8: Proteins
- Page ID
- 342710
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Proteins may be defined as compounds of high molar mass consisting largely or entirely of chains of amino acids. Their masses range from several thousand to several million daltons (Da). In addition to carbon, hydrogen, and oxygen atoms, all proteins contain nitrogen and sulfur atoms, and many also contain phosphorus atoms and traces of other elements. Proteins serve a variety of roles in living organisms and are often classified by these biological roles. 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.
- 8.1: Amino Acids
- Amino acids can be classified based on the characteristics of their distinctive side chains as nonpolar, polar but uncharged, negatively charged, or positively charged. The amino acids found in proteins are L-amino acids.
- 8.2: Reactions of Amino Acids
- Amino acids can act as both an acid and a base due to the presence of the amino and carboxyl functional groups. The pH at which a given amino acid exists in solution as a zwitterion is called the isoelectric point (pI).
- 8.3: Peptides
- The amino group of one amino acid can react with the carboxyl group on another amino acid to form a peptide bond that links the two amino acids together. 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).
- 8.4: Proteins
- Proteins can be divided into two categories: fibrous, which tend to be insoluble in water, and globular, which are more soluble in water. A protein may have up to four levels of structure. The primary structure consists of the specific amino acid sequence. The peptide chain can form an α-helix or β-pleated sheet, which is known as secondary structure and are incorporated into the tertiary structure of the folded polypeptide. The quaternary structure describes the arrangements of subunits.
- 8.5: Enzymes - Biological Catalysts
- Most chemical reactions within organisms would be impossible under the conditions in cells. e.g., the body temperature of most organisms is too low for reactions to occur quickly enough to carry out life processes. Reactants may also be present in such low concentrations that it is unlikely they will meet and collide. Therefore, the rate of most biochemical reactions must be increased by a catalyst, which speeds up chemical reactions. In organisms, catalysts are called enzymes.
- 8.6: Enzyme Activity
- Most chemical reactions within organisms would be impossible under the conditions in cells. e.g., the body temperature of most organisms is too low for reactions to occur quickly enough to carry out life processes. Reactants may also be present in such low concentrations that it is unlikely they will meet and collide. Therefore, the rate of most biochemical reactions must be increased by a catalyst, which speeds up chemical reactions. In organisms, catalysts are called enzymes.
- 8.7: Enzyme Inhibition
- Most chemical reactions within organisms would be impossible under the conditions in cells. e.g., the body temperature of most organisms is too low for reactions to occur quickly enough to carry out life processes. Reactants may also be present in such low concentrations that it is unlikely they will meet and collide. Therefore, the rate of most biochemical reactions must be increased by a catalyst, which speeds up chemical reactions. In organisms, catalysts are called enzymes.
- 8.8: Proteins (Summary)
- To ensure that you understand the material in this chapter, you should review the meanings of the bold terms in the following summary and ask yourself how they relate to the topics in the chapter.
- 8.9: E- Proteins (Exercises)
- Problems and select solutions for the chapter.