Basic Building Blocks The Amino Acids

The 20 amino acids (AA) used for protein synthesis are the proteinogenic amino acids (A). They are exclusively Lamino acids; their D-enantiomers are biologically inactive. They can be divided into four categories, according to the chemical nature of their R-groups (exception: glycine). AA with nonpolar side chains consist of a straight or branched hydrocarbon chain, or—in methionine—of a thioether group. They make up the hydrophobic core of proteins or are found in those protein sites that touch membrane lip-ids. Polar AA can engage in hydrogen bonding and other bonds and thus stabilize or contribute to the tertiary structure. The acidic and basic AA are usually dissociated at physiological pHs and are, therefore, able to form ionic bonds. Results of recent research challenge the classic distinction between essential and nonessential AA. Of the eight AA termed "essential," six can be synthesized endogenously from the respective keto acids—they are, therefore, not truly essential. Only lysine (Lys) and threo-nine (Thr) are truly essential since their transamination is irreversible. The other AA used to be considered simply as nonspecific N donors. However, some of them may become temporarily, or at least conditionally, essential under specific pathological conditions.

Consensus exists that histidine (His) is essential in children as well as in case of chronic renal failure. In case of prolonged His deficiency, plasma His levels drop even in healthy people and normalize after His supplementation. His must, therefore, be essential to a certain degree even in healthy adults.

Tyrosine (Tyr) is synthesized endogenously from the essential AA phenylalanine (Phe). In neonates, especially in preemies, endogenous Tyr synthesis is insufficient. Tyr becomes essential in all diseases affecting the liver enzyme phe-nylalanine hydroxylase (PAH). The classic example is phenylketonuria (PKU) but sepsis and cirrhotic diseases should also be mentioned here. The liver of adults can make cysteine (Cys) from methionine (Met). Patients with homocysteinuria or liver cirrhosis, preemies, and neonates have insufficient or are completely lacking endogenous Cys synthesis. Serine (Ser) is synthesized from glycine (Gly) and formaldehyde. Renal disorders impair sufficient endogenous Ser synthesis.

Nitrogen monoxide (NO) with its numerous effects on the vascular, nervous, and immune systems is derived from arginine (Arg, see p. 128). This suggests that arginine supplementation may have positive effects on certain severe pathologies. Quantitatively, glutamine (Gln) is the body's most important nonessential N source. There is substantial research indicating that in case of trauma, intestinal disorders etc., Gln requirements exceed endogenous synthesis.

|- A. Proteinogenic Amino Acids

COO I

Alanine Ala (A)

COO"

Isoleucine

CHw CHw

Methionine Met (M)

CH I

Valine Val (V)

Proline Pro (P)

Aspartic acid Asp (D)

Leucine Leu (L)

COO

COO

i

H,N+ C H ^ 1

H3N+—C—H

CH2

CH2

CH2

C=CH

SH

NH

u

o

OH

Tryptophan

Cysteine

Tyrosine

Trp (W)

Cys(C)

Tyr (Y)

COO

COO

COO

h,n+—c—h

H,N+ C H

H,N+—C—H

ch2oh

CHOH

ch2oh

Phenylalanine Phe (F)

Serine Ser(S)

Threonine Thr (T)

Glycine Gly (G)

COO I

CHw_ COO

Glutamic acid Glu (E)

NH I

H,N NH2

Arginine Arg (R)

CHw CHw CHw CHw

Lysine Lys (K)

H2N O

Asparagine Asn (N)

H2N XO

Glutamine Gln (Q)

Histidine His (H)

Nonessential

Essential

Conditionally essential

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