# 25: Amino Acids, Peptides, and Proteins

The chemistry of life is largely the chemistry of polyfunctional organic compounds. The functional groups usually are of types that interact rather strongly as, for example, the hydroxyl and carbonyl functions of carbohydrates (Chapter 20). The interaction between amino and carboxyl functions of amino acids figures greatly in the present chapter. We will approach the very important chemistry of amino acids and their derivatives in three stages. First, simple a-amino acids will be considered with emphasis on how the properties of amine functions and of acid functions are modified in molecules that possess both groups. Then we shall discuss some important properties of peptides and proteins, which are substances made up of amino acids linked together by amide bonds. Attention also will be given to the chemical problems presented by enzymes, which are protein molecules able to act as efficient catalysts for specific chemical reactions, and to the role of nucleic acids in protein synthesis.

• 25.1: Types of Biologically Important Amino Acids
The amino acids that occur naturally as constituents of proteins have an amino group (NH2)(NH2) and a carboxylic acid group (CO2H)(CO2H) attached to the same carbon. They are called α -amino acids and differ only in the nature of the R group on the α carbon and, with few exceptions, they are chiral molecules with the L configuration at the chiral α carbon.
• 25.2: Acid-Base Properties of $$\alpha$$-Amino Acids
The behavior of glycine is reasonably typical of that of the simplest amino acids. Because glycine is neither a strong acid nor a strong base, we shall expect a solution of glycine in water to contain four species in rapid equilibrium. The proportions of these species are expected to change with pH, the cationic conjugate acid being the predominant form at low pH and the anionic conjugate base being favored at high pH
• 25.3: Physical and Spectroscopic Properties
The α-amino acids crystallize as the dipolar forms and the strong intermolecular electrical forces in the crystals lead to higher melting points than those of simple amines or monocarboxylic acids. The melting points are so high that decomposition often occurs on melting. The solubility characteristics of amino acids in water are complex because of the acid-dissociation equilibria involved, but they are least soluble at their isoelectric points.
• 25.4: Analysis of Amino Acids
In many kinds of research it is important to have simple and sensitive means for analysis of amino acids, particularly in small quantities.
• 25.5: Reactions of Amino Acids
To some degree the reactions of amino acids are typical of isolated carboxylic acid and amine functions. Thus the carboxyl function can be esterified with an excess of an alcohol under acidic conditions, and the amine function can be acylated with acid chlorides or anhydrides under basic conditions. The amine function of α-amino acids and esters reacts with nitrous acid in a manner similar to that described for primary amines.
• 25.6: Synthesis of α-Amino Acids
Many of the types of reactions that are useful for the preparation of amino acids have been discussed previously in connection with separate syntheses of carboxylic acids and amino compounds. Examples include the SN2 displacement of halogen from α-halo acids by ammonia,  and the Strecker synthesis, which, in its first step, bears a close relationship to cyanohydrin formation
• 25.7: Peptides and Proteins
Amino acids are the building blocks of the polyamide structures of peptides and proteins. Each amino acid is linked to another by an amide (or peptide) bond formed between the amine group of one and the acid group of the other. In this manner a polymeric structure of repeating amide links is built into a chain or ring. The amide groups are planar and configuration about the C−NC−N bond is usually, but not always, trans.
• 25.8: Structure and Function of Proteins
The biological functions of proteins are extremely diverse. The primary structure and presence or absence of special functional groups, metals, and so on, are of paramount importance is making proteins,  made from a common basis set of amino acids, so remarkably heterogeneous and exhibit such varied yet specific functions.
• 25.9: Enzymes
Virtually all biochemical reactions are catalyzed by proteins called enzymes. The catalytic power and specificity of enzymes is extraordinarily high. The reactions that they catalyze are generally enhanced in rate many orders of magnitude, often as much as 10 million, over the nonenzymatic process. Consequently enzymatic reactions may occur under much milder conditions than comparable laboratory reactions.
• 25.E: Amino Acids, Peptides, and Proteins (Exercises)
These are the homework exercises to accompany Chapter 25 of the Textmap for Basic Principles of Organic Chemistry (Roberts and Caserio).
• 25.10: Coenzymes
Many enzymes only operate in combination with organic molecules that are actually reagents for the reaction. These substances are called coenzymes or cofactors. Some coenzymes function with more than one enzyme and are involved in reactions with a number of different substrates.
• 25.11: Enzyme Regulation
You may have wondered how the proteolytic enzymes such as trypsin, pepsin, chymotrypsin, carboxypeptidase, and others keep from self-destructing by catalyzing their own hydrolysis or by hydrolyzing each other. An interesting feature of the digestive enzymes is that they are produced in an inactive form in the stomach or the pancreas - presumably to protect the different kinds of proteolytic enzymes from attacking each other or other proteins.
• 25.12: Enzyme Technology
Because enzymes function nearly to perfection in living systems, there is great interest in how they might be harnessed to carry on desired reactions of practical value outside of living systems. The potential value in the use of enzymes (separate from the organisms that synthesize them) is undeniable, but how to realize this potential is another matter.
• 25.13: Biosynthesis of Proteins
One of the most interesting and basic problems connected with the synthesis of proteins in living cells is how the component amino acids are induced to link together in the sequences that are specific for each type of protein. There also is the related problem of how the information as to the amino-acid sequences is perpetuated in each new generation of cells. We now know that the substances responsible for genetic control in plants and animals are present in and originate from the chromosomes.
• 25.14: Chemical Evolution
A problem of great interest to those curious about the evolution of life concerns the origins of biological molecules. When and how were the molecules of life, such as proteins, nucleic acids, and polysaccharides, first synthesized? In the course of geological history, there must have been a prebiotic period when organic compounds were formed and converted to complex molecules similar to those we encounter in living systems.