Lipids, Carbohydrates, Amino Acids & Peptides in Biochemistry
Lipids
Esfingoglucolipids
In sphingolipids, the polar group that binds to the ceramide is a carbohydrate that can be a monosaccharide or a branched oligosaccharide. The sphingolipids are arranged in the outer zone of the plasma membrane with glycoproteins forming the glycocalyx. Depending on the carbohydrate portion, they are classified into:
- Cerebrosides: Consist of a ceramide and a monosaccharide. They are abundant in the membranes of nerve cells in the brain and peripheral nervous system.
- Gangliosides: Contain a branched oligosaccharide. They are on the outside of cell membranes, particularly of neurons.
Functions:
- Gangliosides appear to be involved in the reception of nerve impulses across the synapse.
- Some cell surface sphingolipids appear to be associated with blood group specificity.
- Some act as sites of attachment of viruses, microorganisms, and toxins in the plasma membrane, allowing entry into the cell.
Terpenes, Steroids, and Prostaglandins
These are unsaponifiable lipids, meaning they cannot form soaps in the absence of fatty acids. They are less abundant than saponifiable lipids, but among them are some that carry out important biological functions such as vitamins or hormones.
Terpenes:
Also called isoprenoids, they are chemically derived from the polymerization of isoprene.
- Monoterpenes: These are substances with two molecules of isoprene.
- Diterpenes: They contain four molecules of isoprene.
- Triterpenes: They consist of six molecules of isoprene.
- Tetraterpenes: They are associations of eight molecules of isoprene.
Steroids:
Steroids are derivatives of a cyclic compound called cyclopentanoperhydrophenanthrene. Steroids are distinguished by the position of double bonds, the type of functional groups, and ring substituents. The most important are:
- Sterols: Cholesterol is one of the most biologically interesting. It is part of the plasma membrane of animal cells, is found in the blood and binds to plasma lipoproteins. It influences plasma membrane properties in a way that maintains its fluidity against fluctuations in temperature and degree of unsaturation. It also affects the permeability of lipid bilayers, diminishing it versus small water-soluble molecules.
- Steroid Hormones: e.g., sex hormones.
- Bile Acids: The most important in humans are cholic acid and deoxycholic acid, the latter also present in many other mammals. They are composed of bile, which is found forming salts that act as detergents in the small intestine, causing an emulsion of fat, which is subsequently degraded by the action of intestinal lipase.
Prostaglandins:
Functions:
- Can act as vasodilators, regulating blood pressure.
- Involved in inflammatory processes that cause fever, redness, swelling, and pain.
- Involved in blood clotting processes.
Carbohydrates
Holósidos:
Consisting only of carbohydrates.
- Oligosaccharides: Between 2 and 10 monosaccharides (e.g., disaccharides).
- Polysaccharides: Multiple monosaccharides (homopolysaccharides: repetition of a single monosaccharide; heteropolysaccharides: more diverse composition).
Glycosides:
Combination of a series of monosaccharides with non-carbohydrate molecular fractions.
Amino Acids
- Hydrophobic: The radicals are of a nonpolar hydrocarbon nature (e.g., alanine).
- Polar Hydrophilic: Radicals are polar but uncharged (e.g., serine and glycine).
- Basic: Positively ionized (e.g., lysine).
- Acidic: Negatively ionized (e.g., aspartic acid).
Peptide Bond:
When amino acids form chains, they are linked by peptide bonds. These covalent bonds are formed between the carboxyl group of one amino acid and the amino group of another, resulting in a loss of one water molecule (dehydration reaction). The amino acids joined by peptide bonds are called residues due to the loss of atoms in the peptide bond formation. A dipeptide is a chain that consists of two amino acid residues, a tripeptide is formed by three, an oligopeptide contains less than 50 residues, and longer chains are generally called polypeptides. The free amino and carboxyl groups at opposite ends of the polypeptide chain are named as N-terminal (amino terminal) and C-terminal (carboxyl terminal), respectively. By convention, residues are numbered from the N-terminal amino acid and are written from left to right in the order they occupy in the chain.
Characteristics of the Peptide Bond:
- The peptide bond is a covalent bond shorter than most other C-N bonds.
- It has some double bond character, which prevents it from spinning freely.
- The four atoms (C=O and N-H) of the peptide group and the two alpha-carbon atoms are located on the same plane, maintaining fixed distances and angles.
- The only bonds that can rotate, and not all freely, are those formed by Cα and N-Cα.
Peptides and Peptides of Biological Interest:
- With stress hormone function, oxytocin regulates uterine contractions.
- Insulin and glucagon regulate blood sugar levels.