Cholesterol synthesis:

Hydroxymethylglutaryl-coenzyme A (HMG-CoA) is the precursor for cholesterol synthesis. 

HMG-CoA is also an intermediate on the pathway for synthesis of ketone bodies from acetyl-CoA. The enzymes for ketone body production are located in the mitochondrial matrix. HMG-CoA destined for cholesterol synthesis is made by equivalent, but different, enzymes in the cytosol.

HMG-CoA is formed by condensation of acetyl-CoA and acetoacetyl-CoA, catalyzed by HMG-CoA Synthase.

HMG-CoA Reductase, the rate-determining step on the pathway for synthesis of cholesterol.

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.

HORMONES

A hormone is a chemical that acts as a messenger transmitting a signal from one cell to another. When it binds to another cell which is the target of the message, the hormone can alter several aspects of cell function, including cell growth, metabolism, or other function.

Hormones can be classified on three primary ways as following:

1.  Autocrine: An autocrine hormone is one that acts on the same cell that released it.

2.  Paracrine: A paracrine hormone is one that acts on cells which are nearby relative to the cell which released it. An example of paracrine hormones includes growth factors, which are proteins that stimulate cellular proliferation and differentiation.

3. Endocrine: An endocrine hormone is one that is released into the bloodstream by endocrine glands. The receptor cells are distant from the source. An example of an endocrine hormone is insulin, which is released by the pancreas into the bloodstream where it regulates glucose uptake by liver and muscle cells.

Amino acids

Proteins are linear polymers of amino acids. Participate in virtually every biological process. Perform diverse functions:
       1. Enzymes: catalyze all reactions in living organisms
       2. Storage and transport
       3. Structural
       4. Mechanical work ( flagella, muscles, separation of chromosomes)
       5. Decoding information (translation, transcription, DNA replication)
       6. Cell-signalling (hormones and receptors)
       7. Defence (antibodies)

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.

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.