tailieunhanh - PDQ BIOCHEMISTRY - PART 5

Việc lắp ráp và liên kết ngang của tiểu đơn vị cung cấp cho fibrin khả năng phục hồi cơ khí và sức mạnh. Sử dụng một phép loại suy bóng đá (Bắc Mỹ trò chơi có sử dụng bóng hình elip và thường tích tụ điểm lớn, vì lợi ích của độc giả từ Vương quốc Anh), fibrin là một cầu thủ tuyến đầu khó khăn và to lớn. | 136 PDQ BIOCHEMISTRY which the rather complex fibrin monomers or subunits polymerize into the tough fibrin of hard clots. The assembly and cross-linking of subunits give fibrin its mechanical resilience and strength. Using a football analogy the North American game that uses the ellipsoidal ball and usually accumulates large scores for the benefit of readers from the United Kingdom fibrin is a tough and massive frontline player. In contrast the subunit structure of the soluble protein hemoglobin lends a remarkably dynamic quality to this protein that allows it to respond readily to its environment. In our football analogy hemoglobin is the quarterback superbly mobile and flexible picking up oxygen holding on to it in certain cases while in other circumstances delivering it to other players by hand-offs or passing. As subunit interaction is the key to understanding hemoglobin function the first topic for discussion is subunit structure. It is quite common for textbooks to introduce the protein myoglobin before considering hemoglobin. This arises because myoglobin is very similar in structure to a hemoglobin subunit yet myoglobin exists as a simple monomer and thus shows many functional differences when compared with the more complex hemoglobin. Myoglobin is a protein of muscle cells and it can bind and store oxygen and also facilitate the distribution of oxygen within these cells. Remember oxygen is vital for cellular energy metabolism and oxygen is needed for the mitochondrial production of ATP the nucleotide derivative that is a fundamental requirement for muscle contraction. We will discuss ATP in more detail when we outline metabolism in Chapter 7. Myoglobin is a single polypeptide and is made up of 153 amino acids with a molecular mass of 17 kDa. Myoglobin is a soluble globular protein that consists of eight a-helical regions folded into a very compact roughly spherical shape Figure 5-1 . Remember that globular proteins generally have amino acid residues with .