To study the effects of phosphoenolpyruvate (PEP) and Mg2+ on the activity of the non-phosphorylated and phosphorylated forms of phosphoenolpyruvate carboxylase (PEPC) from Zea mays leaves, steady-state measurements have been carried out with the free forms of PEP (fPEP) and Mg2+ (fMg2+), both in a near-physiological concentration range. At pH 7.3, in the absence of activators, the initial velocity data obtained with both forms of the enzyme are consistent with the exclusive binding of MgPEP to the active site and of fPEP to an activating allosteric site. At pH 8.3, and in the presence of saturating concentrations of glucose 6-phosphate (Glc6P) or Gly, the free species also combined with the active site in the free enzyme, but with dissociation constants at least 35-fold that estimated for MgPEP. The latter dissociation constant was lowered to the same extent by saturating Glc6P and Gly, to approx. one-tenth and one-sixteenth in the non-phosphorylated and phosphorylated enzymes respectively. When Glc6P is present, fPEP binds to the active site in the free enzyme better than fMg2+, whereas the metal ion binds better in the presence of Gly. Saturation of the enzyme with Glc6P abolished the activation by fPEP, consistent with a common binding site, whereas saturation with Gly increased the affinity of the allosteric site for fPEP. Under all the conditions tested, our results suggest that fPEP is not able to combine with the allosteric site in the free enzyme, i.e. it cannot combine until after MgPEP, fPEP or fMg2+ are bound at the active site. The physiological role of Mg2+ in the regulation of the enzyme is only that of a substrate, mainly as part of the MgPEP complex. The kinetic properties of maize leaf PEPC reported here are consistent with the enzyme being well below saturation under the physiological concentrations of fMg2+ and PEP, particularly during the dark period; it is therefore suggested that the basal PEPC activity in vivo is very low, but highly responsive to even small changes in the intracellular concentration of its substrate and effectors.
López-Valentín, D. M. y Guerra-Ramos, M. T. (2013). Análisis de las actividades de aprendizaje incluidas en libros de texto de ciencias naturales para educación primaria utilizados en México, Enseñanza de las Ciencias, 31(2), pp. 173-191. análisis de las actividades de aprendizaje incluidas en libros de texto de ciencias naturales para educación primaria utilizados en méxico analYsis oF tHe learninG activities included in natural sciences textbooKs For primarY education in mexico
We propose a classroom experiment to complement a classroom activity described by Ellis et al. in this Journal. In the Ellis activity students extracted long polymer fibers from the cells of common foods. In this experiment, the identity of the long polymer fibers as either DNA or protein can be determined by doing three accessible, inexpensive, and easy tests. Two of the tests, temperature and the acid-medium effect, are based on the physicochemical properties of DNA (reversible denaturalization), and the third test is a qualitative determination of proteins (absent in DNA). The results of these three tests provide evidence to distinguish the isolated molecule of DNA from proteins that might look like DNA.KEYWORDS: First-Year Undergraduate/General, High School/Introductory Chemistry, Biochemistry, Physical Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Food Science Nucleic Acids/DNA/RNA, Proteins/Peptides E llis et al.1 described a classroom activity that illustrates DNA extraction from animal and plant tissues. In this activity, students extract DNA from the cells of common foods (beef, liver, and onions) in three steps: (i) breaking the cell membrane with mechanical crushing and by the addition of a buffered detergent solution, (ii) separating and removing the cell debris by centrifugation, and (iii) precipitating the DNA by adding cold ethanol. Students obtain long polymer fibers of DNA forming a fluff-like layer in the last step. What is the evidence that supports that the long polymer fibers are DNA? How can we prove that those white fluffs are DNA without using a spectrophotometer? Students may wonder about the presence of proteins in the extract or even think that the precipitate is protein instead of DNA, so how can we distinguish DNA from proteins?Proteins are the most abundant biological macromolecules, occurring in all cells and in all parts of cells. Proteins are polymers of amino acids, with each amino acid residue joined to its neighbor by a specific type of covalent bond, the peptide bond. Proteins can be easily broken down (hydrolyzed) to their constituent amino acids by a variety of methods, so that the earliest studies of proteins focused on the free amino acids derived from them. Twenty different amino acids are commonly found in proteins and they have a carboxyl (−COOH) group and one amino (−NH 2 ) group attached to the same carbon atom (the α-atom). Amino acids differ from one another in their side chains, or R groups, which vary in structure, size, and electrical charge. Proteins are described in terms of four levels of organization; primary, secondary, tertiary, and quaternary. A description of all covalent bonds (mainly peptide bonds and disulfide bonds) linking amino acid residues in a polypeptide chain defines its primary structure. The most important element of the primary structure is the sequence of amino acid residues. The secondary structure refers to particularly stable arrangements of amino acid residues giving rise to recurring structural patterns...
Este estudio propone la implementación de una secuencia didáctica basada en el modelo de aprendizaje como investigación orientada, para la enseñanza del concepto de elemento químico en la educación secundaria. Su diseño se ha basado en las ideas previas de los estudiantes y en el análisis histórico sobre este concepto. La evaluación se ha realizado al comparar grupos de estudiantes que han seguido esta secuencia didáctica con grupos de estudiantes que han mantenido una enseñanza tradicional. Los resultados revelan que los estudiantes de los grupos experimentales respecto a los grupos control: 1) presentan un mejor manejo en la definición procedimental de sustancia en el nivel macroscópico; 2) definen mejor el concepto de elemento químico en el marco de la teoría daltoniana; 3) distinguen mejor los conceptos de sustancia simple y elemento químico; y 4) presentan una mejor distinción del concepto microscópico de mezcla —de sustancias simples o de sustancias compuestas— cuando es utilizada una secuencia didáctica de orientación constructivista.
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