Chapter Proteins and Amino Acids

DWIGHT E. MATTHEWS

AminoAcids BasicDefinitions

AminoAcjd.Pools.and., .Distribution Amin.o.A.c.id.Tran.sR.orf

PathwaysofAmino ..AcM.Synt.h.esisn and Degradation Am!ino..Acid...Degradat,ion .Pathways

Synth.e.s.i.s. . .of . . Non.essent.i.a,!, . . Am.ino. . Acids incorporation ..of Amino.. Acids. ..into..,Q,t.heLC,omRo,Mn,ds

Turnover of Proteins in the Body

  1. of .Measuring Protein. Turnover..and. Amino. Acid. Kinetics NitrogenBaJance
  2. ArteriovenoM5..Diffe.rencC5 ..to. Define.. Prgan.BaJances Tracer .nMethods.„Defi.nin,g ..Amin,o..Acid...K,ine,tics Contribution..of .Specific..Organs..to ..Protein.., MetaboJism

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Protein ..and.Amino Acid.. Requirements Protein. Reguirements

  1. Acid...Reguirements
  2. .of..Protelin...QuaM!lty Protein, and ..Amino..Ac.id...N.ecds ..in. Dis.ease Chapter.. References Se!ected Readings

Proteins are associated with all forms of life, and much of the effort to determine how life began has centered on how proteins were first produced. Amino acids joined together in long strings by peptide bonds form proteins, which twist and fold in three-dimensional space, producing centers to facilitate the biochemical reactions of life that either would run out of control or not run at all without them. Life could not have begun without enzymes, of which there are thousands of different types in the body. Proteins are prepared and secreted to act as cell-cell signals in the form of hormones and cytokines. Plasma proteins produced and secreted by the liver stabilize the blood by forming a solution of the appropriate viscosity and osmolarity. These secreted proteins also transport a variety of compounds through the blood.

The largest source of protein in higher animals is muscle. Through complex interactions, entire sheets of proteins slide back and forth to form the basis of muscle contraction and all aspects of our mobility. Muscle contraction provides for pumping oxygen and nutrients throughout the body, inhalation and exhalation in our lungs, and movement. Many of the underlying causes of noninfectious diseases are due to derangements in proteins. Molecular biology has provided much information about DNA and RNA, not so much to understand DNA per se, but to understand the purpose and function of the proteins that are translated from the genetic code.

Three major classes of substrates are used for energy: carbohydrate, fat, and protein. The amino acids in protein differ from the other two primary sources of dietary energy by inclusion of nitrogen (N) in their structures. Amino acids contain at least one N in the form of an amino group, and when amino acids are oxidized to CO 2 and water to produce energy, waste N is produced that must be eliminated. Conversely, when the body synthesizes amino acids, N must be available. The synthetic pathways of other N-containing compounds in the body usually require donation of N from amino acids or incorporation of amino acids per se into the compound being synthesized. Amino acids provide the N for DNA and RNA synthesis. Therefore, when we think of amino acid metabolism, we must think of N metabolism.

Protein and amino acids are also important to the energy metabolism of the body. As Cahill pointed out (1), protein is the second largest store of energy in the body after adipose tissue fat stores (T.a.b.l.§ 2.:.,1). Carbohydrate is stored as glycogen, and while important for short-term energy needs, has very limited capacity for meeting energy needs beyond a few hours. Amino acids from protein are converted to glucose by the process called gluconeogenesis, to provide a continuing supply of glucose after the glycogen is consumed during fasting. Yet, protein stores must be conserved for numerous critical roles in the body. Loss of more than about 30% of body protein results in such reduced muscle strength for breathing, reduced immune function, and reduced organ function that death results. Hence, the body must adapt to fasting by conserving protein, as is seen by a dramatic decrease in N excretion within the first week of starvation.

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Table 2.1 Body Composition of a Normal Man in Terms of Energy Components

Body protein is made up of 20 amino acids, each with different metabolic fates in the body, different activities in different metabolic pathways in different organs, and differing compositions in different proteins. When amino acids are released after absorption of dietary protein, the body makes a complex series of decisions concerning the fate of those amino acids: to oxidize them for energy, to incorporate them into proteins, or to use them in the formation of a number of other N-containing compounds. This chapter elucidates the complex pathways and roles amino acids play in the body, with a focus on nutrition. Since the inception of this book, this chapter has been authored by the late Hamish Munro, an excellent teacher who spent much of his life refining complex biochemical concepts into understandable terms. Professor Munro brought order into the apparently chaotic world of amino acid and protein metabolism through his classic four-volume series (2, 3, 4 and 5). Readers familiar with former versions of this chapter will find many of his views carried forward into the present chapter.

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