Muscle tissue is largely protein, as are skin and hair. Proteins are present in the blood, in the brain, and even in tooth enamel. Each type of cell in our bodies makes its own specialized proteins, as well as proteins common to all or most cells.
We begin our study of proteins by looking at the properties and reactions of amino acids, which is followed by a discussion of how amino acids link covalently to form peptides and proteins. We end the chapter with a discussion of enzymes—the proteins that act as catalysts in the body.
The amino acids found in proteins are L-amino acids. The pH at which a given amino acid exists in solution as a zwitterion is called the isoelectric point pI. Additional amino acids can be added on through the formation of addition peptide amide bonds. A sequence of amino acids in a peptide or protein is written with the N-terminal amino acid first and the C-terminal amino acid at the end writing left to right. Protein families may have many members, and they likely evolved from ancient gene duplications.
These duplications led to modifications of protein functions and expanded the functional repertoire of organisms over time. This page appears in the following eBook. Aa Aa Aa. Protein Structure. What Are Proteins Made Of? Figure 1: The relationship between amino acid side chains and protein conformation. The defining feature of an amino acid is its side chain at top, blue circle; below, all colored circles. Figure 2: The structure of the protein bacteriorhodopsin.
Bacteriorhodopsin is a membrane protein in bacteria that acts as a proton pump. What Are Protein Families? Proteins are built as chains of amino acids, which then fold into unique three-dimensional shapes.
Bonding within protein molecules helps stabilize their structure, and the final folded forms of proteins are well-adapted for their functions.
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It is at this level of structure that many proteins take on their cellular role or function. As a polypeptide chain, the molecule had no special properties, but as a three dimensional protein, the molecule is capable of performing an astonishing variety of feats.
Globular proteins catalyze chemical reactions, others act as defending antibodies within the immune system, and yet others trigger violent bodily reactions as they travel through the blood. Some globular proteins come together in complexes consisting of two or more subunits. Attached to these subunits can be other, non protein, molecules such as polysaccharides.
These higher levels of structure can be seen in a protein molecule such as hemoglobin, a large, four subunit globular protein with four additional non protein additions. This protein carries oxygen around in the blood. Most of the special properties of proteins stem from their unique three dimensional shapes. When this shape is lost, the protein ceases to function. The process of changing the shape of a protein so that the function is lost is called denaturation.
Proteins are easily denatured by heat. When protein molecules are boiled their properties change. For example, they frequently become insoluble and remain so even when the solution is cooled. Boiling an egg causes the irreversible denaturation of all the proteins it contains, the "white" of the egg, a globular protein, changes shape and hardens into a solid. This denatured protein has the same primary structure as the original protein, but the tertiary structure has been lost; so have all the critical properties of the original or native protein.
Denaturation can be brought about in other ways as well as heat. All proteins can be denatured by extremes of pH; alkaline or acid. They are sensitive to organic solvents and soaps. Beating an egg white will cause mechanical denaturation by increasing the surface area of the liquid. Surface tension then pulls the protein out of shape. Science at a Distance.
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