Digestion and Absorption of Amino Acids Flashcards
What is the first place digested amino acids go after being absorbed by enterocytes?
the hepatic portal vein to be transported to the liver
Once in the liver, what are the major fates of absorbed amino acids?
1) protein synthesis
2) synthesis of N-containing compounds
3/4) split into carbon and nitrogen- carbon can be broken down for energy or stored; nitrogen can be broken down to urea
5) biosynthesis of amino acids
What are essential amino acids?
AAs that the body cannot synthesis. Thus, they are requires in your diet. Non-essential AAs can be made by the body and, thus, are not needed exogenously
Which non-essential AAs require input from essential AAs to be made?
cysteine and tyrosine synthesis depend on adequate amount of methionine (sulfur) and phenylalanine, respectively
What is an AA that is essential in children but not really necessary in adults (although still considered ‘essential’)
arginine
What is the recommended daily allowance of protein?
0.8g of high quality protein per kilogram of ideal body-weight (50g/day for women and 60g/day for men)
What is ‘high-quality’ protein?
just means that the protein contains all of the essential AAs in adequate amounts (generally, protein of animal origin-milk, egg, meat)
What is ‘low-quality’ protein?
low in one or more essential AA (generally protein from plant origin- easy to compensate with mixtures of low-quality protein)
What is nitrogen balance?
Means that the dietary nitrogen intake is equal to the excreted nitrogen. Someone who has a positive nitrogen balance has more dietary N than excreted N, while someone who has a negative nitrogen balance excretes more nitrogen than they ingest
When would someone be nitrogen positive and it be normal?
growing children or pregnant women (steroid users would too)
What is Kwashiorkor?
a disease caused by a deficiency of protein in a diet that is adequate in calories; common in Africa and third world countries
clinically: muscle wasting and decreased concentrations of muscle plasma
Why is there muscle wasting associated with Kwashiorkor? What happens when you consume none or insufficiency quantities of one or more essential amino acid?
non-essential AAs are going to be used to synthesis the essential ones, resulting in muscle wasting
What would you see in a patient that has decreased plasma proteins?
distended/bloated stomach
Where does proteolysis begin?
in the stomach, indicating that none occurs in the mouth or esophagus
What are the two phases of proteolysis in the stomach?
1) denaturation in acidic conditions that unfold the protein to make them a bettie substrate for breakdown by protease
2) Pepsin- a protease- breakdown
What are zymogens?
(Aka proenzymes) proteases that are produced as inactive precursors
require some change to become active form (eg. cleavage or conformational change)
How is pepsin activated?
pepsin is made as a zymogen called pepsinogen (secreted by chief cells). Under acidic conditions, the structure of pepsinogen changes and it cleaves itself (auto cleavage) generating pepsin
NOTE: Pepsin is not denatured by the pH in the stomach
What happens after proteolysis is initiated in the stomach?
the products of the initial breakdown continue on to the small intestine where they continue to be broken down by proteases secreted from the pancreas (secreted as zymogens)
What are the main zymogens/proteases secreted by the pancreas?
1) trypsinogen
2) chymotrypsinogen
3) proelastase
4) procarboxypeptidases
What is trypsinogen activated by?
enteropeptidase cleaves it to become trypsin, which acts as a digestive protease itself AND activates the other three pancreatic proteases
How does the pancreas regulate the activity of trypsin?
1) trypsin is primarily found in its zymogen form
2) the pancreas also makes an inhibitor as a safety net
Why are there so many proteases in the pancreas?
they have different specificities for where they cleave
How does cystic fibrosis affect proteolysis?
defective chloride channels cause drying and thickening of pancreatic exocrine secretions, leading to obstructed ducts so proteases can’t be released
How are single amino acids transported from the intestinal lumen into cells for transport to the liver?
Na+-dependent active transport by carriers (secondary active transport) into the intestinal cell, followed by facilitated passage into the hepatic portal vein from the basal side of the enterocyte
Can also occur by the gamma-glutamyl cycle (not covered in class)
How are single amino acids transported from the liver cells into peripheral tissue?
mostly Na+-dependent active transport by carriers (secondary active transport) out of the bloodstream with some facilitated passage
T or F. Most AAs are transported by more than one transporter
T
What is Hartnup Disease (neutral amino aciduria)?
relatively rare autosomal recessive disorder caused by the inability of intestinal and/or kidney epithelial cells to absorb NEUTRAL amino acids (Leu, Val, The, Ile, Ala, Ser) and aromatic amino acids (Phe, Trp, Tyr)- just know the classes
What happens to the kidney as a consequence of Hartnup Disease?
increased AAs in the urine (hyperaminoaciduria)
What happens to the intestine as a consequence of Hartnup Disease?
malabsorption -> deficiency of essential AAs
What are the clinical manifestations of Hartnup’s Disease?
“Pellagra-like” symptoms:
1) photosensitive rash
2) ataxia
3) neuropsychiatric symptoms
Pellagra results from deficiency of Trp or niacin which impairs synthesis of NAD+ or NADP+
What is the treatment for Hartnup’s Disease?
Exogenous Niacin (vitamin B3)
What is Cystinuria?
A rare autosomal disorder caused by the inability of intestine and/or kidney to absorb cystine (two fused cysteine AAs by sulfide bond) and BASIC amino acids (Arg, Lys, Ornithine)
Will also cause associated hyperaminoaciduria
What does cystine cause if not absorbed?
the insolubility of cystine can generate kidney stones that block the ureter and can cause urinary bleeding and severe pain (renal colic)
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What could be used to treat cystinuria?
can’t do much since cysteine are non-essential amino acids but you can treat with high fluid intake and oral alkalinizing agents
