Clinical significance

Primary hyperparathyroidism is due to autonomous, abnormal hypersecretion of PTH in the parathyroid gland

Secondary hyperparathyroidism is an appropriately high PTH level seen as a physiological response to hypocalcemia.

A low level of PTH in the blood is known as hypoparathyroidism and is most commonly due to damage to or removal of parathyroid glands during thyroid surgery.

Weak Acids and pKa

• The strength of an acid can be determined by its dissociation constant, Ka.

• Acids that do not dissociate significantly in water are weak acids.

• The dissociation of an acid is expressed by the following reaction: HA = H⁺ + A^- and the dissociation constant Ka = [H⁺ ][A^- ] / [HA]  

• When Ka < 1, [HA] > [H⁺ ][A^- ] and HA is not significantly dissociated. Thus, HA is a weak acid when ka < 1.

• The lesser the value of Ka, the weaker the acid.

• Similar to pH, the value of Ka can also be represented as pKa.

• pKa = -log Ka.

• The larger the pKa, the weaker the acid.

• pKa is a constant for each conjugate acid and its conjugate base pair.

• Most biological compounds are weak acids or weak bases.

Anaerobic organisms lack a respiratory chain. They must reoxidize NADH produced in Glycolysis through some other reaction, because NAD⁺ is needed for the Glyceraldehyde-3-phosphate Dehydrogenase reaction (see above). Usually NADH is reoxidized as pyruvate is converted to a more reduced compound, that may be excreted.

The complete pathway, including Glycolysis and the re-oxidation of NADH, is called fermentation.

For example, Lactate Dehydrogenase catalyzes reduction of the keto group in pyruvate to a hydroxyl, yielding lactate, as NADH is oxidized to NAD⁺.

Skeletal muscles ferment glucose to lactate during exercise, when aerobic metabolism cannot keep up with energy needs. Lactate released to the blood may be taken up by other tissues, or by muscle after exercise, and converted via the reversible Lactate Dehydrogenase back to pyruvate

Fermentation Pathway, from glucose to lactate (omitting H⁺):

   glucose + 2 ADP + 2 P_i → 2 lactate + 2 ATP

Anaerobic catabolism of glucose yields only 2 “high energy” bonds of ATP.

These amino acids have aromatic side chains, which contribute to protein structure, UV absorbance, and biochemical signaling.

1. Phenylalanine (Phe, F)

• Structure: Contains a benzyl side chain (a phenyl group attached to a CH₂).

• Properties: Non-polar, hydrophobic.

• Function: Precursor to tyrosine, dopamine, norepinephrine, and epinephrine.

2. Tyrosine (Tyr, Y)

• Structure: Similar to phenylalanine but with a hydroxyl group (-OH) on the aromatic ring.

• Properties: Polar, can participate in hydrogen bonding.

• Function: Precursor to catecholamines and thyroid hormones; involved in signal transduction via phosphorylation.

3. Tryptophan (Trp, W)

• Structure: Contains an indole ring (a fused double ring with nitrogen).

• Properties: Slightly polar, bulky.

• Function: Precursor to serotonin, melatonin, and niacin; absorbs UV light strongly at 280 nm.

4. Histidine (His, H) (sometimes included due to its aromatic-like imidazole ring)

• Structure: Has an imidazole ring (five-membered ring with two nitrogen atoms).

• Properties: Polar, positively charged at physiological pH.

• Function: Key role in enzyme active sites; acts as a proton donor/acceptor.

Bonus Insight: UV Absorbance

• Tryptophan and Tyrosine absorb UV light at ~280 nm, which is useful for measuring protein concentration.

• Phenylalanine absorbs weakly at ~260 nm.

BIOLOGICAL ROLES OF LIPID

Lipids have the common property of being relatively insoluble in water and soluble in nonpolar solvents such as ether and chloroform. They are important dietary constituents not only because of their high energy value but also because of the fat-soluble vitamins and the essential fatty acids contained in the fat of natural foods

Nonpolar lipids act as electrical insulators, allowing rapid propagation of depolarization waves along myelinated nerves

Combinations of lipid and protein (lipoproteins) are important cellular constituents, occurring both in the cell membrane and in the mitochondria, and serving also as the means of transporting lipids in the blood.

Enzyme assays measure the activity or concentration of specific enzymes in blood or tissue samples. Elevated or reduced levels often indicate organ dysfunction or cellular damage.

Enzyme Groups & Their Clinical Significance