Encyclopedia of Earth and Physical Sciences - Vol. 9

By Joyce Tavolacci | Go to book overview

PROTEINS
Proteins are biological polymers that consist of long chains of amino acids joined together by peptide bondsProteins are biological polymers. They consist of long chains of amino acids that are necessary for the chemical processes that occur in every living; organism. According to their function, proteins are divided into two categories: structural proteins and biologically active proteins. Most structural proteins are fibrous; they include collagen, which is the protein of skin, and keratin, the protein of hair. Biologically active proteins regulate metabolism and include enzymes, hormones, and antibodies.
Amino acids
Although amino acids may have other formulas, those in proteins invariably have the general formula RCH(NH2)COOH, where C is the alpha (a) carbon, H is hydrogen, NHS is an amino group, COOH is the carboxyl group, and R is a group that defines the amino acid. The simplest protein is glycine, where R is another hydrogen atom. Naturally occurring proteins are comprised solely of alpha- (CX-) amino acids, in which the amino group and the carboxylic acid are separated by only one carbon atom.With the exception of glycine, all naturally occurring amino acids are chiral they can exist in either ot twoforms that are mirror images of each other (see STEREOCHEMISTRY). Only 20 different amino acids are commonly found in proteins. Called the common, or essential, amino acids, they are always in the same chiral form, called the L-form.
Proteins or polypeptides?
Proteins are sometimes referred to as macromolecular polypeptides because they are very large molecules joined together by peptide bonds. (A peptide bond is a link between the amino group of one amino acid to the carboxyl group of the next amino acid in the chain.) In general, a shorter chain of up to about 50 amino acids is called a polypeptide, and a longer chain is termed a protein. To define a protein or polypeptide, the amino acid molecules (called residues) are listed, working from the amino group on the left to the carboxyl
CORE FACTS
Proteins are made up of long chains of amino acids
that are joined together by peptide bonds.
Naturally occurring proteins are initially synthesized from
only 20 different amino acids—these amino acids
are called the common, or essential, amino acids.
There are four levels of protein structure: primary,
secondary, tertiary, and quaternary.
In the process of denaturation, the protein molecule
unfolds and becomes a disorganized tangle; as a result,
the protein molecule loses biological activity.
Hormones and enzymes are examples of proteins that are
essential to nearly every chemical reaction in the body.

This light micrograph
shows the structure of hair
filaments taken from sheep
wool (magnified x 400). The
thick hairs are composed
of a protein called keratin.

group on the right. Each amino acid has a three-letter abbreviation, with an H at the amino end and an —OH to denote the acid. For example, a polypeptide that consists of glycine bonded to alanine bonded to asparlic acid is written as H-Glv-Ala-Asp OH.
Three-dimensional structure of proteins
There are four levels of protein structure, representing increasingly complex three-dimensional shapes.
Primary structure: The sequence of amino acids in each type of protein is unique. This sequence, called the primary structure, determines the shape and function of the protein.
Secondary structure: Regular hydrogen-bond interactions within stretches of the polypeptide chain give rise to alpha-helices and beta-pleated sheets (the protein’s secondary structure). The most common secondary structure is the alpha-helix, in which the protein coils up to form a right-handed helix (imagine a spiral staircase that turns to the right) with the side chains pointing outward. In the beta-pleated sheet, protein chains line up side by side. A long chain can arrange itself in this way by forming a hairpin bend, which turns the chain back on itself. A large, complicated protein may exhibit several different forms of secondary structure over its entire length.
Tertiary structure: Certain combinations of alpha-helices and beta-pleated sheets pack together to form tightly folded globular units, each of which is called a domain. Tertiary structure refers to covalent bonding between side groups of the amino acids. The most important of these cross bridges are links between two cysteine residues to give a disulfide
CONNECTIONS
The chemical
structure of many
complex proteins, such
as hemoglobin, have
been deduced through
a process called X-
RAY
crystallography.
Naturally occurring
proteins include
enzymes, which
are biological
CATALYSTS.
Enzymes are essential
to nearly every
CHEMICAL
REACTION
in
the human body.

-1269-

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Encyclopedia of Earth and Physical Sciences - Vol. 9
Table of contents

Table of contents

  • Title Page 1153
  • Contents 1155
  • Paleontology 1157
  • Paleozoic Era 1163
  • Pangaea 1165
  • Particle Accelerators 1167
  • Particle Physics 1169
  • Pcbs 1175
  • Pennsylvanian Period 1177
  • Periodic Table 1181
  • Permafrost 1186
  • Permian Period 1188
  • Petrochemicals 1194
  • Phase Transitions 1200
  • Phenols 1202
  • Phosphorus 1204
  • Photoelectric Effect 1206
  • Photography 1208
  • Physical Chemistry 1214
  • Physics 1218
  • Piezoelectricity 1226
  • Planet X 1228
  • Plasma 1229
  • Plastics 1230
  • Plate Tectonics 1236
  • Platinum Metals 1241
  • Pluto 1243
  • Plutonium 1245
  • Polarity 1247
  • Polarization 1249
  • Polar Regions 1252
  • Pollution 1255
  • Polymers 1259
  • Precambrian 1263
  • Precipitation 1266
  • Proteins 1269
  • Proterozoiceon 1271
  • Protons 1274
  • Pulsars 1276
  • Purines and Pyrimidines 1278
  • Quantum Theory 1280
  • Quarks 1287
  • Quartz 1291
  • Quasars 1293
  • Index 1295
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