Protein Digestion and Amino Acid Metabolism (7 Oct 2024) PDF

Digestion of protein and Absorption of amino acids  The word protein is derived from Greek word, “proteios” which means primary  Out of the total dry bodyweight, 3/4ths are made up of proteins  All the major structural and functional aspects of the body are carrie...

Digestion of protein and Absorption of amino acids  The word protein is derived from Greek word, “proteios” which means primary  Out of the total dry bodyweight, 3/4ths are made up of proteins  All the major structural and functional aspects of the body are carried out by protein molecules.  Proteins contain Carbon, Hydrogen, Oxygen and Nitrogen as the major components while Sulphur and Phosphorus are minor constituents.  Digestion and …………  Proteins are made by polymerization of amino acids through peptide bonds  Two amino acids are combined to form a dipeptide; three amino acids form a tripeptide; four will make a tetra peptide  Combination of 10 to 50 amino acids is called as a polypeptide  By convention, big polypeptide chains containing more than 50 amino acids are called proteins.  Digestion and …………  The digestion of proteins involves the gradual breakdown of this polypeptide by enzymatic hydrolysis in to amino acid molecules which are absorbed in the blood stream  The main role of amino acids is in the synthesis of structural and functional proteins. Unlike carbohydrates and fats, there is no storage form of proteins in the body  The digestion of protein is effected by enzymes in: stomach, pancreas and intestinal cells. The process of protein digestion can be divided, depending on the sources of peptidases  Gastric Digestion…………  Entry of a protein in to stomach stimulates the gastric mucosa to secrete a hormone gastrin which in turn stimulates the secretion of HCl by the parietal cells of the gastric glands and pepsinogen by the chief cells.  The HCL thus produced, lower the pH of stomach to (pH1.5 – 2.5) and acts as an antiseptic and kills most of the bacteria and other foreign cells ingested along with.  The acid denatures the protein and the whole protein susceptible to hydrolysis by the action of other proteolytic enzymes  Gastric Digestion…………  Proteases are endopeptidases which attack the internal bonds and liberate large fragments of peptides  Pepsinogen, having MW 40,000, an inactive precursor or zymogen is converted in to active pepsin in the stomach itself  This active pepsin cleaves the ingested protein at their amino terminus  The major products of pepsin action are large peptide fragments and some free amino acids.  Pancreatic Digestion  Pancreatic zymogens proceed digestion as the acidic stomach contents pass in to the small intestine  A low pH triggers the secretion of a hormone Secretin in the blood  Secretin stimulates the pancreas to secrete bicarbonate, which in the small intestine neutralizes the gastric HCL and abruptly change the pH to 7.0.  The entry of large peptide fragments and some free amino acids in the upper part of the small intestine (Duodenum), excites the release of a hormone cholecytokinin (CCK)  EFFECTS of cholecytokinin (CCK)  Stimulates gall bladder contraction  Stimulate secretion of several pancreatic enzymes whose activity is between pH 7and 8 in proenzyme forms  Three of these pro-enzyme are trypsinogen, chymotrypsinogen and procarboxy peptidase  When the proenzyme reach the lumen of the small intestine, activates pancreatic trypsinogen to trypsin proteolytic enzymes  Enzyme - Specificity ENZYME SPECIFICITY Trypsin Basic amino acids (Arg, Lys) Chymotrypsin Aromatic amino acids (Phe, Trp, Tyr) Carboxypeptidas Most C-terminal amino eA acids Carboxypeptidas C-terminal Arg and Lys eB Elastase Ala, Gly, Ser  Intestinal Digestion  Since pancreatic juice does not contain appreciable aminopeptidase activity final digestion of di and Oligopeptides depends on the small intestinal enzymes  The lumenal surface of epithelial cells is rich in endopeptidase, and dipptidase aminopeptidase activity  The end products of the cell surface digestion are free amino acids and di and tripeptides  These are passed in to the interior