1.1 El equipo tendrá 1 año mínimo de garantía
LISTA DE BIENES Y PLAN DE ENTREGA
MOST OF THEENZYMESAREPROTEINS
Proteins are susceptible to heat, strong acids and bases, heavy metals, and detergents. They are hydro- lyzed to amino acids by heating in acidic solution and by the proteolytic action of enzymes on peptide bonds. Enzymes give positive results on typical pro- tein tests, such as the Biuret, Millions, Hopkins- Cole, and Sakaguchi reactions. X-ray crystallo- graphic studies revealed that there are peptide bonds between adjacent amino acid residues in proteins. The majority of the enzymes fulfill the above criteri- on; therefore, they are proteins in nature. However, the catalytic element of some well-known rib- ozymes is just RNAs in nature (Steitz and Moore 2003, Raj and Liu 2003).
CHEMICALCOMPOSITION OFENZYMES
For many enzymes, protein is not the only compo- nent required for its full activity. On the basis of the chemical composition of enzymes, they are catego- rized into several groups, as follows:
1. Polypeptide, the only component, for example, lysozyme, trypsin, chymotrypsin, or papain; 2. Polypeptide plus one to two kinds of metal
ions, for example, -amylase containing Ca2,
kinase containing Mg2, and superoxide dis-
mutase having Cu2and/or Zn2;
3. Polypeptide plus a prosthetic group, for exam- ple, peroxidase containing a heme group; 4. Polypeptide plus a prosthetic group and a metal
ion, for example, cytochome oxidase (aa3)
containing a heme group and Cu2; 5. Polypeptide plus a coenzyme, for example,
many dehydrogenases containing NADor
NADP;
6. Combination of polypeptide, coenzyme, and a metal ion, for example, succinate dehydroge- nase containing both the FAD and nonheme iron.
ENZYMESARESPECIFIC
In the life cycle of a unicellular organism, thousands of reactions are carried out. For the multicellular higher organisms with tissues and organs, even more kinds of reactions are progressing in every moment. Less than 1% of errors that occur in these reactions will cause accumulation of waste materials (Drake 1999), and sooner or later the organism will not be able to tolerate these accumulated waste materials. These phenomena can be explained by examining ge- netic diseases; for example, Phenylketonuria (PKU) in humans, where the malfunction of phenylalanine hydroxylase leads to an accumulation of metabolites such as phenylalanine and phenylacetate, and oth- ers, finally causing death (Scriver 1995, Waters 2003). Therefore, enzymes catalyzing the reactions bear the responsibility for producing desired metab- olites and keeping the metabolism going smoothly.
Enzymes have different types of specificities. They can be grouped into the following common types:
1. Absolute specificity. For example, urease (Sirko and Brodzik 2000) and carbonate anhydrase
(Khalifah 2003) catalyze only the hydrolysis and cleavage of urea and carbonic acid, respec- tively. Those enzymes having small molecules as substrates or that work on biosynthesis path- ways belong to this category.
2. Group specificity. For example, hexokinase prefers D-glucose, but it also acts on other hex- oses (Cardenas et al. 1998).
3. Stereo specificity. For example, D-amino acid oxidase reacts only with D-amino acid, but not with L-amino acid; and succinate dehydroge- nase reacts only with the fumarate (trans form), but not with maleate (cis form).
4. Bond specificity. For example, many digestive enzymes. They catalyze the hydrolysis of large molecules of food components, such as pro- teases, amylases, and lipases. They seem to have broad specificity in their substrates; for example, trypsin acts on all kinds of denatured proteins in the intestine, but prefers the basic amino acid residues at the C-terminal of the peptide bond. This broad specificity brings up an economic effect in organisms: they are not required to produce many digestive enzymes for all kinds of food components.
The following two less common types of enzyme specificity are impressive: (1) An unchiral com- pound, citric acid is formed by citrate synthase with the condensation of oxaloacetate and acetyl-CoA. An aconitase acts only on the moiety that comes from oxaloacetate. This phenomenon shows that, for aconitase, the citric acid acts as a chiral compound (Barry 1997). (2) If people notice the rare mutation that happens naturally, they will be impressed by the fidelity of certain enzymes, for example, amino acyl- tRNA synthethase (Burbaum and Schimmel 1991, Cusack 1997), RNA polymerase (Kornberg 1996), and DNA polymerase (Goodman 1997); they even correct the accidents that occur during catalyzing reactions.
ENZYMESAREREGULATED
The catalytic activity of enzymes is changeable under different conditions. These enzymes catalyze the set steps of a metabolic pathway, and their activ- ities are responsible for the states of cells. Inter- mediate metabolites serve as modulators on enzyme activity; for example, at high concentrations of ADP,
the catalytic activity of the phosphofructokinase, pyruvate kinase, and pyruvate dehydrogenase com- plex are enhanced; as [ATP] becomes high, they will be inhibited. These modulators bind at allosteric sites on enzymes (Hammes 2002). The structure and mechanism of allosteric regulatory enzymes such as aspartate transcarbamylase (Cherfils et al. 1990) and ribonucleoside diphosphate reductase (Scott et al. 2001) have been well studied. However, allosteric regulation is not the only way that the enzymatic activity is influenced. Covalent modification by pro- tein kinases (Langfort et al. 1998) and phosphatases (Luan 2003) on enzymes will cause large fluctua- tions in total enzymatic activity in the metabolism pool. In addition, sophisticated tuning phenomena on glycogen phosphorylase and glycogen synthase through phosphorylation and dephosphorylation have been observed after hormone signaling (Nuttall et al. 1988, Preiss and Romeo 1994).
ENZYMESAREPOWERFULCATALYSTS
The compound glucose will remain in a bottle for years without any detectable changes. However, when glucose is applied in a minimal medium as the only carbon source for the growth of Salmonella
typhimurium(or Escherichia coli), phosphorylation of this molecule to glucose-6-phosphate is the first chemical reaction that occurs as it enters the cells. From then on, the activated glucose not only serves as a fuel compound to be oxidized to produce chem- ical energy, but also goes through numerous reac- tions to become the carbon skeleton of various micro- and macrobiomolecules. To obtain each end product, multiple steps have to be carried out. It may take less than 30 minutes for a generation to go through all the reactions. Only the existence of en- zymes guarantees this quick utilization and disap- pearance of glucose.