Cutin structure

Cutin. Structural component of the outer lipophilic protective layer (cuticle) of the aerial parts of plants, especially leaves. Suberin serves similar functions in roots and bark. C. is a natural polyester, formed enzymatically from hydroxyfatty acids with 16 and 18 C atoms. o+Hydroxy- and dihydroxyfatty acids, e.g., 10,16-dihydroxypalmitic acid, as well as epoxy- and oxofatty acids, and a,o>-dicarboxylic acids are the main components of cutin. Cutinases (C.-cleaving enzymes) occur especially in pollen and in plant-pathogenic fungi, e.g., Fusarium solani (while rot in potatoes). 

In the cutin structure, a polyester intramolecular structure exists where cross-linking is mainly influenced by the availability of secondary hydroxyl groups. Thus cutins which contain large amounts of epoxy, 0X0 and cy-hydroxy monomers must be predominantly linear (Table 2.7) (Deas and Holloway, 1977). Esterification appears to occur chiefly through... 

Plants were probably the first to have polyester outerwear, as the aerial parts of higher plants are covered with a cuticle whose structural component is a polyester called cutin. Even plants that live under water in the oceans, such as Zoestra marina, are covered with cutin. This lipid-derived polyester covering is unique to plants, as animals use carbohydrate or protein polymers as their outer covering. Cutin, the insoluble cuticular polymer of plants, is composed of inter-esterified hydroxy and hydroxy epoxy fatty acids derived from the common cellular fatty acids and is attached to the outer epidermal layer of cells by a pectinaceous layer (Fig. 1). The insoluble polymer is embedded in a complex mixture of soluble lipids collectively called waxes [1], Electron microscopic examination of the cuticle usually shows an amorphous appearance but in some plants the cuticle has a lamellar appearance (Fig. 2). 

Fig. 4. Structure of the most common major monomers of cutin...

Fig. 5. Models showing the type of structures present in the polymers cutin (top) and sube-rin (bottom)...

Fig. 6. Proposed chemical structures of isolated soluble products of lime cutin depolymerization with TMSil (bottom) and pancreatic lipase (top)...

The major function of cutin is to serve as the structural component of the outer barrier of plants. As the major component of the cuticle it plays a major role in the interaction of the plant with its environment. Development of the cuticle is thought to be responsible for the ability of plants to move onto land where the cuticle limits diffusion of moisture and thus prevents desiccation [141]. The plant cuticle controls the exchange of matter between leaf and atmosphere. The transport properties of the cuticle strongly influences the loss of water and solutes from the leaf interior as well as uptake of nonvolatile chemicals from the atmosphere to the leaf surface. In the absence of stomata the cuticle controls gas exchange. The cuticle as a transport-limiting barrier is important in its physiological and ecological functions. The diffusion across plant cuticle follows basic laws of passive diffusion across lipophylic membranes [142]. Isolated cuticular membranes have been used to study this permeability and the results obtained appear to be valid... 

Kolattukudy PE (1980) Cutin, suberin and waxes. In Stumpf PK (ed) The biochemistry of plants vol 4 - lipids structure and function. Academic Press, London, p 571... 

Kolattukudy PE (1981) Structure, biosynthesis and biodegradation of cutin and suberin. In Briggs WR (ed) Annual reviews of plant physiol, vol. 32. Annual Reviews, Palo Alto CA, p 539... 

Deas AHB, Holloway PJ (1977) The intermolecular structure of some plant cutins. In Tevini M, Lichtenthaler HK (eds) Lipids and lipid polymers in higher plants. Springer, Berlin Heidelberg New York, p 293... 

Due to the structure of the corn kernel (cutinized outer layer of the pericarp surrounding the corn kernel), the diffusion of water and chemicals inside the kernel is through a very specific pathway. Initial results with the use of enzymes during steeping (Figure 1) indicated that enzymes were not able to penetrate the kernels and break down the protein matrix surrounding starch particles. For enzymes to penetrate the corn kernel, it was necessary... 

High-resolution 13C NMR studies have been conducted on intact cuticles from limes, suberized cell walls from potatoes, and insoluble residues that remain after chemical depolymerization treatments of these materials. Identification and quantitation of the major functional moieties in cutin and suberin have been accomplished with cross-polarization magic-angle spinning as well as direct polarization methods. Evidence for polyester crosslinks and details of the interactions among polyester, wax, and cell-wall components have come from a variety of spin-relaxation measurements. Structural models for these protective plant biopolymers have been evaluated in light of the NMR results. 

Cutin-Wax Interactions. In order to obtain a more complete structural picture of plant cuticle, 13C CPMAS data were also obtained for the polymeric assembly prior to removal of waxes (Figure 5). A second (CH2)n peak appeared in the spectrum, and additional signal intensity in the carboxyl region produced a single broadened peak. Bulk methylene carbons from cutin and wax components exhibited identical values of Tip(H), indicating that they were mixed intimately and shared a common 1H spin reservoir... 

Solid-state 13C NMR was employed to characterize intact samples of cutin and suberin biopolyesters. Although a considerable degree of structural heterogeneity was observed for both materials, it was possible nonetheless to resolve and assign many NMR peaks, even when the polyesters were accompanied by waxes or cell walls. Quantitative estimates for the various aliphatic, aromatic, and carbonyl carbon types indicated that cutin was primarily aliphatic in composition, whereas suberin had more aromatic and olefinic moieties. Additional analysis should be facilitated by the biosynthetic incorporation of selectively 13C-enriched precursors (26,27). 

For the study of complex cuticular mixtures, measurements of cross-polarization dynamics proved to be especially informative. The equality of Ti/>(H) values in cutin-wax assemblies demonstrated that these cuticular materials were mixed intimately. By contrast, Ti >(H) measurements showed that the polymeric components of suberized cell walls were present in distinct domains, suggesting that suberin was attached at a few structural sites rather than being embedded in the polysaccharide wall. 

Preliminary structural studies of cutin and suberin breakdown involved examination of 13C NMR spectra for insoluble residues that were resistant to chemical depolymerization. In cutin samples, flexible CH2 moieties in particular were removed by such treatments, but CHOCOR crosslinks and polysaccharide impurities were retained preferentially. A concomitant narrowing of NMR spectral lines suggested that the treatments produced more homogeneous polyester structures in both cases. Our current studies of cu-ticular breakdown also employ selective depolymerization strategies with appropriate enzymes (1,28). 

These results demonstrated the usefulness of 13C NMR in studies of molecular structure and dynamics for the polymeric constituents of plant cuticle. Although these materials are insoluble and sometimes present as interpenetrating phases, CPMAS and spin relaxation techniques helped identify important carbon types and provided structural clues to the protective functions of cutin and suberin in terrestrial plants. 

In addition to water and inorganic solids (salts dissolved in cell fluids, shells, and bones), organisms consist of a mix of organic substances. Some of these are macromolecules (e.g., globular proteins, cellulose). Some combine to form subcellular and tissue structures built with combinations of lipids, proteins, carbohydrates, and some specialized polymers like cutin or lignin (Fig. 10.2). These diverse organic materials cause organisms to have diverse macromolecular, cellular, and tissue portions that may be apolar, monopolar, and/or bipolar. 

---