of the epithelial cell where other specific peptidases convert almost all of them to single amino acids that are transported to the blood stream Absorption of Amino Acids and Peptides  The mechanism by which amino acids are absorbed is conceptually identical to that of monosaccharides  The lumenal plasma membrane of the absorptive cell bears at least four sodium- dependent amino acid transporters - one each for acidic, basic, neutral and imino amino acids  These transporters bind amino acids only after binding sodium. The fully loaded transporter then undergoes a conformational change that dumps sodium and the amino acid into the cytoplasm, followed by its reorientation back to the original form Absorption of Amino Acids and Peptides  The brush border of the small intestine is equipped with a family of peptidases. They function to further the hydrolysis of lumenal peptides, converting them to free amino acids and very small peptides.  Thus, absorption of amino acids is absolutely dependent on the electrochemical gradient of sodium across the epithelium  The basolateral membrane of the enterocyte contains additional transporters which export amino acids from the cell into blood. These are not dependent on sodium gradients.  Absorption of Peptides  There is virtually no absorption of peptides longer than four amino acids  However, there is abundant absorption of di- and tripeptides in the small intestine. These small peptides are absorbed into the small intestinal epithelial cell by cotransport with H+ ions via a transporter called PepT1.  Once inside the enterocyte, the vast bulk of absorbed di- and tripeptides are digested into amino acids by cytoplasmic peptidases and exported from the cell into blood. Only a very small number of these small peptides enter blood intact. Protein digestion and absorption 1. What is the basic building block of proteins? a) Carbohydrates b) Fatty acids c) Amino acids d) Nucleotides 2. How many essential amino acids are there? a) 10 b) 15 c) Which 3. 20 of the following is not an essential amino acid? d) 25 a) Leucine b) Tryptophan c) Histidine 11/13/2024 15 d) Asparagine 4. What is the primary structure of a protein? a) The overall three-dimensional shape b) The sequence of amino acids c) The arrangement of multiple polypeptide chains d) The presence of secondary structures like alpha helices and beta sheets 5. Which of the following is a function of proteins in the body? a) Energy storage b) Transport of oxygen c) Nervous system regulation d) All of the above 11/13/2024 16 6. Which amino acid is responsible for forming disulfide bonds in proteins? a) Glycine b) Serine c) Cysteine d) Alanine 7. Which type of protein helps to speed up chemical reactions in the body? a) Structural proteins b) Transport proteins c) Enzymes d) Hormones 11/13/2024 17 11/13/2024 18  Catabolism of amino acids Amino acids are mainly degraded by separating amino group from their carbon skeleton by:  Hydrolysis of amide Asn + H 2O → Asp + NH3 E E= Asperaginase Transamination AT Deamination : Oxidative deamination / non oxidative deamination Oxidative deamination is a major system delivering 11/13/2024 free ammonia to urea cycle. 19  All amino acids except threonine, lysine & proline can be transaminated. 11/13/2024 22 11/13/2024 23 11/13/2024 24 11/13/2024 25 11/13/2024 26 11/13/2024 27  Glucose- Alanine cycle: Its role in recycling α-keto acids between muscle and liver and transporting NH4+ to the liver, liver convert NH4+ to urea 11/13/2024 28 Glucose- alinine cycle:  Ammonia Ammonia is a toxic compound. Ammonia is toxic to nerve cells (Hyperammonemia,>60μM ) Inhibit action of glycine and glutamate Decreases α-ketoglutarate and glutamate ( glutamine) Causes brain edema (osmotic edema) Symptoms like slurring of speech, blurring of vision , coma and death. It is rapidly removed from circulation by the kidneys and liver Liver disorders such as cirrhosis, interferes with urea formation 11/13/2024 29 Brain glutamate-glutamine 11/13/2024 By Abdisa T  Fig. Relationship between FH4, B12, and SAM. A. Overall scheme. B. 11/13/2024 By Abdisa T  Dopamine, Norepinephrine, and Epinephrine 1. SYNTHESIS OF THE CATECHOLAMINE NEUROTRANSMITTERS  These three neurotransmitters are synthesized in a common pathway from the amino acid L- tyrosine.  