Plant peptides, examples

Lipid transfer proteins, a family of homologous antifungal peptides containing eight disulfide-linked cysteines produced by some plants. For example, onion seeds (Allium cepa. L) produce the lipid transfer peptide ACE-AMPi, which inhibited F. oxysporum [B. P. A. Cammune et al.. Plant Pathol. 1996, 109, 445]. 

Nicotinic acid is found in plants associated with both peptides and polysaccharides. For example in wheat bran, two forms are described a peptide with a molecular weight of approximately 12,000 and a carbohydrate complex that is called niacytin. Polysaccharides isolated from wheat bran have been found to contain 1.05% nicotinic acid in bound form. Hydrolysis yielded a fragment identified as P-3-O-nicotinoyl-D-glucose (25). 

Mammals, fungi, and higher plants produce a family of proteolytic enzymes known as aspartic proteases. These enzymes are active at acidic (or sometimes neutral) pH, and each possesses two aspartic acid residues at the active site. Aspartic proteases carry out a variety of functions (Table 16.3), including digestion pepsin and ehymosin), lysosomal protein degradation eathepsin D and E), and regulation of blood pressure renin is an aspartic protease involved in the production of an otensin, a hormone that stimulates smooth muscle contraction and reduces excretion of salts and fluid). The aspartic proteases display a variety of substrate specificities, but normally they are most active in the cleavage of peptide bonds between two hydrophobic amino acid residues. The preferred substrates of pepsin, for example, contain aromatic residues on both sides of the peptide bond to be cleaved. 

Although carotenogenesis in plants takes place in plastids, all of the carotenoid biosynthesis genes are nuclear encoded and their polypeptide products are imported into the plastids. Therefore, they contain a N-terminal transit peptide sequence. For example, the size of the transit peptide of PSY from ripe tomato fruit is approximately 9 kDa, corresponding to about 80 amino acid residues (Misawa et al, 1994). 

Polar organic compounds such as amino acids normally do not polymerize in water because of dipole-dipole interactions. However, polymerization of amino acids to peptides may occur on clay surfaces. For example, Degens and Metheja51 found kaolinite to serve as a catalyst for the polymerization of amino acids to peptides. In natural systems, Cu2+ is not very likely to exist in significant concentrations. However, Fe3+ may be present in the deep-well environment in sufficient amounts to enhance the adsorption of phenol, benzene, and related aromatics. Wastes from resinmanufacturing facilities, food-processing plants, pharmaceutical plants, and other types of chemical plants occasionally contain resin-like materials that may polymerize to form solids at deep-well-injection pressures and temperatures. 

Many enzymes have absolute specificity for a substrate and will not attack the molecules with common structural features. The enzyme aspartase, found in many plants and bacteria, is such an enzyme [57], It catalyzes the formation of L-aspartate by reversible addition of ammonia to the double bond of fumaric acid. Aspartase, however, does not take part in the addition of ammonia to any other unsaturated acid requiring specific optical and geometrical characteristics. At the other end of the spectrum are enzymes which do not have specificity for a given substrate and act on many molecules with similar structural characteristics. A good example is the enzyme chymotrypsin, which catalyzes hydrolysis of many different peptides or polypeptides as well as amides and esters. 

Ferritins have been found in a wide range of species, and sequence data - some, as in the first ever sequence of horse spleen apoferritin (Heusterspreute and Crichton, 1981) determined by direct methods, but many now by DNa sequencing 1, have been deposited for more than 70 ferritins. They vary in length from 154-185 residues per subunit. Some ferritins have N-terminal extensions which lie on the outside of the assembled shell and target the ferritin to a specific destination such as plastids in plants and yolk sac in snails (Andrews etah, 1992 Lobreaux etah, 1992). For example, pea ferritin is synthesized with an N-terminal extension of 75 residues, which is missing from the mature protein. The first part of this extension is a chloroplast-targetting sequence of 47 residues, which is lost on entry into the plastid. The second part, an extension peptide, is lost prior to assembly of the... 

This example belongs to chemotaxonomy, a discipline that tries to classify and identify organisms (usually plants, but also bacteria, and even insects) by the chemical or biochemical composition (e.g., fingerprint of concentrations of terpenes, phenolic compounds, fatty acids, peptides, or pyrolysis products) (Harbome and Turner 1984 Reynolds 2007 Waterman 2007). Data evaluation in this field is often performed by multivariate techniques. 

Figure 1 Examples of functions of different classes of plant defense peptides.

The arsenal of plant defense peptides contains members capable of binding carbohydrate residues, namely /31-4 linked A -acetyl glucosamine residues that form the biopolymer chitin. The actual mode of action remains unclear. Antifungal and antimicrobial activity has been shown in vitro. For example Ac-AMP2 is a small disulfide-rich chitin-binding peptide isolated from the seeds of Amaranthus caudatus with antimicrobial activity. It differs from Ac-AMP 1 by one additional arginine residue at the C-terminus. The structure was determined by NMR and contains a cystine knot motif. Ac-AMP2 displays a so-called hevein domain partly... 

Some of the peptide families here have been successfully expressed in standard laboratory bacterial or yeast expression systems. This approach allows the production of larger amounts of peptides not amenable to chemical synthesis and opens the potential of large-scale production for biotechnological uses, for example, as plant protection agent applied externally. 

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