Carbohydrates, Lipids, and Proteins: Essential Biomolecules

Posted on Dec 25, 2024 in Chemistry

Carbohydrates

Carbohydrates are compounds composed of carbon (C), hydrogen (H), and oxygen (O). Their general formula is Cn(H2O)n. They are derived from polyalcohols, aldehydes (aldoses), and ketones (ketoses).

Classification

  • Monosaccharides: Consist of one molecule, containing 3-7 carbon atoms. They are sweet, soluble, and crystallize (sugars).
  • Disaccharides: Formed by the binding of two monosaccharide molecules.
  • Oligosaccharides: Composed of several monosaccharide molecules (approximately 10-12).
  • Polysaccharides: Made up of thousands of monosaccharide molecules. Oligosaccharides and polysaccharides are not sweet, not soluble, and do not crystallize.

Functions

  • Provide energy for animals and plants.
  • Structural components (e.g., cellulose).
  • Reserve elements (starch in plants, glycogen in animals).
  • Form the exoskeleton of many invertebrates.

Monosaccharides

Monosaccharides are white, sweet, soluble, and crystallize. Their general formula is Cn(H2O)n. Polyols are substituted by a keto or aldehyde group. Nomenclature uses the suffix -ose, -keto/-aldo, and the number of carbon atoms. They have hydroxyl (OH) groups and either an aldehyde or a ketone group (only one). The simplest are trioses, derived from glycerol (glyceraldehyde and dihydroxyacetone).

Isomerism

Isomers are compounds with the same empirical formula but different structural formulas, resulting in different physical and chemical properties. For example, C6H12O6.

Spatial Isomerism: All monosaccharides, except dihydroxyacetone, have one or more asymmetric carbon atoms. An asymmetric carbon atom is one whose four valences are saturated by different constituents.

  • Spatial isomers have the same empirical formula.
  • Enantiomers are mirror images.
  • Epimers differ by only one carbon atom.

Optical Isomerism: All monosaccharides have an optical configuration (due to asymmetric carbon atoms), except dihydroxyacetone. A solution of monosaccharides rotates the polarization plane of polarized light.

Cyclization of Monosaccharides: Trioses, tetroses, pentoses (ribose, deoxyribose, ribulose), and hexoses (glucose, galactose, mannose, fructose) can form cyclic structures. Pyran forms a hexagonal structure, and furan forms a pentagonal structure.

Derivatives of Monosaccharides

Chemical changes are caused by the monosaccharides, often resulting from the replacement of one of the OH groups by other functional groups. There are three main groups:

  • Reduction: Deoxy sugars (e.g., deoxyribose).
  • Oxidation: Acids (e.g., glucuronic acid).
  • Substitution: Amines (e.g., glucosamine).

Disaccharides

Disaccharides are formed by joining two monosaccharides with the loss of a water molecule. The bond is called an O-glycosidic bond. There are two types:

  • Monocarbonyl: Involves one hemiacetal carbon atom.
  • Dicarbonyl: Involves two carbon atoms.

Examples include lactose (β-galactopyranosyl-glucose), sucrose (glucose-fructose), maltose (α-glucose x 2), and cellobiose (β-glucose).

Oligosaccharides

Oligosaccharides consist of 8-10 branched monosaccharide monomers. They form the glycocalyx, which acts as a fingerprint of cells, aiding in their identification and providing information. For example, glycoproteins.

Polysaccharides

Polysaccharides are composed of many monosaccharides linked by O-glycosidic bonds with the loss of water in each case. They have a high molecular weight. Hydrolysis yields disaccharides and then monosaccharides. They lose the properties of sugar, are insoluble, do not crystallize, and are not sweet. They have two main functions:

  • Storage: Monomers linked by α-bonds (all).
  • Structural: Monomers linked by β-bonds (all).

There are three groups:

  • Homopolysaccharides: Formed by a single type of monomer. Examples include starch, glycogen, cellulose, and chitin.
  • Heteropolysaccharides: Composed of various types of monomers. Examples include mucopolysaccharides (lubricants like mucilages and hyaluronic acid), pectin, and hemicellulose.

Glycosides

Glycosides consist of a carbohydrate group and a non-carbohydrate group. Examples include glycoproteins (membrane proteins, antibodies), glycolipids (brain, ganglia), peptidoglycans, and teichoic acids (part of bacterial walls).

Flavones

Flavones are non-carbohydrate compounds. Isoflavones have antioxidant effects and are often colored (orange-red). They are abundant in plants (beets, blackberries) and strengthen the action of vitamin C.