Tyrosine is supplied in the diet or is synthesized in the liver from the essential amino acid phenylalanine by phenylalanine hydroxylase. 11/13/2024 By Abdisa T 11/13/2024 By Abdisa T 11/13/2024 By Abdisa T 11/13/2024 By Abdisa T 1) Ring hydroxylation 2) Decarboxylation – formation of dopamine 3) Side chain hydroxylation – formation of noradrenaline 4) N-methylation – formation of adrenaline  2. STORAGE AND RELEASE OF CATECHOLAMINES  only low concentrations of catecholamines are free in the cytosol, whereas high concentrations are found within the storage vesicles.  Conversion of tyrosine to L-DOPA and that of L-DOPA to dopamine occurs in the cytosol.  Dopamine is then taken up into the storage vesicles.  In norepinephrine-containing neurons, the final hydroxylation reaction occurs within the vesicles.  The catecholamines are transported into vesicles by the protein VMAT2 (vesicle membrane associated11/13/2024 transporter By Abdisa T 2).  The intravesicular concentration of catecholamines is approximately 0.5M, roughly 100 times the cytosolic concentration.  In the vesicles, the catecholamines exist in a complex with ATP and acidic proteins known as chromogranins.  Chromogranins are required for the biogenesis of the secretory vesicle. When released from the vesicle, chromogranins can be proteolytically clipped to form bioactive peptides.  Elevated levels of chromogranins in the circulation may be found in patients harboring neuroendocrine tumors, such as a pheochromocytoma 11/13/2024 By Abdisa T  Fig. Transport of catecholamines into storage vesicles. This is a secondary active transport based on the generation of a proton gradient across the vesicular membrane. NT positively charged neurotransmitter (catecholamine); DBH dopamine -hydroxylase; VMAT2 vesicle membrane transporter 2; ByV-ATPase 11/13/2024 Abdisa T vesicular 3. INACTIVATION AND DEGRADATION OF CATECHOLAMINE NEUROTRANSMITTERS The action of catecholamines is terminated through reuptake into the presynaptic terminal and diffusion away from the synapse. Degradative enzymes are present in the presynaptic terminal, and in adjacent cells, including glial cells and endothelial cells. 11/13/2024 By Abdisa T Release and re-uptake of norepinephrine 11/13/2024 By Abdisa T Two of the major reactions in the process of inactivation and degradation of catecholamines are catalyzed by monamine oxidase (MAO) and catechol-O-methyltransferase (COMT). MAO is present on the outer mitochondrial membrane of many cells and oxidizes the carbon containing the amino group to an aldehyde, thereby releasing ammonium ion. 11/13/2024 By Abdisa T In the presynaptic terminal, MAO inactivates catecholamines that are not protected in storage vesicles. (Thus, drugs that deplete storage vesicles indirectly increase catecholamine degradation.) There are two isoforms of MAO with different specificities of action: ◦ MAO-A preferentially deaminates norepinephrine and serotonin, whereas ◦ MAO-B acts on a wide spectrum of phenylethylamines (phenylethyl refers to a – CH2- group linked to a phenyl ring). MAO in the liver and other sites protects against the ingestion of dietary 11/13/2024 By Abdisa T biogenic   Fig. Inactivation of catecholamines. Methylation and oxidation may occur in any order. Methylated and oxidized derivatives of norepinephrine and epinephrine are produced, and 3-methoxy-4- hydroxymandelic acid is the final product. These compounds are excreted in the urine. MAO monoamine oxidase; COMT catechol O- methyltransferase; SAM S- adenosylmethionine; SAH S-adenosylhomocysteine. 11/13/2024 By Abdisa T Comment Tyramine is a degradation product of tyrosine that can lead to headaches, palpitations, nausea and vomiting, and elevated ABP if present in large quantities. Tyramine mimics norepinephrine and binds to norepinephrine receptors, stimulating them. Tyramine is inactivated by MAO-A, but if a person is taking a MAO inhibitor, foods containing tyramine should be avoided. 