NEET MDS Lessons
Biochemistry
Amino Acid Biosynthesis
Glutamate and Aspartate
Glutamate and aspartate are synthesized from their widely distributed a-keto acid precursors by simple 1-step transamination reactions. The former catalyzed by glutamate dehydrogenase and the latter by aspartate aminotransferase, AST. Aspartate is also derived from asparagine through the action of asparaginase. The importance of glutamate as a common intracellular amino donor for transamination reactions and of aspartate as a precursor of ornithine for the urea cycle is described in the Nitrogen Metabolism page.
Alanine and the Glucose-Alanine Cycle
Role in protein synthesis,
Alanine is second only to glutamine in prominence as a circulating amino acid.. When alanine transfer from muscle to liver is coupled with glucose transport from liver back to muscle, the process is known as the glucose-alanine cycle. The key feature of the cycle is that in 1 molecule, alanine, peripheral tissue exports pyruvate and ammonia (which are potentially rate-limiting for metabolism) to the liver, where the carbon skeleton is recycled and most nitrogen eliminated.
There are 2 main pathways to production of muscle alanine: directly from protein degradation, and via the transamination of pyruvate by alanine transaminase, ALT (also referred to as serum glutamate-pyruvate transaminase, SGPT).
glutamate + pyruvate <-------> a-KG + alanine
Cysteine Biosynthesis
The sulfur for cysteine synthesis comes from the essential amino acid methionine. A condensation of ATP and methionine catalyzed by methionine adenosyltransferase yields S-adenosylmethionine
Tyrosine Biosynthesis
Tyrosine is produced in cells by hydroxylating the essential amino acid phenylalanine. This relationship is much like that between cysteine and methionine. Half of the phenylalanine required goes into the production of tyrosine; if the diet is rich in tyrosine itself, the requirements for phenylalanine are reduced by about 50%.
Phenylalanine hydroxylase is a mixed-function oxygenase: one atom of oxygen is incorporated into water and the other into the hydroxyl of tyrosine. The reductant is the tetrahydrofolate-related cofactor tetrahydrobiopterin, which is maintained in the reduced state by the NADH-dependent enzyme dihydropteridine reductase (DHPR).
Ornithine and Proline Biosynthesis
Glutamate is the precursor of both proline and ornithine, with glutamate semialdehyde being a branch point intermediate leading to one or the other of these 2 products. While ornithine is not one of the 20 amino acids used in protein synthesis, it plays a significant role as the acceptor of carbamoyl phosphate in the urea cycle
Serine Biosynthesis
The main pathway to serine starts with the glycolytic intermediate 3-phosphoglycerate. An NADH-linked dehydrogenase converts 3-phosphoglycerate into a keto acid, 3-phosphopyruvate, suitable for subsequent transamination. Aminotransferase activity with glutamate as a donor produces 3-phosphoserine, which is converted to serine by phosphoserine phosphatase.
Glycine Biosynthesis
The main pathway to glycine is a 1-step reaction catalyzed by serine hydroxymethyltransferase. This reaction involves the transfer of the hydroxymethyl group from serine to the cofactor tetrahydrofolate (THF), producing glycine and N5,N10-methylene-THF. Glycine produced from serine or from the diet can also be oxidized by glycine cleavage complex, GCC, to yield a second equivalent of N5,N10-methylene-tetrahydrofolate as well as ammonia and CO2.
Glycine is involved in many anabolic reactions other than protein synthesis including the synthesis of purine nucleotides, heme, glutathione, creatine and serine.
Aspartate/Asparagine and Glutamate/Glutamine Biosynthesis
Glutamate is synthesized by the reductive amination of a-ketoglutarate catalyzed by glutamate dehydrogenase; it is thus a nitrogen-fixing reaction. In addition, glutamate arises by aminotransferase reactions, with the amino nitrogen being donated by a number of different amino acids. Thus, glutamate is a general collector of amino nitrogen.
Aspartate is formed in a transamintion reaction catalyzed by aspartate transaminase, AST. This reaction uses the aspartate a-keto acid analog, oxaloacetate, and glutamate as the amino donor. Aspartate can also be formed by deamination of asparagine catalyzed by asparaginase.
Asparagine synthetase and glutamine synthetase, catalyze the production of asparagine and glutamine from their respective a-amino acids. Glutamine is produced from glutamate by the direct incorporation of ammonia; and this can be considered another nitrogen fixing reaction. Asparagine, however, is formed by an amidotransferase reaction.
Aminotransferase reactions are readily reversible. The direction of any individual transamination depends principally on the concentration ratio of reactants and products. By contrast, transamidation reactions, which are dependent on ATP, are considered irreversible. As a consequence, the degradation of asparagine and glutamine take place by a hydrolytic pathway rather than by a reversal of the pathway by which they were formed. As indicated above, asparagine can be degraded to aspartate
Pentose Phosphate Pathway (Hexose Monophosphate Shunt)
The pentose phosphate pathway is primarily an anabolic pathway that utilizes the 6 carbons of glucose to generate 5 carbon sugars and reducing equivalents. However, this pathway does oxidize glucose and under certain conditions can completely oxidize glucose to CO2 and water. The primary functions of this pathway are:
- To generate reducing equivalents, in the form of NADPH, for reductive biosynthesis reactions within cells.
