Thyroid Hormones

Thyroid hormones (T4 and T3) are tyrosine-based hormones produced by the follicular cells of the thyroid gland and are regulated by TSH made by the thyrotropes of the anterior pituitary gland, are primarily responsible for regulation of metabolism. Iodine is necessary for the production of T3 (triiodothyronine) and T4 (thyroxine).

A deficiency of iodine leads to decreased production of T3 and T4, enlarges  the thyroid tissue and will cause the disease known as goitre.

Thyroid hormones are transported by Thyroid-Binding Globulin

Thyroxine binding globulin (TBG), a glycoprotein binds T4 and T3 and has the capacity to bind 20 μg/dL of plasma.

Diseases

1. Hyperthyroidism (an example is Graves Disease) is the clinical syndrome caused by an excess of circulating free thyroxine, free triiodothyronine, or both. It is a common disorder that affects approximately 2% of women and 0.2% of men.

2 Hypothyroidism (an example is Hashimoto’s thyroiditis) is the case where there is a deficiency of thyroxine, triiodiothyronine, or both.

Glycogen Storage Diseases are genetic enzyme deficiencies associated with excessive glycogen accumulation within cells.

• When an enzyme defect affects mainly glycogen storage in liver, a common symptom is hypoglycemia (low blood glucose), relating to impaired mobilization of glucose for release to the blood during fasting.
• When the defect is in muscle tissue, weakness and difficulty with exercise result from inability to increase glucose entry into Glycolysis during exercise.

Various type of Glycogen storage disease are

FLUORIDE

The safe limit of fluorine is about 1PPM in water. But excess of fluoride causes Flourosis

Flourosis is more dangerous than caries. When Fluoride content is more than 2 PPM, it will cause chronic intestinal upset, gastroenteritis, loss of weight, osteosclerosis, stratification and discoloration of teeth

The Phosphate Buffer System

This system, which acts in the cytoplasm of all cells, consists of H₂PO₄^–  as proton donor and HPO₄ ^2– as proton acceptor :

H₂PO₄^–  = H⁺ + H₂PO₄^–

The phosphate buffer system works exactly like the acetate buffer system, except for the pH range in which it functions. The phosphate buffer system is maximally effective at a pH close to its pKa of 6.86 and thus tends to resist pH changes in the range between 6.4 and 7.4. It is, therefore, effective in providing buffering power in intracellular fluids.

Biotin

 Biotin helps release energy from carbohydrates and aids in the metabolism of fats, proteins and carbohydrates from food.

RDA The Adequate Intake (AI) for Biotin is 30 mcg/day for adult males and females

Biotin Deficiency Biotin deficiency is uncommon under normal circumstances, but symptoms include fatigue, loss of appetite, nausea, vomiting, depression, muscle pains, heart abnormalities and anemia.

The Effects of Enzyme Inhibitors

Enzymes can be inhibited

• competitively, when the substrate and inhibitor compete for binding to the same active site or
• noncompetitively, when the inhibitor binds somewhere else on the enzyme molecule reducing its efficiency.

The distinction can be determined by plotting enzyme activity with and without the inhibitor present.

Competitive Inhibition

In the presence of a competitive inhibitor, it takes a higher substrate concentration to achieve the same velocities that were reached in its absence. So while V_max can still be reached if sufficient substrate is available, one-half V_max requires a higher [S] than before and thus K_m is larger.

Noncompetitive Inhibition

With noncompetitive inhibition, enzyme molecules that have been bound by the inhibitor are taken out

• enzyme rate (velocity) is reduced for all values of [S], including
• V_max and one-half V_max but
• K_m remains unchanged because the active site of those enzyme molecules that have not been inhibited is unchanged.