11/13/2024 By Abdisa T  COMT is also found in many cells, including the erythrocyte. It works on a broad spectrum of extraneuronal catechols and those that have diffused away from the synapse.  COMT transfers a methyl group from SAM to a hydroxyl group on the catecholamine or its degradation product.  Because the inactivation reaction requires SAM, it is indirectly dependent on vitamins B12 and folate. 11/13/2024 By Abdisa T  The action of MAO and COMT can occur in almost any order, thereby resulting in a large number of degradation products and intermediates, many of which appear in the urine.  Cerebrospinal homovanillylmandelic acid (HVA) is an indicator of dopamine degradation. 11/13/2024 By Abdisa T Fig. Pathways of norepinephrine degradation. COMT, catechol-O-methyltransferase. 11/13/2024 By Abdisa T Dopamine Metabolism 11/13/2024 By Abdisa T Catecholamine Receptors The brain contains many different catecholamine receptors. The effects of dopamine are mediated through interaction with five different receptors; usually referred to as D1-like (D1, D5) and D2- like (D2, D3, D4). The effects of norepinephrine and epinephrine are mediated through nine distinct receptors grouped into three families (α1, T α2, β), each 11/13/2024 By Abdisa  D1-Like D2-Like D1 D5 D2 D3 D4 Amino acids 446 477 415/444 400 387 Effecter ↑cAMP ↑cAMP ↓cAMP ↓cAMP ↓cAMP pathways ↑K+ channel ↑K+ channel ↑K+ channel ↓Ca2+ ↓Ca2+ channel ↓Ca2+ channel channel Distributions Olfactory Hippocampus, Nucleus Olfactory Frontal cortex, tubericle,cerebral hypothalamus, accumbens, tubercle, medulla, midbrain, nucleus accumbens cortex cerebral cortex midbrain hypothalamus, 11/13/2024 By Abdisa T 11/13/2024 By Abdisa T  Dopamine Pathways & Function 1. Mesostriatal (or nigrostriatal) pathway: 75% of the dopamine in brain. Neurons in the substantia nigra pars compacta (SNc) project to the dorsal striatum (upward dashed blue arrows); motor control-this is the pathway that degenerates in Parkinson disease. 2. Tuberohypophyseal system: is a group of short neurons running from the ventral hypothalamus to the median eminence and pituitary gland (red arrows). Regulate secretions of pituitary gland  Dopamine Pathways & Function 3.Mesolimbic pathway: The third pathway projects from the ventral tegmentum to the mesolimbic forebrain, especially the nucleus accumbens and the amygdaloid nucleus. 4.Mesocortical pathway: whose cell bodies also lie in the VTA and which project via the medial forebrain bundle to the frontal cortex (solid blue arrows). These pathways - cognitive, reward and emotional behavior Mesocortical dopamine deficiency - ADHD Mesolimbic dopamine deficiency – Schizophrenia Adrenergic Receptors  Since NE & EP are important messengers in both the peripheral sympathetic nervous system & the brain, adrenergic receptors are widely distributed in peripheral tissues as well as existing in high concentrations in the brain.  Adrenergic receptor subclassified into 3 major families (α1, α2, β) based on their pharmacological properties, amino acid sequences and signaling mechanisms.  Like the dopamine receptors, are members of the GCPR 11/13/2024 By Abdisa T  The actions of noradrenaline are mainly inhibitory (β-receptors), but some are excitatory (α- or β-receptors) Noradrenergic transmission -controls wakefulness and alertness -regulates blood pressure -regulates response to stress -takes a part in the control of mood (functional deficiency contributing to depression)  Clinical Comment In albinism, either the copper dependent tyrosine hydroxylase of melanocytes (which is distinct from the tyrosine hydroxylase found in the adrenal medulla) or other enzymes that convert tyrosine to melanins may be defective. Individuals with albinism suffer from a lack of pigment in the skin, hair, and eyes, and they are sensitive to sunlight. 11/13/2024 By Abdisa T 11/13/2024 By Abdisa T 11/13/2024 By Abdisa T  Treatment  There is no cure for albinism  Treatments only ease the symptoms  Visual Rehabilitation  Wear dark glasses to shield light  Always wear sunscreen  Special UV proof clothing/swimsuits  Does not effect life span  There will never be a cure (genetics) 11/13/2024 By Abdisa T