proteolysis X.pro-te-'a-lo-sosX Process in which a protein is broken down partially, into peptides, or completely, into amino acids, by proteolytic enzymes, present in bacteria and in plants but most abundant in animals. Proteins in food are attacked in the stomach by pepsin and in the small intestine mainly by trypsin and chymotrypsin from the pancreas. Pro¬ teolytic enzymes are secreted as zymogens, which are themselves con¬ verted by proteolysis to their active forms. Many other zymogens or precursors undergo proteolysis to form active enzymes or proteins (e.g., fibrinogen to fibrin). In cells, proteolytic degradation of old proteins is part of cellular maintenance.
Proterozoic V.pra-to-ro-'zo-ik, .pro-ts-ro-'zo-ikN Eon Younger of the two divisions of Precambrian time, from 2.5 billion to 543 million years ago. Proterozoic rocks have been identified on all the continents and often constitute important sources of metallic ores, notably of iron, gold, cop¬ per, uranium, and nickel. The many small protocontinents that had formed during early Precambrian time had coalesced into one or several large landmasses by the beginning of the Proterozoic. Rocks of the Proterozoic contain many traces of primitive life forms, such as the fossil remains of bacteria and blue-green algae (see cyanobacteria).
Protestant ethic Value attached to hard work, thrift, and self-discipline under certain Protestant doctrines, particularly those of Calvinism. Max Weber, in The Protestant Ethic and the Spirit of Capitalism (1904-05), held that the Protestant ethic was an important factor in the economic success of Protestant groups in the early stages of European capitalism, in that worldly success came to be interpreted as a sign of the individual’s election to eternal salvation. Weber’s thesis was variously criticized and expanded throughout the 20th century. See also Protestantism; Richard H. Tawney.
Protestantism One of the three major branches of Christianity, origi¬ nating in the 16th-century Reformation. The term applies to the beliefs of Christians who do not adhere to Roman Catholicism or Eastern Orthodoxy. A variety of Protestant denominations grew out of the Reformation. The
followers of Martin Luther established the evangelical churches of Ger¬ many and Scandinavia; John Calvin and more radical reformers such as Huldrych Zwingli founded Reformed churches in Switzerland, and Calvin’s disciple John Knox established a church in Scotland (Presbyterianism). Another important branch of Protestantism, represented by the Church of England and Episcopal Church, had its origins in 16th-century England and is now the Protestant denomination closest to Roman Catholicism in the¬ ology and worship. The doctrines of the various Protestant denominations vary considerably, but all emphasize the supremacy of the Bible in mat¬ ters of faith and order, justification by grace through faith and not through works, and the priesthood of all believers. In the early 21st century there were nearly 350 million Protestants in the world. See also Adventist, Bap¬ tist, Society of Friends, Mennonite, Methodism.
Proteus \'pro-te-3s\ In Greek mythology, the prophetic old man of the sea and the shepherd of sea animals such as seals. He was subject to Poseidon. He knew all things—past, present, and future—but disliked telling what he knew. Those who wanted information from him had to catch him sleeping and bind him. He would try to escape by changing his form, but if a captor held him fast he gave the wished-for answer and plunged into the sea.
prothrombin \pro-'thram-bin\ Carbohydrate-protein compound in plasma essential to coagulation. In response to bleeding, a complex series of clotting-factor interactions leads to its conversion by thromboplastin to thrombin, which transforms fibrinogen in plasma into fibrin. Fibrin and platelets combine to form a clot. Hemophilia is caused by a hereditary lack of one of the clotting factors. Vitamin K is needed to synthesize prothrom¬ bin, so conditions that impair the vitamin’s absorption result in prothrom¬ bin deficiency and a tendency to prolonged bleeding.
protist \'pro-t9St\ Any member of a kingdom (Protista) of diverse eukary¬ otes, including algae, protozoans, and lower fungi (see fungus). Most are single-celled organisms, though the algae tend to be multicellular. Many can move, mainly by using flagella (see flagellum), cilia (see cilium), or footlike extensions (pseudopodia). The kingdom was developed to accom¬ modate intermediate organisms that, even though they possessed some plant or animal characteristics, did not exhibit the specialized features indicative of those groups. Some protists are considered the ancestors of multicellular plants, animals, and fungi. The term was first suggested in 1866 by Ernst Haeckel. With the development of advanced biochemical, genetic, and imaging techniques, previously established relationships have come under scrutiny, and it is now thought that some groups are less closely related to one another than once believed. As a result, the clas¬ sification of protists, while convenient, is no longer entirely satisfactory.
Proto-Geometric style Visual art style of ancient Greece that sig¬ naled the reawakening of technical proficiency and conscious creative spirit after the collapse of the Minoan and Mycenaean civilizations, around the 12th century bc. The vocabulary of the style was limited to circles, arcs, triangles, and wavy lines, all derived from Minoan- Mycenaean representations of aquatic and plant life. On pottery, these design elements were carefully placed in horizontal bands, mainly at a vase’s shoulder or belly. Its lower portion was usually either left plain or painted in a solid glossy black pig¬ ment inherited from Bronze Age art¬ ists.
protoceratops
N.pro-ts-'ser-o-.tapsN Any member of a genus of quadrupedal dinosaurs found as fossils in Gobi deposits of the Cretaceous period (144-65 million years ago). The hind limbs were more strongly developed than the forelimbs; the back was arched. Adults were about 7 ft (2 m) long and probably weighed about 400 lbs (180 kg). The skull was about one-fifth the body length. Bones in the skull grew backward into a perforated frill. The jaws were beaklike and toothed. There may have been a hornlike structure on top of the snout. Long spines on the well-developed tail suggest that protoceratops was semiaquatic.
Proto-Geometric amphora from Ath¬ ens, early 10th century bc; in the Ker- ameikos Museum, Athens.