Sugar-amino acid mechanism

When melanoidins are formed, many factors come into play. Here, several aspects will be considered (cfi ref. 466) differences between sugars and between amino acids, the molar ratio of amino acid to sugar, use of peptides and of hydrolysates, sugar-amino acid mixtures versus MRPs, properties of MRPs, and the mechanisms of antioxidant activity. 

By examination of the recent literature as reviewed by Tissot and Welte (1978), one gets the impression that the diagenesis of humic substances to form kerogen is well understood. In effect, many consider kerogen to be formed via condensation mechanisms in which aquatic plant substances are microbially degraded to form soluble monomers that condense to form humic polymers that eventually condense to form kerogen. The melanoidin pathway (sugar-amino acid condensation products) has been invoked by some to explain the structures formed (Nissenbaum and Kaplan, 1972 Hue and Durand, 1973, 1977 Welte, 1973 Nissenbaum, 1974 Stuermer et al. 1978 Tissot and Welte, 1978). 

Reverse Osmosis Asymmetric membrane with homogeneous skin and microporous sub- or support structure Hydrostatic pressure 10-100 bar Solut ion-d if fusion mechanism, solubility, and dif-fusivity of individual components in the homogeneous polymer matrix determine separation characteristics. Concentration of microsolutes, such as salts, sugars, amino acids, etc., recovery of water from microbiological processes. 

It does not seem that we have gained overwhelming new evidence on mechanisms for transport of excreted compounds across algal membranes during the last 10-20 years. We still think that small molecules like sugars, amino acids, organic acids and fatty acids mainly diffuse through the cell membrane [34]. Bjornsen [42] made calculations of exudation based on assumptions that... 

The mechanism of the entry of fructose into the Maillard reaction (a series of sugar/amino acid processes in vivo) has been studied by DFT the order of reactivity for the isomers is predicted as a- > /3- > open-chain. Heyns rearrangement products are most favourable under basic conditions, possible under neutral conditions, but unfeasible at or below glycine s isoelectric point. Kinetic and activation parameters have been reported for the corresponding glucose/proline reaction. ... 

In its role as a regulatory mechanism, the plasma membrane also controls the exchange of such metabolites as sugars, amino acids, and ions between the cell interior and extracellular fluids. Transport of such metabolites is thought to be mediated in large part by some carrier mechanism. Specific sites for carriers and pores for the exchange of metabolites have been postulated but have not been identified. Transport systems for sugars, amino acids, and ions will be discussed later in this chapter. 

In a less straightforward way, D-glucose (393) underwent a Maillard-type reaction with an excess of glycine (394) under microwave irradiation to afford 5-hydroxy-l,3-dimethyl-2(l/f)-quinoxahnone (395) as a major product, Repetition with labeled reactants suggested that the product contained six carbon atoms from the sugar and four from the amino acid on this evidence, a detailed mechanism has been postulated. 

As the name implies, the odor of urine in maple syrup urine disease (brancbed-chain ketonuria) suggests maple symp or burnt sugar. The biochemical defect involves the a-keto acid decarboxylase complex (reaction 2, Figure 30-19). Plasma and urinary levels of leucine, isoleucine, valine, a-keto acids, and a-hydroxy acids (reduced a-keto acids) are elevated. The mechanism of toxicity is unknown. Early diagnosis, especially prior to 1 week of age, employs enzymatic analysis. Prompt replacement of dietary protein by an amino acid mixture that lacks leucine, isoleucine, and valine averts brain damage and early mortality. 

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