Lipids

Lipids are composed of carbon (C), hydrogen (H), and oxygen (O), and sometimes phosphorus (P), nitrogen (N), and sulfur (S). They are characterized by having fatty acids in their molecules (not all). They are very heterogeneous, insoluble in water (due to many C-H or C-C bonds), and soluble in organic solvents.

Functions

  • Energy sources.
  • Intracellular energy storage.
  • Oxidize/metabolize only in the presence of oxygen.
  • Important structural functions.
  • Insulation.
  • Protection.
  • Waterproofing.
  • Water reserve.
  • Produce metabolic heat.

Classification

  • Saponifiable:
    • Simple (glycerides and waxes).
    • Complex (phospholipids, phosphoaminolipids, sphingolipids, proteolipids, glycolipids).
  • Non-saponifiable:
    • Derivatives of isoprene (terpenes, steroids).
    • Prostaglandins.

Fatty Acids

Fatty acids are volatile organic compounds with a carbonate chain and a carboxyl group. There are two types:

  • Saturated: Linear chain without isomerism, single bonds.
  • Unsaturated: One or more double or triple bonds, with isomerism.

They are solid at ambient temperature.

Glycerides/Neutral Fats

Glycerides are esters of glycerol (a trialcohol) with different fatty acids. Alcohol + acid → ester + water. Triglycerides are the main components of animal and plant fats. Their melting point determines whether they are liquid (oils) or solid (tallow). Saponification is the hydrolysis of ester bonds, yielding glycerol and fatty acids. Lipases perform this process.

Waxes

Waxes are esterifications of fatty acids with very long-chain monovalent alcohols. They are solid at ambient temperature.

Complex Lipids

Phospholipids

The basic compound is phosphoric acid. Glycerol is esterified with a fatty acid at C1, another fatty acid at C2, and phosphoric acid at C3. When the polar molecule has nitrogen, it is called a phosphoaminolipid. When it has a carbohydrate, it is called a phosphoglycolipid.

Sphingolipids

Sphingolipids are formed by a molecule of sphingosine, a fatty acid that joins the NH2 group of sphingosine, and phosphoric acid that joins the terminal CH2OH and another OH group to be polar.

Proteolipids

Proteolipids are lipids associated with proteins. Important examples are transport proteins like chylomicrons, HDL, LDL, and VLDL, which transport cholesterol.

Isoprene Derivatives

Isoprene is 2-methyl-1,3-butadiene. Terpenes are polymerizable isoprene molecules forming linear or cyclic structures. They can be diterpenes, tetraterpenes, or thousands of molecules.

Steroids

Steroids are derived from terpenes forming cyclic structures. The basic structure is cyclopentane perhydro phenanthrene with different R groups. Cholesterol is a key component of cell membranes, providing rigidity while preventing crystallization. Sex hormones (testosterone, progesterone) and bile acids are synthesized from cholesterol.

Prostaglandins

Prostaglandins are lipids derived from the cyclization of fatty acids. They function in smooth muscle stimulation, the menstrual cycle, allergies, and inflammation.

Proteins

Proteins are building blocks and macromolecules with high molecular weight. They exhibit specificity, meaning each species synthesizes its own proteins, and there is variability among individuals within a species.

Functions

  • Structural: They make up more than half of the cell’s dry weight.
  • Catalytic: Enzymes.
  • Transport: Hemoglobin, active transport.
  • Defense.
  • Hormonal.
  • Recognition of chemical signals.
  • Motility.
  • Lubricant.
  • Reserve.
  • Regulatory: Regulate the cell cycle.

Amino Acids

Amino acids (AAs) are simple carbonated molecules with an amino group and an acid group. The amino acids that are part of proteins have the amino group and the carboxyl group attached to the α-carbon, the one next to the COOH group.

Properties

  • Crystalline solids.
  • Water-soluble.
  • Amphoteric behavior.
  • High melting point.
  • Optical activity.

Optical Activity

Amino acids can bend light to the right or left. There are two types of spatial isomers: enantiomers and stereoisomers. All amino acids exhibit optical isomerism.

Amphoteric Behavior

Amino acids have acid-base properties. In solutions where the pH is close to 7, amino acids are ionized. At pH 7, the R group can behave as polar, apolar, positively charged, or negatively charged. By changing the pH, they behave as acids or bases. All amino acids can balance their charges so that their net charge is 0. This pH is called the isoelectric point.

There are 20 common amino acids, each with a three-letter abbreviation, such as glycine (Gly).