The input to fatty acid synthesis is acetyl-CoA, which is carboxylated to malonyl-CoA.

The ATP-dependent carboxylation provides energy input. The CO₂ is lost later during condensation with the growing fatty acid. The spontaneous decarboxylation drives the condensation. 

 fatty acid synthesis
acetyl-CoA + 7 malonyl-CoA + 14 NADPH → palmitate + 7 CO₂ + 14 NADP⁺ + 8 CoA

ATP-dependent synthesis of malonate:
8 acetyl-CoA + 14 NADPH + 7 ATP → palmitate + 14 NADP⁺ + 8 CoA + 7 ADP + 7 P_i

Fatty acid synthesis occurs in the cytosol. Acetyl-CoA generated in the mitochondria is transported to the cytosol via a shuttle mechanism involving citrate

Enzyme Kinetics

Enzymes are protein catalysts that, like all catalysts, speed up the rate of a chemical reaction without being used up in the process. They achieve their effect by temporarily binding to the substrate and, in doing so, lowering the activation energy needed to convert it to a product.

The rate at which an enzyme works is influenced by several factors, e.g.,

• the concentration of substrate molecules (the more of them available, the quicker the enzyme molecules collide and bind with them). The concentration of substrate is designated [S] and is expressed in unit of molarity.
• the temperature. As the temperature rises, molecular motion - and hence collisions between enzyme and substrate - speed up. But as enzymes are proteins, there is an upper limit beyond which the enzyme becomes denatured and ineffective.
• the presence of inhibitors.
  • competitive inhibitors are molecules that bind to the same site as the substrate - preventing the substrate from binding as they do so - but are not changed by the enzyme.
  • noncompetitive inhibitors are molecules that bind to some other site on the enzyme reducing its catalytic power.
• pH. The conformation of a protein is influenced by pH and as enzyme activity is crucially dependent on its conformation, its activity is likewise affected.

The study of the rate at which an enzyme works is called enzyme kinetics.

Function of Calcium

The major functions of calcium are

(a) Excitation and contraction of muscle fibres needs calcium. The active transport system utilizing calcium binding protein is called Calsequestrin. Calcium decreases neuromuscular irritability.
(b) Calcium is necessary for transmission of nerve impulse from presynaptic to postsynaptic region.
(c) Calcium is used as second messenger in system involving protein and inositol triphosphate.
(d) Secretion of insulin, parathyroid hormone, calcium etc, from the cells requires calcium.
(e) Calcium decrease the passage of serum through capillaries thus, calcium is clinically used  to reduce allergic exudates.
(f) Calcium is also required for coagulation factors such as prothrombin.
(g) Calcium prolongs systole.
(h) Bone and teeth contains bulk quantity of calcium.

Amino Acid Catabolism

Glutamine/Glutamate and Asparagine/Aspartate Catabolism

Glutaminase is an important kidney tubule enzyme involved in converting glutamine (from liver and from other tissue) to glutamate and NH₃⁺, with the NH₃⁺ being excreted in the urine. Glutaminase activity is present in many other tissues as well, although its activity is not nearly as prominent as in the kidney. The glutamate produced from glutamine is converted to a-ketoglutarate, making glutamine a glucogenic amino acid.

Asparaginase is also widely distributed within the body, where it converts asparagine into ammonia and aspartate. Aspartate transaminates to oxaloacetate, which follows the gluconeogenic pathway to glucose.

Glutamate and aspartate are important in collecting and eliminating amino nitrogen via glutamine synthetase and the urea cycle, respectively. The catabolic path of the carbon skeletons involves simple 1-step aminotransferase reactions that directly produce net quantities of a TCA cycle intermediate. The glutamate dehydrogenase reaction operating in the direction of a-ketoglutarate production provides a second avenue leading from glutamate to gluconeogenesis.

Alanine Catabolism

Alanine is also important in intertissue nitrogen transport as part of the glucose-alanine cycle. Alanine's catabolic pathway involves a simple aminotransferase reaction that directly produces pyruvate. Generally pyruvate produced by this pathway will result in the formation of oxaloacetate, although when the energy charge of a cell is low the pyruvate will be oxidized to CO₂ and H₂O via the PDH complex and the TCA cycle. This makes alanine a glucogenic amino acid.

Arginine, Ornithine and Proline Catabolism

The catabolism of arginine begins within the context of the urea cycle. It is hydrolyzed to urea and ornithine by arginase.

Ornithine, in excess of urea cycle needs, is transaminated to form glutamate semialdehyde. Glutamate semialdehyde can serve as the precursor for proline biosynthesis as described above or it can be converted to glutamate.

Proline catabolism is a reversal of its synthesis process.

The glutamate semialdehyde generated from ornithine and proline catabolism is oxidized to glutamate by an ATP-independent glutamate semialdehyde dehydrogenase. The glutamate can then be converted to α-ketoglutarate in a transamination reaction. Thus arginine, ornithine and proline, are glucogenic.
 

Methionine Catabolism

The principal fates of the essential amino acid methionine are incorporation into polypeptide chains, and use in the production of α -ketobutyrate and cysteine via SAM as described above. The transulfuration reactions that produce cysteine from homocysteine and serine also produce α -ketobutyrate, the latter being converted to succinyl-CoA.

Regulation of the methionine metabolic pathway is based on the availability of methionine and cysteine

Phenylalanine and Tyrosine Catabolism

Phenylalanine normally has only two fates: incorporation into polypeptide chains, and production of tyrosine via the tetrahydrobiopterin-requiring phenylalanine hydroxylase. Thus, phenylalanine catabolism always follows the pathway of tyrosine catabolism. The main pathway for tyrosine degradation involves conversion to fumarate and acetoacetate, allowing phenylalanine and tyrosine to be classified as both glucogenic and ketogenic.

Tyrosine is equally important for protein biosynthesis as well as an intermediate in the biosynthesis of several physiologically important metabolites e.g. dopamine, norepinephrine and epinephrine

COENZYMES

 Enzymes may be simple proteins, or complex enzymes.

A complex enzyme contains a non-protein part, called as prosthetic group (co-enzymes).

Coenzymes are heat stable low molecular weight organic compound. The combined form of protein and the co-enzyme are called as holo-enzyme. The heat labile or unstable part of the holo-enzyme is called as apo-enzyme. The apo-enzyme gives necessary three dimensional structures required for the enzymatic chemical reaction.

Co-enzymes are very essential for the biological activities of the enzyme.

Co-enzymes combine loosely with apo-enzyme and are released easily by dialysis. Most of the co-enzymes are derivatives of vitamin B complex

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.