The Cross-bridge Cycle
picture quote from © 2000, All Rights Reserved, SDSU & Joseph M. Mahaffy San Diego State University
肌肉生理-橫橋循環
MuMuch of our understanding of the mechanism of muscle contraction has come from excellent biochemical studies performed from the 1950s to the mid-1970s(Webb and Trentham, 83). It was during this period that methods for isolating specific muscle proteins were developed as well as the methods for measuring their physicochemical and biochemical properties.
我們對肌肉收縮機制的了解大多來自50’s到70’s年代中期傑出的生物力學研究
就在這個時期發展出了獨立研究肌肉蛋白的方法以及測量它們生理化學與生物化學特性的方法。
In its simplest form, biochemical experiments on muscle contractile proteins ave shown that, during the cross-bridge cycle, actin (A) combines with myosin (M) and ATP to produce force, adenosine diphosphate (ADP) and inorganic phosphate, Pi This can be represented as a chemical reaction in the form
其最簡易的型態，以生物化學在肌肉收縮蛋白上的研究顯示出，在橫橋循環中
肌動蛋白結合肌凝蛋白與ATP產生力量，ADP與無機磷酸鹽、磷酸，可以下列化學反應式來表示
A + M + ATP -> A + M + ADP + Pi + Force (Equation 1)
ADP
產生－當ATP分子的磷酸根水解斷裂時，會產生二磷酸腺苷，並釋放出7.3仟卡的能量
功用－當ADP與磷酸基結合並獲得8仟卡能量，可行成ATP。
ATP
備註:
ATP：三磷酸腺苷，也就是說它帶有三個磷酸根(Pi)
ADP：二磷酸腺苷，帶有兩個磷酸根(Pi)
AMP：單磷酸腺苷，帶有一個磷酸根(Pi)
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However, we also know that upon the death of a muscle, a rigor state is entered whereby actin and myosin interact to form a very stiff connection. This can be represented as
然而，我們同時也知道由於肌肉的衰死，肌凝蛋白與肌動蛋白形成非常僵硬的聯結而進入僵直的狀態，以下列化學式來表示－
：A + M -> A.M "rigor" complex (Equation 2)
If actin and myosin can interact by themselves, where does ATP come into the picture during contraction? Experiments have demonstrated that the myosin molecule can hydrolyze ATP into ADP and Pi. In other words,
若Actin與Myosin彼此可自行互動，那ATP是在收縮期間的哪個階段參與的呢？
研究顯示肌凝蛋白分子能夠水解ATP成ADP與PiM + ATP -> M + ADP + Pi (Equation 3)換句話說
Scientists now agree that ATP serves at least two functions in skeletal muscle systems: First, ATP disconnects actin from myosin, and second, ATP is hydrolyzed by the myosin molecule to produce the energy required for muscle contraction. This description of the different biochemical steps involved in muscle contraction is referred to as the Lymn-Taylor actomyosin ATPase hydrolysis mechanism. (Webb and Trentham, 83)
科學家現在同意ATP在肌肉系統中起碼有兩個功能
第一：解開Actin與Myosin的結合
第二：ATP被Actin分子所水解以產生收縮時所需要的能量
其中描述肌肉收縮的生化程序的差異是依據Lymn-Taylor：actomyosin ATPase hydrolysis mechanism
─肌動球蛋白-ATPase-水解機制
註解：
※actomyosin：結合成肌動球蛋白（actomyosin）的作用，在活體動物的肌肉中，三磷酸腺苷（ATP）除了提供能量還有防止肌纖維中的肌球蛋白和肌動蛋白結合成肌動球蛋白（actomyosin）的作用。在死亡後，由於體內的氧化磷酸化過程逐漸停止。ATP不再被合成，肌肉中現存的ATP因為水解急劇減少。這導致細胞壁上的鈣離子-ATP酶泵開啟細胞內鈣離子濃度上升，肌纖維凝結成肌動球蛋白，導致肌肉失去彈性攣縮，再過一段時間（幾小時到幾天），組織內部的酶開始消化肌肉本身，稱為自溶現象這個過程導致屍僵的自然緩解
ATPase：F型ATP酶（ATPase）ATP其中之ㄧ的合成酶，也稱為『Phosphorylation Factor』存在在各種生物中，利用電化學勢進行ATP的合成
The relationship between the Lymn-Taylor kinetic scheme and the mechanical cross-bridge cycle is not fully known. However, Lymn and Taylor proposed that their biochemical data could be incorporated into a four-step cross-bridge cycle that could be envisioned thus:
The actin-myosin bridge very rapidly dissociates due to ATP binding to myosin.
The free myosin bridge moves into position to attach to actin, during which ATP is hydrolyzed. (Eq. 3)
The free myosin bridge along with its hydrolysis products rebinds to the actin filament. (Eq. 2)
The cross-bridge generates force, and actin displaces the reaction products
(ADP and Pi) from the myosin cross-bridge. This is the rate-limiting step of contraction. The actin-myosin cross-bridge is now ready for the ATP binding.
