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Biochemistry Study Guide (page 2)

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Updated on Sep 25, 2011

Ketone Bodies

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

Phospholipids are lipids containing a phosphate group. They are the main constituents of cellular membranes.

Table 20.1 The Natural Occurring Amino Acids

Table 20.1 The Natural Occurring Amino Acids

Steroids

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).

Proteins

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

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

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

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