Biochemistry Basics: Carbohydrates, Lipids, Proteins and Enzymes

Carbohydrates

Carbohydrates have the general formula C_nH_2nO_n, also written as CHO or C_n(H₂O)_n.

Monosaccharides

Monosaccharides, also called simple sugars, include:

• Ribose
• Deoxyribose
• Glucose
• Fructose
• Galactose

Oligosaccharides

Oligosaccharides have the general formula C_nH_2n-2O_n-1, for example C₁₂H₂₂O₁₁.

Disaccharides

• Sucrose (glucose + fructose)
• Lactose (glucose + galactose)
• Maltose (glucose + glucose)
• Cellobiose (glucose + glucose)

Trisaccharides

• Raffinose

Polysaccharides

Polysaccharides have the general formula (C₆H₁₀O₅)_n.

Homopolysaccharides

• Starch
• Glycogen
• Cellulose
• Chitin

Heteropolysaccharides

• Agar agar
• Pectin
• Hyaluronic acid
• Chondroitin
• Heparin

Lipids

Lipids are composed mainly of C, H, O, P and N.

Saponifiable Lipids

Saponifiable lipids can be hydrolyzed to form soap.

Hololipids (glycerol + fatty acids)

• Glycerides: oils, tallow, lard
• Ceridos (waxes): beeswax, lanolin, spermaceti, plant waxes, and carnauba palm wax

Heterolipids (lipid + other non-lipid substances)

• Phosphoglycerides: cephalin, lecithin, cardiolipin
• Sphingolipids: sphingomyelins
• Glycolipids: cerebrosides, gangliosides

Unsaponifiable Lipids

• Steroids: cholesterol, sex hormones, vitamin D
• Carotenoids: retinene, limonene, pimentol
• Prostaglandins: regulate blood pressure and muscle contraction

Proteins

Proteins are composed of C, H, O, N and sometimes S.

Holoproteins (yield only amino acids on hydrolysis)

Soluble Holoproteins

• Albumins: ovalbumin, lactalbumin, serum albumin
• Globulins: gamma globulins (antibodies), fibrinogen
• Protamines and histones: nucleoproteins

Insoluble Holoproteins

• Prolamins: zein (maize), gliadin (wheat), hordein (barley)
• Glutelins: glutenin (wheat), oryzanin (rice)
• Scleroproteins: collagen, elastin, fibroin, myosins, keratins

Heteroproteins (amino acids + prosthetic group)

• Phosphoproteins: casein
• Glycoproteins: mucin
• Lipoproteins: membrane proteins
• Chromoproteins: hemoglobin, hemerythrin, chlorophyll
• Nucleoproteins: protamines and histones

Enzymes

Enzymes are biological catalysts that reduce the activation energy of biochemical reactions.

Structure

• Holoenzyme (functional enzyme) = apoenzyme (protein part) + coenzyme (e.g., vitamins) or cofactor (e.g., chemical elements)

Properties

• Soluble
• Denatured by heat or extreme conditions
• High specificity for substrates
• High efficiency: a small amount of enzyme can transform large amounts of substrate

Mechanism

Enzymes do not combine permanently with the substrate; they only lower the activation energy.

Nomenclature

Enzyme names are formed from the name of the substrate plus the suffix -ase.

Classification

• Oxidoreductases
• Transferases
• Lyases
• Ligases
• Hydrolases
• Isomerases

Zymogens

An apoenzyme or inactive zymogen is an enzyme precursor that must be activated before it becomes functional.