Biochemistry Study Guide (page 2)
Biochemistry is the study of the chemistry of living things. The biomolecules in this lesson give an insight into all living things, including humans.
Carbohydrates (or sugars) serve as the main source of energy for living organisms. They are composed of carbon, hydrogen, and oxygen with the general molecular formula Cx(H2O)y, a hydrate of carbon. Carbohydrate names have the suffix –ose (e.g., such as glucose or fructose).
Monosaccharides are the simplest carbohydrate structure composed of one ring that can contain five C atoms (pentose, such as ribose, which is a constituent of RNA) or six C atoms (hexose, such as galactose derived from milk sugar lactose).
Disaccharides are dimeric sugar made of two monosaccharides joined together in a reaction that releases a molecule of water (dehydration). The bond between the two sugar molecules is called glycosidic linkage and can have either an axial (α-glycoside) or an equatorial (β-glycoside) orientation with respect to the ring conformation. Examples include the following:
- Maltose (two glucose molecules joined together), found in starch
- Lactose (one galactose joined to one glucose), found in milk
- Sucrose (one fructose joined to one glucose), table sugar
Polysaccharides are polymers or a long chain of repeating monosaccharide units. Examples include the following:
- Starch is a mixture of two kinds of polymers of α-glucose (linear amylose and amylopectin).
- Amylose contains glucose molecules joined together by α-glycosidic linkages, and amylopectin additionally has a branching at C-6. They are storage polysaccharides in plants.
- Glycogen consists of glucose molecules linked by α-glycosidic linkage (C-1 and C-4) and branched (C-6) by α-glycosidc linkage. Glycogen is the storage form of glucose in animals (liver and skeletal muscle).
- Cellulose consists of glucose molecules joined together by β-glycosidic linkage. Cellulose is found in plants and is not digested by humans (lacking the necessary enzyme).
Condensation and Hydrolysis
Condensation is the process of bonding together separate monosaccharide subunits into a disaccharide and/or a polysaccharide. It is also called dehydration synthesis as one molecule of water is lost in the process. It is carried out by specific enzymes.
Hydrolysis is the reverse process of condensation as a water molecule and specific enzymes break all the glycosidic linkages in disaccharides and polysaccharides into their constituting monosaccharides.
Lipids are a diverse group of compounds that are insoluble in water and polar solvents but are soluble in nonpolar solvents. Lipids are stored in the body as a source of energy (twice the energy provided by an equal amount of carbohydrates).
Triglycerides are lipids formed by the condensation of glycerol (one molecule) with fatty acids (three molecules). They can be saturated (from fatty acid containing only C-C single bonds) or unsaturated (the presence of one or more C = C double bonds). Triglycerides are found in the adipose cells of the body (neutral fat) and are metabolized by the enzyme lipase (an esterase) during hydrolysis, producing fatty acids and glycerol.
Three ketone bodies are formed during the breakdown (metabolism) of fats: acetoacetate, β-hydroxybutyrate, and acetone. They are produced to meet the energy requirements of other tissues. Fatty acids, produced by the hydrolysis of triglycerides, are converted to ketone bodies in the liver. They are removed by the kidneys (ketosuria), but if found in excess in the blood (ketonemia), ketone bodies can cause a decrease of the blood pH and ketoacidosis may result. The ketone body acetone is exhaled via the lungs (this process is called ketosis). Ketosuria and ketonemia are common in diabetes mellitus patients and in cases of prolonged starvation.
Phospholipids are lipids containing a phosphate group. They are the main constituents of cellular membranes.
Steroids are organic compounds characterized by a core structure known as gonane (three cyclohexane or six-carbon rings and one cyclopentane or a fivecarbon ring fused together). Steroids differ by the functional groups attached to the gonane core. Cholesterol is an example of a steroid and is a precursor to the steroid hormones such as the sex hormones (the androgens and estrogens) and the corticosteroids (hormones of the adrenal cortex).
Every organism contains thousands of different proteins with a variety of functions: structure (collagen or histones), transport (hemoglobin or serum albumin), defense (the antibodies or fibrinogen for blood coagulation), control and regulation (insulin), catalysis (the enzymes), and storage. Proteins (also called polypeptides) are long chains of amino acids joined together by covalent bonds of the same type (peptide or amide bonds). Twenty naturally occurring amino acids exist, each characterized by an amino group at one end and a carboxylic acid group at the other end. Different proteins are characterized by different amino acids and/or a difference order of amino acids. Please refer to Table 20.1 on pages 121–122.
The sequence of amino acids in the long chain defines the primary structure of a protein. A secondary structure is determined when several residues, linked by hydrogen bonds, conform to a given combination (e.g., the α-helix, pleated sheet, and β-turns). Tertiary structure refers to the three-dimensional folded conformation of a protein. This is the biologically active conformation (crystal structure). A quaternary structure can result when two or more individual proteins assemble into two or more polypeptide chains. Conjugated proteins are complexes of proteins with other biomolecules, such as glycoproteins (sugar-proteins).
Enzymes are biological catalysts whose role is to increase the rate of chemical (metabolic) reactions without being consumed in the reaction. They do so by lowering the activation energy of a reaction by binding specifically (i.e., in the active site) to their substrates in a "lock-and-key" or "induced-fit" mechanism. They do not change the nature of the reaction (in fact, any change is associated with a malfunctioning enzyme), the onset of a disease, or its outcome.
- enzyme + substrate → enzyme-substrate
- complex → enzyme + product
Enzyme activity is influenced by
- Temperature: Proteins can be destroyed at high temperatures and their action is slowed at low temperature.
- pH: Enzymes are active in a certain range of the pH.
- concentration of cofactors and coenzymes (vitamins).
- concentration of enzymes and substrates.
- feedback reactions.
Enzymes names are derived from their substrate names with the addition of the suffix –ase, such as sucrase (substrate: sucrose). Categories of enzymes are created according to the reactions they catalyze, such as the kinases (phosphorylation). Enzymes are often found in multienzyme systems that operate by simple negative feedback.
Protein denaturation occurs when the protein configuration is changed by the destruction of the secondary and tertiary structures (reduced to the primary structure). Common denaturing agents are alcohol, heat, and heavy metal salts.
Practice problems for these concepts can be found at - Biochemistry Practice Questions