6. (C)General Amino Acid Metabolism Flashcards
Digestions of proteins?
This is just for getting the context:
The dietary proteins are denatured on cooking and therefore more easily digested. All these enzymes are hydrolases (class 3 enzymes) in nature. Proteolytic enzymes are secreted as inactive zymogens which are converted to their active form in the intestinal lumen. This would prevent auto- digestion of the secretory acini. The proteolytic enzymes include:
1. Endopeptidases. They act on peptide bonds inside the protein molecule, so that the protein becomes successively smaller and smaller units. This group includes Pepsin, Trypsin, Chymotrypsin, and Elastase.
2. Exopeptidases, which act at the peptide bond only at the end region of the chain. This group includes:
2-A. Carboxypeptidase acts on the peptide bond only at the carboxy terminal end on the chain.
2-B. Aminopeptidase, which acts on the peptide bond only
Answer:
A. Gastric Digestion of Proteins
In the stomach, hydrochloric acid is secreted (Chapter 26). It makes the pH optimum for the action of pepsin and also activates pepsin. The acid also denatures the proteins.
1. Rennin
Rennin otherwise called Chymosin, is active in infants and is involved in the curdling of milk. It is absent in adults. Milk protein, casein is converted to paracasein by the action of rennin. This denatured protein is easily digested further by pepsin.
2. Pepsin
It is secreted by the chief cells of stomach as inactive pepsinogen. The conversion of pepsinogen to pepsin is brought about by removal of 44 amino acids from the N-terminal end, by the hydrochloric acid. The optimum pH for activity of pepsin is around 2. Pepsin is an endopeptidase, (Table 14.1). Pepsin catalyses hydrolysis of the bonds formed by carboxyl groups of Phe, Tyr, Trp and Met. By the action of pepsin, proteins are broken into proteoses and peptones.
B. Pancreatic Digestion of Proteins
The optimum pH for the activity of pancreatic enzymes (pH 8) is provided by the alkaline bile and pancreatic juice. The secretion of pancreatic juice is stimulated by the peptide hormones, Cholecystokinin and Pancreozymin.
Pancreatic juice contains the important endo-peptidases, namely Trypsin, Chymotrypsin, Elastase and Carboxypeptidase.
These enzymes are also secreted as zymogens (trypsinogen, chymotrypsinogen and pro-elastase), so that the pancreatic acinar cells are not autolysed. All the three are serine proteases, i.e. the active centers of these enzymes contain serine residues.
3. Trypsin
Trypsinogen is activated by enterokinase (entero- peptidase) present on the intestinal microvillus membranes. Once activated, the trypsin activates other enzyme molecules. Trypsin is activated by the removal of a hexapeptide from N-terminal end. Trypsin catalyses hydrolysis of the bonds formed by carboxyl groups of Arg and Lys.
Acute pancreatitis: Premature activation of trypsinogen inside the pancreas itself will result in the autodigestion of pancreatic cells. The result is acute pancreatitis. It is a life-threatening condition.
4. Chymotrypsin
Trypsin will act on chymotrypsinogen, in such a manner that A , B and C peptides are formed. These 3 segments are approximated, so that the active site is formed. Thus, selective proteolysis produces the catalytic site.
5. Carboxypeptidases
Trypsin and chymotrypsin degrade the proteins into small peptides; these are further hydrolysed into dipeptides and tripeptides by carboxypeptidases present in the pancreatic juice. The procarboxy peptidase is activated by trypsin. They are metallo- enzymes requiring zinc.
C. Intestinal Digestion of Proteins
Complete digestion of the small peptides to the level of amino acids is brought about by enzymes present in intestinal juice (succus entericus). The luminal surface of intestinal epithelial cell contains the following enzymes:
6. Leucine aminopeptidase
It releases the N-terminal basic amino acids and glycine.
7. Proline amino peptidase
It removes proline from the end of polypeptides.
