
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2
Q

What is the recommended daily protein intake?

A

0.8 grams per kilogram body weight per day
40/50 grams per person per day
-Nursing and athletes need more protein

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3
Q

What is net protein utilization (NPU)?

A
  • Measure of the ability of a protein to sustain growth/ quantify nutritional value of protein
  • weight. amino acids incorporated into protein / wt. amino acids supplied in diet
  • %= human milk (95), egg (87), cow’s milk (81)
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4
Q

What are the essential and non-essential amino acids?

A 
-Essential= cannot be synthesised within body (diet)- carbon skeletons too complex
=Leucine
=Lysine
=Valine
=Methionine
=Tryptophan
=Isoleucine
=Phenylalanine
=threonine 
-Non-essential= can be synthesised from other amino acids in diet
-Arginine and histidine= capacity to synthesise but not at rate required for protein turnover   
-High NPU
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5
Q

What are the ketogenic amino acids?

A

-Leu
-Lys
Also glucogenic
-Ile
-Phe
-Trp
-Tyr (not essential)

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6
Q

What is Kwashiorkor?

A

Children having body weight between 60 and 80% of that expected for their age

  • Swollen bellies (enlarged liver, oedema due to albumin deficiency)
  • Muscle wasting, diarrhoea
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7
Q

What is Marasmus?

A
  • Body weight less than 60% of the expected value for the person’s age
  • Protein and calorie deficiency
  • Monoculture sources of protein in diet
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8
Q

What are the different imbalances of nitrogen?

A
  • Positive= growth, pregnancy (more body protein synthesised than degraded)
  • Negative= protein deprivation, essential amino acid deficiency, trauma/ disease
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9
Q

What are the half-lives of proteins?

A 
Insulin= mins
Intestinal= 1-2 days
Fibrinogen= 3 days
Albumin= 20 days
Haemoglobin= 120 days
Collagen= months
Crystallin (life)
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10
Q

Describe degradation of extracellular proteins

A
  • Taken up by cell in endocytosis
  • Binds to a receptor or series of receptors, receptors then internalised to form closed compartment (endosome)
  • Fuse with lysosomes (degradative organelles, low internal pH)
  • Membrane returned to surface, releasing amino acids
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11
Q

Describe degradation of intracellular proteins

A
  • Tagged by the attachment at the C-terminus of ubiquitin (76 amino acids long)= label
  • Degradation by protein complex (proteasome)= ATP-dependent process
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12
Q

What was the action of thalidomide?

A

Alter specificities of the enzymes which attach ubiquitin to proteins for degradation

  • Some transcription factors degraded prematurely, other survive too long
  • Altered expression of lots of different proteins
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13
Q

Describe degradation of dietary protein

A
  • Chief cells in gastric pit of stomach produce pepsin= gastric acidification
  • Pepsin prefers large hydrophobic amino acids like phenylalanine or leucine
  • Stomach contents neutralised by pancreatic secretions- high concentrations of bicarbonate
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14
Q

How is gastric acid produced by parietal cells?

A
  • Structure maximised plasma membrane area
  • Proton concentration high at apical side/ lumen (0.8)- different protein composition= gastric ATPase (proton pump)
  • At basolateral side, pH= 7.4, transport proteins for anions
  • Protons come originally from carbonic acid (hydration of CO2, formed by oxidative metabolism)- to gastric lumen and bicarbonate
  • Bicarbonate stays within cells= alkaline= exported into plasma in exchange for Cl-
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15
Q

What drugs treat gastric ulcers?

A
  • Omeprazole, vonoprazan

- Inhibitors of gastric ATPase

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16
Q

What is achlorhydria?

A

Secretion of HCl fails

Uptake of vitamin B12, binds to protein called intrinsic factor

17
Q

Describe secretion and activation of pepsinogen

A
  • Hazardous to synthesise degradative enzymes in cell so secreted as inactive precursors (zymogens)
  • Chief cells
  • Regulated exocytosis
  • Protein made in cytoplasm, introduced into lumen of ER, passes to Golgi apparatus, buds off in secretory vesicles
  • Stimulus= acetylcholine
  • Converted to pepsin when encounters H+ (autocatalysis)
18
Q

Examples of proteinases

A
  • Trypsin hydrolyses bonds on carbonyl side of basic amino acids arginine or lysine
  • Chymotrypsin= large, neutral amino acids phenylalanine
  • Elastase= small neutral
  • Carboxypeptidases= release C terminal amino acids- A (neutral), B (basic)
19
Q

