Enzymatic modification

All of the 20 amino acids have in common a central carbon atom (Co) to which are attached a hydrogen atom, an amino group (NH2), and a carboxyl group (COOH) (Figure 1.2a). What distinguishes one amino acid from another is the side chain attached to the Ca through its fourth valence. There are 20 different side chains specified by the genetic code others occur, in rare cases, as" the products of enzymatic modifications after translation. 

Before we leave the enzymatic modification of terpenoids, we should point out that enzymes are also employed to resolve racemic mixtures of terpenoids. The principles of Bus are similar to those employed in the resolution of racemic mixtures of amino acids (see Chapter 8). 

It is not only the activity that can be altered by incorporation of noncoded amino acids. Introduction of structures possessing certain chemical functions leads to the possibility of highly regioselective modification of enzymes. For example, selective enzymatic modification of cystein residues with compounds containing azide groups has led to the preparation of enzymes that could be selectively immobilized using click chemistry methods [99]. 

Very few post-translational modifications have been found on tropoelastin. However, hydroxylation of 25% of the proline residues is observed [10]. The enzymatic modification of proline to hydroxyproline (Hyp) is performed by prolyl hydroxylase [11]. The purpose of this hydroxylation remains unclear and it is even proposed that Hyps in tropoelastin are a by-product of collagen hydroxylation as this occurs in the same cellular compartment [8]. 

Enzymatic modification Some aminoglycoside Alteration of the molecule by... 

Sparrow, C.P., Parthasarathy, S. and Steinberg, D. (1988). Enzymatic modification of LDL by purified lipoxygenase plus phospholipase Ai mimics cell mediated oxidative modification. J. Lipid Res. 29, 745-733. 

Some form of chemical labeling process must be used regardless of whether the final oligo conjugate is created by enzymatic or strictly chemical means. If enzymatic modification is to be done, the initial label still must be incorporated into an individual nucleoside triphosphate, which then is polymerized into an existing oligonucleotide strand (Section 1, this chapter). Fortunately, many useful modified nucleoside triphosphates are now available from commercial sources, often eliminating the need for custom derivatization of individual nucleotides. 

Van Lenten, L., and Ashwell, G. (1971) Studies on the chemical and enzymatic modification of glycoproteins. A general method for the tritiation of sialic acid-containing glycoproteins. J. Biol. Chem. 46, 1889-1894. 

Soya Proteins. Early attempts to make albumen substitutes from soya protein also ran into problems. A bean flavour tended to appear in the finished product. A solution to these problems has been found. Whipping agents based on enzyme modified soy proteins are now available. The advantage of enzymatic modification is that by appropriate choice of enzymes the protein can be modified in a very controlled way. Chemical treatment would be far less specific. In making these materials the manufacturer has control of the substrate and the enzyme, allowing the final product to be almost made to order. The substrates used are oil-free soy flakes or flour or soy protein concentrate or isolate. The enzymes to use are chosen from a combination of pepsin, papain, ficin, trypsin or bacterial proteases. The substrate will be treated with one or more enzymes under carefully controlled conditions. The finished product is then spray dried. 

Table 7 includes many structurally unique nonprotein amino acids with heterocyclic units as part of the structure. These amino acids are isolated from various sources and exhibit interesting bioactivities. In most cases, enzymatic modification of proline occurs through installation of hydroxyl groups, halogenated side chains, and alkyl side chains to give rise to the observed structural diversity. 

Mode of action Activation of macrophages/monocytes release of endogenous mediators such as lipids from arachidonic acid, reduced oxygen species, proteins 1. Pore formation in cell membranes 2. Enzymatic modification of specific substrates in the cytosol of host cells (AB-type toxins) 3. Superantigen stimulation of the immune system... 

In addition to the 20 proteinogenic amino acids (see p. 60), there are also many more compounds of the same type in nature. These arise during metabolic reactions (A) or as a result of enzymatic modifications of amino acid residues in peptides or proteins (B). The biogenic amines (C) are synthesized from a-amino acids by decarboxylation. 

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