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Amino Acids, Peptides, and Proteins Help (page 2)

By — McGraw-Hill Professional
Updated on Aug 16, 2011

Proteins

Proteins are polypeptides with molecular weights from ca. 10,000 up to several million, and are a major constituent of living cells. Simple proteins are hydrolyzed to amino acids. Conjugated proteins are composed of amino acids and nonpeptide substances known as prosthetic groups. These prosthetic groups include nucleic acids of nucleoproteins, carbohydrates of glycoproteins, pigments (such as hemin and chlorophyll) of chromoproteins, and fats or lipids of lipoproteins.

Amphoteric Properties. Isoelectric Points and Electrophoresis. Proteins have different isoelectric points, and in an electrochemical cell they migrate to one of the electrodes (depending on their charge, size, and shape) at different speeds. This difference in behavior is used in electrophoresis for the separation and analysis of protein mixtures.

Proteins

Protein Structure

The primary structure of a protein consists of the sequence of the constituent amino acids. The secondary structure arises from different conformations of the protein chains; these conformations are best determined by X-ray analysis. There are three types.

  • The α-helix is a mainly right-handed coiled arrangement maintained by H-bonds between an N–H and O=C that are four peptide bonds apart.
  • The pleated sheet has chains lying side by side and linked through N–H – – – – O=C H bonds. The α C's rotate slightly out of the plane of the merger (to minimize repulsions between their bulky R groups), which gives rise to the "pleats."
  • Random structures have no repeating geometric pattern. However, there are structural constraints on the randomness leading to a constrained random orientation.

Tertiary structure is determined by any folding of the chains. There are two types.

  • Fibrous proteins are water-insoluble, elongated, threadlike helixes (occasionally pleated sheets) made up of chains which are bundled together intermolecularly through N–H – – – – O=C H-bonding. They include fibroin (found in silk), keratin (in hair, skin, feathers, etc.), and myosin (in muscle tissue).
  • Globular proteins, or globulins, are folded into compact spheroid shapes such that hydrophilic R groups point outward toward the water solvent and the hydrophobic (lipophilic) R groups turn inward. As a result, globulins can dissolve in or easily emulsify with water. The shape is maintained by intramolecular H bonding. The secondary structure is a combination of random (always present), helical, and pleated structures. Globulins include all enzymes, antibodies, albumin of eggs, hemoglobin, and many hormones such an insulin.

Quaternary structure exists when two or more polypeptide chains are linked only by weak forces of attraction among R groups at the surface of the chains. Such proteins are called oligomers (dimers, trimers, and so on).

Denaturation. Heat, strong acids or bases, ethanol, or heavy-metal ions irreversibly alter the secondary structure of proteins (see below). This process, known as denaturation, is exemplified by the heat-induced coagulation and hardening of egg white (albumin). Denaturation destroys the physiological activity of proteins.

 Practice problems for these concepts can be found at:  Amino Acids, Peptides, and Proteins Practice Problems

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