Nonproteins

The three levels of structure listed above are also useful categories for describing nonprotein polymers. Thus details of the microstructure of a chain is a description of the primary structure. The overall shape assumed by an individual molecule as a result of the rotation around individual bonds is the secondary structure. Structures that are locked in by chemical cross-links are tertiary structures. 

Acetamide has been used experimentally as a source of nonprotein nitrogen for sheep and dairy cattie (13). It does not appear to be toxic in amounts of about 2—3% of ration. Buffering the diet with dibasic acids serves to allow higher levels of intake because the ammonia Hberated in the digestive process is then scavenged. 

Crude protein = %nitrogen X 6.25. Does not accurately reflect true protein content. Algal cells may contain nonprotein nitrogen substances, eg, 4—6% nucleic acids, dry wt basis. 

The presence of many nonprotein amino acids has been reported in various living metaboUtes, such as in antibiotics, some other microbial products, and in nonproteinaceous substances of animals and plants (7). Plant amino acids (8) and seleno amino acids (9) have been reviewed. 

Gofactors. Frequendy proteins exist in their native state in association with other nonprotein molecules or cofactors, which are cmcial to their function. These may be simple metal ions, such as Fe " in hemerythrin or Ca " in calmodulin a heme group, as for the globins nucleotides, as for dehydrogenases, etc. 

Proteins and Meals. Nutritional properties of the oilseed protein meals and their derived products are deterrnined by the amino acid compositions, content of biologically active proteins, and various nonprotein constituents found in the defatted meals. Phytic acid (3), present as salts in all four meals, is beheved to interfere with dietary absorption of minerals such as 2inc, calcium, and iron (67) (see Food toxicants, naturally occurring Mineral nutrients). ... 

Many enzymes carry out their catalytic function relying solely on their protein structure. Many others require nonprotein components, called cofactors (Table 14.2). Cofactors may be metal ions or organic molecules referred to as coenzymes. Cofactors, because they are structurally less complex than proteins, tend to be stable to heat (incubation in a boiling water bath). Typically, proteins are denatured under such conditions. Many coenzymes are vitamins or contain vitamins as part of their structure. Usually coenzymes are actively involved in the catalytic reaction of the enzyme, often serving as intermediate carriers of functional groups in the conversion of substrates to products. In most cases, a coenzyme is firmly associated with its enzyme, perhaps even by covalent bonds, and it is difficult to... 

In addition to the twenty amino acids commonly found in proteins, two others—selenocysteine and pyrrolysine—are found in some organisms, and more than 700 nonprotein amino acids are also found in nature. y-Amino-butyric acid (GABA), for instance, is found in the brain and acts as a neurotransmitter homocysteine is found in blood and is linked to coronary heart disease and thyroxine is found in the thyroid gland, where it acts as a hormone. 

In addition to their protein part, most enzymes also contain a small nonprotein part called a cofactor. A cofactor can be either an inorganic ion, such as Zn2+, or a small organic molecule, called a coenzyme. A coenzyme is not a catalyst but is a reactant that undergoes chemical change during the reaction and... 

Lipids are naturally occurring organic molecules that have limited solubility in water and can be isolated from organisms by extraction with nonpolar organic solvents. Fats, oils, waxes, many vitamins and hormones, and most nonprotein cell-meznbrane components are examples. Note that this definition differs from the sort used for carbohydrates and proteins in that lipids are defined by a physical property (solubility) rather than by structure. Of the many kinds of lipids, we ll be concerned in this chapter only with a few triacvlglycerols, eicosanoids, terpenoids, and steroids. 

Cofactor (Section 26.10) A small nonprotein part of an enzyme that is necessary for biological activity. 

Figure 1. Structures of some nonprotein amino acids isolated from plants...

Azetidine-2-carboxylic acid, the lower homolog of proline, has been isolated from Convallaria majalis (lily of the valley) 40,44), Polygonatum officinalis (Solomon s seal) 153), and Polygonatum multiflorum 45). Fowden and Steward 47) surveyed plants from 56 genera for nitrogenous compounds and found azetidine-2 -carboxylic acid to be restricted to members of the Liliaceae. In some species it was identified in leaf, stem, and root but was more commonly found in the seed. In Polygonatum, azetidine-2-carboxylic acid accounted for 75% or more of the total nonprotein nitrogen in the rhizome 45). There was no evidence that it occurred as a constituent of protein. 

The kynurenine pathway accounts for most of the nonprotein tryptophan metabolism in most tissues. Several metabolites produced by this pathway have been shown... 

An IV fat emulsion contains soybean or safflower oil and a mixture of natural triglycerides, predominately unsaturated fatty acids. It is used in the prevention and treatment of essential fatty acid deficiency. It also provides nonprotein calories for those receiving TPN when calorie requirements cannot be met by glucose. Examples of intravenous fat emulsion include Intralipid 10% and 20%, Liposyn II 10% and 20%, and Liposyn III 10% and 20%. Fat emulsion is used as a source of calories and essential fatty acids for... 

In mammals, peptide hormones typically contain only the a-amino acids of proteins finked by standard peptide bonds. Other peptides may, however, contain nonprotein amino acids, derivatives of the protein amino acids, or amino acids finked by an atypical peptide bond. For example, the amino terminal glutamate of glutathione, which participates in protein folding and in the metabolism of xenobiotics (Chapter 53), is finked to cysteine by a non-a peptide bond (Figure 3—3). The amino terminal glutamate of thyrotropin-... 

Scientists initially approached structure-function relationships in proteins by separating them into classes based upon properties such as solubility, shape, or the presence of nonprotein groups. For example, the proteins that can be extracted from cells using solutions at physiologic pH and ionic strength are classified as soluble. Extraction of integral membrane proteins requires dissolution of the membrane with detergents. 

Many enzymes contain small nonprotein molecules and metal ions that participate directly in substrate binding or catalysis. Termed prosthetic groups, cofactors, and coenzymes, these extend the repertoire of catalytic capabilities beyond those afforded by the limited number of functional groups present on the aminoacyl side chains of peptides. 

From this brief review of marine vertebrate venoms, it is obvious that very few biochemical investigations have been done. The technology to study marine vertebrate venom components is available. There are simply not enough scientists interested enough to enter the field. The first task is to isolate the toxic principles and identify the amino acid sequences. Pharmacological investigation should be done on the purified toxic principle and not on the crude venom, which is a mixture of many proteins and nonproteins. 

Figure 20, Demonstration of disparity between nonprotein nitrogen and urea levels in a patient with terminal kidney disease complicated by liver involvement...

The enzymes are protein molecules having globular structure, as a rule. The molecular masses of the different enzymes have values between ten thousands and hundred thousands. The enzyme s active site, which, as a rule, consists of a nonproteinic organic compound containing metal ions of variable valency (iron, copper, molybdenum, etc.) is linked to the protein globule by covalent or hydrogen bonds. The catalytic action of the enzymes is due to electron transfer from these ions to the substrate. The protein part of the enzyme secures a suitable disposition of the substrate relative to the active site and is responsible for the high selectivity of catalytic action. 

Antibiotic effects of nonprotein amino acids have not been studied in much detail with soil microorganisms. The organs of legumes alone contain more than 300 nonproteinogenic amino acids, such as methylene-glutamine, common to all... 

Table 3 Susceptibility of Yeast Species and Bacterial Strains from Pea Seedlings Against the Heterocyclic Nonprotein Amino Acid P-I.soxylinonyl-Alanine (PlA) ...

Limit to 40 g/d or 0.5 g/kg/d whereas providing appropriate nonprotein calories... 

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