NEET MDS Lessons
Biochemistry
Ampholytes, Polyampholytes, pI and Zwitterion
Many substances in nature contain both acidic and basic groups as well as many different types of these groups in the same molecule. (e.g. proteins). These are called ampholytes (one acidic and one basic group) or polyampholytes (many acidic and basic groups). Proteins contains many different amino acids some of which contain ionizable side groups, both acidic and basic. Therefore, a useful term for dealing with the titration of ampholytes and polyampholytes (e.g. proteins) is the isoelectric point, pI. This is described as the pH at which the effective net charge on a molecule is zero.
For the case of a simple ampholyte like the amino acid glycine the pI, when calculated from the Henderson-Hasselbalch equation, is shown to be the average of the pK for the a-COOH group and the pK for the a-NH2 group:
pI = [pKa-(COOH) + pKa-(NH3+)]/2
For more complex molecules such as polyampholytes the pI is the average of the pKa values that represent the boundaries of the zwitterionic form of the molecule. The pI value, like that of pK, is very informative as to the nature of different molecules. A molecule with a low pI would contain a predominance of acidic groups, whereas a high pI indicates predominance of basic groups.
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 Hemoglobin Buffer Systems
These buffer systems are involved in buffering CO2 inside erythrocytes. The buffering capacity of hemoglobin depends on its oxygenation and deoxygenation. Inside the erythrocytes, CO2 combines with H2O to form carbonic acid (H2CO3) under the action of carbonic anhydrase.
At the blood pH 7.4, H2CO3 dissociates into H+ and HCO3 − and needs immediate buffering.
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.
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b Oxidation Pathway |
Fatty Acid Synthesis |
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pathway location |
mitochondrial matrix |
cytosol |
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acyl carriers (thiols) |
Coenzyme-A |
phosphopantetheine (ACP) & cysteine |
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electron acceptors/donor |
FAD & NAD+ |
NADPH |
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hydroxyl intermediate |
L |
D |
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2-C product/donor |
acetyl-CoA |
malonyl-CoA (& acetyl-CoA) |
STEROIDS
Steroids are the compounds containing a cyclic steroid nucleus (or ring) namely cyclopentanoperhydrophenanthrene (CPPP).It consists of a phenanthrene nucleus (rings A, B and C) to which a cyclopentane ring (D) is attached.
Steroids are the compounds containing a cyclic steroid nucleus (or ring) namely cyclopentanoperhydrophenanthrene (CPPP).It consists of a phenanthrene nucleus (rings A, B and C) to which a cyclopentane ring (D) is attached.
There are several steroids in the biological system. These include cholesterol, bile acids, vitamin D, sex hormones, adrenocortical hormones,sitosterols, cardiac glycosides and alkaloids
3-D Structure of proteins
Proteins are the main players in the life of a cell. Each protein is a unique sequence of amino acid residues, each of which folds into a unique, stable, three dimentional structure that is biologically functional.
Conformation = spatial arrangement of atoms that depends on rotation of bonds. Can change without breaking covalent bonds.
- Since each residue has a number of possible conformations, and there are many residues in a protein, the number of possible conformations for a protein is enormous.
Native conformation = single, stable shape a protein assumes under physiological conditions.
- In native conformation, rotation around covalent bonds in polypeptide is constrained by a number of factors ( H-bonding, weak interactions, steric interference)
- Biological function of proteins depends completely on its conformation. In biology, shape is everything.
- Proteins can be classified as globular or fibrous.
There are 4 levels of protein structure
- Primary structure
- linear sequence of amino acids
- held by covalent forces
- primary structure determines all oversall shape of folded polypeptides (i.e primary structure determines secondary , tertiary, and quaternary structures)
- Secondary structure
- regions of regularly repeating conformations of the peptide chain (α helices, β sheets)
- maintained by H-bonds between amide hydrogens and carbonyl oxygens of peptide backbone.
- Tertiary structure
- completely folded and compacted polypeptide chain.
- stabilized by interactions of sidechains of non-neighboring amino acid residues (fibrous proteins lack tertiary structure)
- Quaternary structure
- association of two or more polypeptide chains into a multisubunit protein.