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INTRODUCTION

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OBJECTIVES

After studying this chapter, you should be able to:

  • Diagram the structures and write the three- and one-letter designations for each of the amino acids present in proteins.

  • Describe the contribution of each type of R group of the protein amino acids to their chemical properties.

  • List additional key functions of amino acids and explain how certain amino acids in plant seeds can severely impact human health.

  • Name the ionizable groups of the protein amino acids and list their approximate pKa values as free amino acids in aqueous solution.

  • Calculate the pH of an unbuffered aqueous solution of a polyfunctional amino acid and the change in pH that occurs following the addition of a given quantity of strong acid or alkali.

  • Define pI and explain its relationship to the net charge on a polyfunctional electrolyte.

  • Explain how pH, pKa and pI can be used to predict the mobility of a polyelectrolyte, such as an amino acid, in a direct-current electrical field.

  • Describe the directionality, nomenclature, and primary structure of peptides.

  • Describe the conformational consequences of the partial double-bond character of the peptide bond and identify the bonds in the peptide backbone that are free to rotate.

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BIOMEDICAL IMPORTANCE

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In addition to providing the monomer units from which the long polypeptide chains of proteins are synthesized, the l-α-amino acids and their derivatives participate in cellular functions as diverse as nerve transmission and the biosynthesis of porphyrins, purines, pyrimidines, and urea. The neuroendocrine system employs short polymers of amino acids called peptides as hormones, hormone-releasing factors, neuromodulators, and neurotransmitters. Humans and other higher animals cannot synthesize 10 of the l-α-amino acids present in proteins in amounts adequate to support infant growth or to maintain adult health. Consequently, the human diet must contain adequate quantities of these nutritionally essential amino acids. Each day the kidneys filter over 50 g of free amino acids from the arterial renal blood. However, only traces of free amino acids normally appear in the urine because amino acids are almost totally reabsorbed in the proximal tubule, conserving them for protein synthesis and other vital functions. Not all amino acids are, however, beneficial. While their proteins contain only l-α-amino acids, some microorganisms secrete mixtures of d-amino acids. Many bacteria elaborate peptides that contain both d- and l-α-amino acids, several of which possess therapeutic value, including the antibiotics bacitracin and gramicidin A and the antitumor agent bleomycin. Certain other microbial peptides are toxic. The cyanobacterial peptides microcystin and nodularin are lethal in large doses, while small quantities promote the formation of hepatic tumors. The ingestion of certain amino acids present in the seeds of legumes of the genus Lathyrus results in lathyrism, a tragic irreversible disease in which individuals lose control of their limbs. Certain other plant seed amino acids have also been implicated in neurodegenerative disease in natives of Guam.

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