- To provide the cell with ribose-5-phosphate (R5P) for the synthesis of the nucleotides and nucleic acids.
- Although not a significant function of the PPP, it can operate to metabolize dietary pentose sugars derived from the digestion of nucleic acids as well as to rearrange the carbon skeletons of dietary carbohydrates into glycolytic/gluconeogenic intermediates
Enzymes that function primarily in the reductive direction utilize the NADP+/NADPH cofactor pair as co-factors as opposed to oxidative enzymes that utilize the NAD+/NADH cofactor pair. The reactions of fatty acid biosynthesis and steroid biosynthesis utilize large amounts of NADPH. As a consequence, cells of the liver, adipose tissue, adrenal cortex, testis and lactating mammary gland have high levels of the PPP enzymes. In fact 30% of the oxidation of glucose in the liver occurs via the PPP. Additionally, erythrocytes utilize the reactions of the PPP to generate large amounts of NADPH used in the reduction of glutathione. The conversion of ribonucleotides to deoxyribonucleotides (through the action of ribonucleotide reductase) requires NADPH as the electron source, therefore, any rapidly proliferating cell needs large quantities of NADPH.
Regulation: Glucose-6-phosphate Dehydrogenase is the committed step of the Pentose Phosphate Pathway. This enzyme is regulated by availability of the substrate NADP+. As NADPH is utilized in reductive synthetic pathways, the increasing concentration of NADP+ stimulates the Pentose Phosphate Pathway, to replenish NADPH
Vitamin B12: Cobalamin
Vitamin B12, also known as cobalamin, aids in the building of genetic material, production of normal red blood cells, and maintenance of the nervous system.
RDA The Recommended Dietary Allowance (RDA) for vitamin B12 is 2.4 mcg/day for adult males and females
Vitamin B12 Deficiency
Vitamin B12 deficiency most commonly affects strict vegetarians (those who eat no animal products), infants of vegan mothers, and the elderly. Symptoms of deficiency include anemia, fatigue, neurological disorders, and degeneration of nerves resulting in numbness and tingling.
Polyprotic Acids
• Some acids are polyprotic acids; they can lose more than one proton.
• In this case, the conjugate base is also a weak acid.
• For example: Carbonic acid (H2CO3 ) can lose two protons sequentially.
• Each dissociation has a unique Ka and pKa value.
Ka1 = [H+ ][HCO3 - ] / [H2CO3]
Ka2 = [H+ ][CO3 -2 ] / [HCO3- ]
Note: (The difference between a weak acid and its conjugate base differ is one hydrogen)
Carbohydrates (glycans) have the basic composition
- Monosaccharides - simple sugars, with multiple hydroxyl groups. Based on the number of carbons (e.g., 3, 4, 5, or 6) a monosaccharide is a triose, tetrose, pentose, or hexose, etc.
- Disaccharides - two monosaccharides covalently linked
- Oligosaccharides - a few monosaccharides covalently linked.
- Polysaccharides - polymers consisting of chains of monosaccharide or disaccharide units
The basic characteristics of enzymes includes
(i) Almost all the enzymes are proteins and they follow the physical and chemical reactions of proteins (ii) Enzymes are sensitive and labile to heat
(iii) Enzymes are water soluble
(iv) Enzymes could be precipitated by protein precipitating agents such as ammonium sulfate and trichloroacetic acid.
Factors regulating blood calcium level
(i) Vitamin D
(a) Vitamin D and absorption of calcium: Active form of calcium is calcitriol. Calcitriol enters intestinal wall and binds to cytoplasmic receptor and then binds with DNA causes depression and consequent transcription of gene code for calbindin. Due to increased availability of calbindin, absorption of calcium increases leading to increased blood calcium level.
(b) Vitamin D and Bone: Vitamin D activates osteoblast, bone forming cells & also stimulates secretion of alkaline phosphatase. Due to this enzyme, calcium and phosphorus increase.
(c) Vitamin D and Kidney: Calcitriol increase reabsorption of calcium and phosphorus by renal tubules.
(ii) Parathyroid hormone (PTH)
Normal PTH level in serum is 10-60ng/l.
(a) PTH and bones: In bone, PTH causes demineralization. It also causes recreation of collagenase from osteoclast leads to loss of matrix and bone resorption. As a result, mucopolysacharides and hydroxyproline are excreted in urine.
(b) PTH and Kidney: In kidney, PTH causes increased reabsorption of calcium but decreases reabsorption of phosphorus from kidney tubules.
(iii) Calcitonin Calcitonin decreases serum calcium level. It inhibits resorption of bone. It decreases the activity of osteoclasts and increases osteoblasts.
Hyper Calcemia When plasma Ca2+ level is more than 11mg/dl is called Hypercalcemia. It is due to parathyroid adenoma or ectopic PTH secreting tumor. calcium excreted in urine decreases excretion of chloride causing hyperchloremic acidosis.
Hypocalcemia Plasma calcium level less than 8mg/dl is called hypocalcemia. Tetany due to accidental surgical removal of parathyroid glands or by autoimmune disease. In tetany, neuromuscular irritability is increased. Increased Q-7 internal in ECG is seen. Main manifestation is carpopedal spasm. Laryngismus and stridor are also observed.