Enzymes are biomolecules that catalyze (i.e., increase the rates of) chemical reactions. Nearly all known enzymes are proteins. However, certain RNA molecules can be effective biocatalysts too. These RNA molecules have come to be known as ribozymes. In enzymatic reactions, the molecules at the beginning of the process are called substrates, and the enzyme converts them into different molecules, called the products. Almost all processes in a biological cell need enzymes to occur at significant rates. Since enzymes are selective for their substrates and speed up only a few reactions from among many possibilities, the set of enzymes made in a cell determines which metabolic pathways occur in that cell. Like all catalysts, enzymes work by lowering the activation energy (Ea or ?Gâ€¡) for a reaction, thus dramatically increasing the rate of the reaction. Most enzyme reaction rates are millions of times faster than those of comparable un-catalyzed reactions. As with all catalysts, enzymes are not consumed by the reactions they catalyze, nor do they alter the equilibrium of these reactions. However, enzymes do differ from most other catalysts by being much more specific. Enzymes are known to catalyze about 4,000 biochemical reactions. A few RNA molecules called ribozymes catalyze reactions, with an important example being some parts of the ribosome. Synthetic molecules called artificial enzymes also display enzyme-like catalysis. Enzyme activity can be affected by other molecules. Inhibitors are molecules that decrease enzyme activity; activators are molecules that increase activity. Many drugs and poisons are enzyme inhibitors. Activity is also affected by temperature, chemical environment (e.g., pH), and the concentration of substrate. Some enzymes are used commercially, for example, in the synthesis of antibiotics. In addition, some household products use enzymes to speed up biochemical reactions (e.g., enzymes in biological washing powders break down protein or fat stains on clothes; enzymes in meat tenderizers break down proteins, making the meat easier to chew). Kashrut (also kashruth or kashrus, ??????????) is the set of Jewish dietary laws. Food in accord with halakha (Jewish law) is termed kosher in English, from the Ashkenazi pronunciation of the Hebrew term kashÃ©r (???????), meaning "fit" (in this context, fit for consumption by Jews according to traditional Jewish law). Jews who keep kashrut may not consume non-kosher food, but there are no restrictions on non-dietary use of non-kosher products, for example, injection of insulin of porcine origin. Food that is not in accordance with Jewish law is called treif (Yiddish: ???? or treyf, derived from Hebrew: ???????? trefÃ¡h). In the technical sense, treif means "torn" and refers to meat which comes from an animal killed by another animal, killed with a dull knife (so that it felt pain) or having a defect that renders it unfit for slaughter. Many of the basic laws of kashrut are derived from the Torah's Books of Leviticus and Deuteronomy, with their details set down in the oral law (the Mishnah and the Talmud) and codified by the Shulchan Aruch and later rabbinical authorities. The Torah does not explicitly state the reason for most kashrut laws, and many varied reasons have been offered for these laws, ranging from philosophical and ritualistic, to practical and hygienic. Islam has a related but different system, named halal, and both systems have a comparable system of ritual slaughter (shechita in Judaism and ?abi?ah in Islam). The Seventh-day Adventist Church, well known for their health message, expects adherence to the kosher laws, which they refer to as clean foods. Adventists believe that adherence to the laws is not only healthy, but also keeps the body, the metaphorical temple, clean. Many members practice vegetarianism and veganism.