Cascade of activation of pancreatic zymogens

A
  • Trypsinogen to trypsin (activated by enteropeptidase/ enterokinase)
  • Chymotrypsinogen to chymotrypsin
  • Proelastase to elastase
20
Q

Describe the transamination reaction

A

-Transamination
=catalysed by transaminases/ aminotransferases
=Amino group transferred onto oxoglutarate, (converts amino acid to oxo-acid) oxoglutarate converted to a different aminoacid glutamate

21
Q

Describe the oxidative deamination reaction

A

=Removed from glutamate in mitochondria via glutamate dehydrogenase (oxidises glutamate back to oxoglutarate, reduces NAD to NADH= oxidative deamination)
=Amino group liberated as ammonia
=Ammonia essentially inhibits TCA cycle

22
Q

How do we remove ammonia?

A

-Reacts with CO2 and is phosphorylated= carbamyl phosphate in mitochondria
=Reacts with intermediate ornithine, transfers amino group to form citrulline
-Citrulline receives another amino group that comes from glutamate indirectly by aspartate (glutamate transaminated with oxaloacetate)
=Arginine broken down to release urea
-Incorporated into urea in liver only

23
Q

Describe amino acid breakdown in peripheral tissues

A
  • Doesn’t synthesise urea so amino groups transported to liver
  • Alanine transported, formed when glutamate transaminated with pyruvate
24
Q

What are the properties of urea?

A
  • Very soluble in water
  • Electrically neutral: neither acidic nor basic
  • 48% N by weight
  • Synthesised in liver so not further metabolised
  • Normal plasma 2.5-7 mmol/l (rises in renal failure- uraemia, falls in liver cirrhosis/ deficiency in urea cycle enzymes)
  • Plasma ammonia normally low (12-60 micromole/l) but rises if urea cycle inhibited (liver cirrhosis)
  • Ammonia is toxic, particularly to CNS
25
Q

How is urea excreted from the kidneys?

A
  • Ammonia liberated by glutamate dehydrogenase
  • Can be used to convert glutamate to its amide, glutamine
  • Glutamine transferred through plasma to kidneys where glutaminase takes amino group off to form ammonia
  • Liberated directly into urine by kidneys
26
Q

Overview of amino acid metabolism

A
  • Amino acids are transaminated to form carbon skeletons which are oxidised to CO2 if they are used as fuel
  • Amino acids undergo transamination, amino group in glutamate and transported to liver from peripheral tissues via alanine
  • Alanine backaminated to glutamate, feeds amino-groups into urea cycle as either ammonia via carbamyl phosphate or transamination to aspartate, excreted in urine by kidneys
  • In starvation= amino acid group in glutamate, liberated as ammonia, amide of glutamate- glutamine, transported through the blood
27
Q

Difference between gluconeogenic and ketogenic amino acids

A
  • If amino acid broken down to acetyl-CoA= ketogenic

- If amino acid broken down to a TCA cycle intermediate, can be converted back to glucose= gluconeogenic

28
Q

What is one-carbon metabolism?

A

-Set of reactions in which single-carbon fragments from aminoacid breakdown are metabolised (glycine, serine, histidine, tryptophan)
-Metabolised by being attached to tetrahydro-folic acid
-Folic acid= vitamin, utilised as a coenzyme, reduced twice
-Tetrahydofolic acid acts as carrier of one-carbon groups (formyl, formamino)
-Oxidised and reduced
=Important ingredients in synthesis of compounds like purine, thiamine, methionine

29
Q

What is pernicious anaemia?

A
  • Autoimmune destruction of parietal cells in digestive tract
  • Fail to secrete a protein called intrinsic factor
  • Vitamin B12 not absorbed
  • Failure of one-carbon pathway
  • Affects DNA replication
30
Q

Use in drugs (folic acid inhibitors)

A
  • Cycloguanil = inhibitor of dihydrofolate reductase
  • Trimethoprim in malarone
  • Methotrexate anti-tumour drug
31
Q

Describe the action of sulphonamide

A
  • Mimics part of the folic acid structure

- Acts as inhibitor of folic acid synthesis in bacteria

32
Q

Describe the action of methotrexate

A
  • Analog of folic acid

- Inhibits dihydrofolate reductase

Gastrointestinal (48 decks)

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