Sphingosine is an amino alcohol present in sphingomyelins (sphingophospholipids).  They do not contain glycerol at all.

Sphingosine is attached by an amide linkage to a fatty acid to produce ceramide. The alcohol group of sphingosine is bound to phosphorylcholine in sphingomyelin structure. .

Sphingomyelins are important constituents of myelin and are found in good quantity in brain and nervous tissues.

PHOSPHORUS

Serum level of phosphate is 3-4 mg/dl for adults and 5-6 mg/dl in children. Consumption of calcitriol increases phosphate absorption.

Functions of phosphorus
(a) Plays key role in formation of tooth and bone

(b) Production of high energy phosphate compounds such as ATP, CTP, GTP etc.,

(c) Synthesis of nucleotide co-enzymes such as NAD and NADP

(d) Formation of phosphodiester backbone structure for DNA and RNA synthesis

Hypophosphatemia is the condition which leads to decrease in absorption of phosphorus. it leads to hypercalcamia

Hyperphosphatemia, increase in absorption of phosphate was noticed. Hyperphosphatemia leads to cell lysis, hypocalcemia and thyrotoxicosis.

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

Glycogen Metabolism

The formation of glycogen from glucose is called Glycogenesis

Glycogen is a polymer of glucose residues linked mainly by a(1→ 4)  glycosidic linkages. There are a(1→6) linkages at branch points. The chains and branches are longer than shown. Glucose is stored as glycogen predominantly in liver and muscle cells

Glycogen Synthesis

Uridine diphosphate glucose (UDP-glucose) is the immediate precursor for glycogen synthesis. As glucose residues are added to glycogen, UDP-glucose is the substrate and UDP is released as a reaction product. Nucleotide diphosphate sugars are precursors also for synthesis of other complex carbohydrates, including oligosaccharide chains of glycoproteins, etc.

UDP-glucose is formed from glucose-1-phosphate and uridine triphosphate (UTP)

glucose-1-phosphate + UTP → UDP-glucose + 2 P_i

Cleavage of PPi is the only energy cost for glycogen synthesis (1P bond per glucose residue)

Glycogenin initiates glycogen synthesis. Glycogenin is an enzyme that catalyzes glycosylation of one of its own tyrosine residues.

Physiological regulation of glycogen metabolism

Both synthesis and breakdown of glycogen are spontaneous. If glycogen synthesis and phosphorolysis were active simultaneously in a cell, there would be a futile cycle with cleavage of 1 P bond per cycle

To prevent such a futile cycle, Glycogen Synthase and Glycogen Phosphorylase are reciprocally regulated, both by allosteric effectors and by covalent modification (phosphorylation)

Glycogen catabolism (breakdown)

Glycogen Phosphorylase catalyzes phosphorolytic cleavage of the →(1→4) glycosidic linkages of glycogen, releasing glucose-1-phosphate as the reaction product.

Glycogen (n residues) + P_i → glycogen (n-1 residues) + glucose-1-phosphate

The Major product of glycogen breakdown is glucose -1-phosphate

Fate of glucose-1-phosphate in relation to other pathways:

Phosphoglucomutase catalyzes the reversible reaction:

Glucose-1-phosphate → Glucose-6-phosphate

Titration of a weak acid with a strong base

• A weak acid is mostly in its conjugate acid form

• When strong base is added, it removes protons from the solution, more and more acid is in the conjugate base form, and the pH increases

• When the moles of base added equals half the total moles of acid, the weak acid and its conjugate base are in equal amounts. The ratio of CB / WA = 1 and according to the HH equation, pH = pKa + log(1) or pH = pKa.

• If more base is added, the conjugate base form becomes greater till the equivalance point when all of the acid is in the conjugate base form.

FATTY  ACIDS

Fatty acids consist of a hydrocarbon chain with a carboxylic acid at one end.

• are usually in esterified form as major components of other lipids

• are often complexed in triacylglycerols (TAGs)

• most have an even number of carbon atoms (usually 14 to 24)

• are synthesized by concatenation of C2 units.

• C16 & C18 FAs are the most common FAs in higher plants and animals

• Are either:

—saturated (all C-C bonds are single bonds) or

—unsaturated (with one or more double bonds in the chain)

—monounsaturated (a single double bond)

1.Example of monounsaturated FA: Oleic acid 18:1(9) (the number in unsaturated FA parentheses indicates that the double bond is between carbons 9 & 10)

2. Double bonds are almost all in the cis conformation

—polyunsaturated (more then one double bond)

Polyunsaturated fatty acids contain 2 or more double bonds. They usually occur at every third carbon atom towards the methyl terminus (-CH3 ) of the molecule. Example of polyunsaturated FA: Linoleic acid 18:2(9,12)

• the number of double bonds in FAs varies from 1 to 4 (usually), but in most bacteria it is rarely more than 1

Saturated FAs are highly flexible molecules that can assume a wide range of conformations because there is relatively free rotation about their C-C bonds.