8. Dipeptidases and tripeptidases
They will bring about the complete digestion of proteins; their specificities are shown in Figure 14.2.
Why ammonia is toxic to humans?
Ammonia toxicity:
NH3 is highly toxic. Even minute amounts of NH3 is toxic to CNS and can cause ammonia intoxication. Its mainly caused due to hyperammonemia and accumulation of NH3 in brain.
Reason for ammonia intoxication:
Accumulated ammonia in brain reacts with a-ketoglutarate to form glutamate, resulting in depletion of
a-ketoglutarate, impairing TCA cycle function in brain.
Symptoms of ammonia intoxication:
Slurred of speech, blurred vision, tremors, vomiting, lethargy, aversion to high protein food, disorientation, irritability, mental retardation. In severe cases coma and death.
Why breast milk is avoided in citrullinemia?
Citrullinemia is Autosomal recessive inheritance. High blood levels of ammonia and citrulline. Citrullinuria (1-2 g/day).
Caused due to deficiency of Arginosuccinate synthetase.
Since Citrulline is present in significant quantities in milk, breast milk is to be avoided in citrullinemia.
Treatment of hyperammonemia
Low protein diet with sufficient arginine and energy by frequent feeding can minimize brain damage since ammonia levels do not increase very high.
Lactulose and neomycin.
Blood levels of urea
Urea Level in Blood
In clinical practice, blood urea level is taken as an indicator of renal function. The normal urea level in plasma is from 20 to 40 mg/dl.
Blood urea level is increased where renal function is inadequate. Urea level in blood may be theoretically increased when protein intake is very high. However in usual conditions, this will be only within the upper limit of the normal values.
Urinary excretion of urea is 15 to 30 g/day (6-15 g nitrogen/day). This corresponds to the breakdown of 40 to 80 g of proteins per day. Urea constitutes 80% of urinary organic solids.
Difference between CPS I AND CPS II
Formation of carbomoyl phosphate from ornithine and citrulline in first step of urea cycle is catalysed by carbomoyl phosphate synthestase I or CPS I. This is not to be confused with an entirely different cytoplasmic enzyme, carbamoyl phosphate synthetase-II, (CPS-II), which is involved in pyrimidine nucleotide synthesis.
CPS I CPS II
- Site Mitochondria Cytosol
- Pathway of Urea Pyrimidine
- +ive effector NAG Nil
- Source for N Ammonia Glutamine
- Inhibitor Nil CTP
Why urea cycle is called urea bicycle?
The urea cycle and TCA cycle are interlinked, and so, it is called as “urea bicycle”.
Energetics of Urea Cycle
During these reactions, 2 ATPs are used in the 1st reaction. Another ATP is converted to AMP + PPi in the 3rd step, which is equivalent to 2 ATPs. The urea cycle consumes 4 high energy phos- phate bonds. However, fumarate formed in the 4th step may be converted to malate. Malate when oxidised to oxaloacetate produces 1 NADH equivalent to 2.5 ATP. So net energy expenditure is only 1.5 high energy phosphates. The urea cycle and TCA cycle are interlinked, and so, it is called as “urea bicycle”.
Regulation of urea cycle
Regulation of the Urea Cycle
1. Coarse Regulation
The enzyme levels change with the protein content of diet. During starvation, the activity of urea cycle enzymes is elevated to meet the increased rate of protein catabolism.
2. Fine Regulation
The major regulatory step is catalyzed by CPS-I where the positive effector is N-acetyl glutamate (NAG). It is formed from glutamate and acetyl CoA (Fig. 14.14). Arginine is an activator of NAG synthase.
3. Compartmentalization
The urea cycle enzymes are located in such a way that the first two enzymes are in the mitochondrial
matrix. The inhibitory effect of fumarate on its own formation is minimized because argininosuccinate lyase is in the cytoplasm, while fumarase is in mitochondria.
Disorders of urea cycle
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