脂肪酸Fatty acid

饱和脂肪酸（如棕榈酸 C16:0、硬脂酸 C18:0）碳链伸直、堆积紧密，熔点高，动物脂肪中多见，常温呈固态。天然不饱和脂肪酸的双键多为顺式（如油酸 C18:1 cis-9），碳链在双键处弯折约 30°，堆积疏松、熔点低，植物油中多见。反式脂肪酸主要由油脂部分氢化产生，会升高低密度脂蛋白胆固醇。命名中 ω-3、ω-6 表示从甲基端数第 3、6 个碳处开始有双键。

Saturated fatty acids such as palmitic (C16:0) and stearic (C18:0) have straight chains that pack tightly and melt high, so animal fats rich in them are solid at room temperature. Natural unsaturated fatty acids mostly have cis double bonds (oleic acid is C18:1 cis-9), which kink the chain by about 30°, loosen packing and lower the melting point, as in vegetable oils. Trans fatty acids come mainly from partial hydrogenation of oils and raise LDL cholesterol. In ω-3 and ω-6 the number gives the position of the first double bond counted from the methyl end.

1823 年谢弗勒尔在油脂研究中分离出多种脂肪酸并命名。1929 年伯尔夫妇发现大鼠缺乏亚油酸会出现皮肤病和生长停滞，提出“必需脂肪酸”概念。1904 年克诺普用苯基标记的脂肪酸喂狗，提出 β-氧化学说；1950 年代林嫩等人阐明了脂肪酸 β-氧化的酶促过程。

Chevreul isolated and named several fatty acids during his work on fats in 1823. In 1929 George and Mildred Burr found that rats deprived of linoleic acid developed skin disease and stopped growing, introducing the idea of essential fatty acids. In 1904 Knoop fed dogs phenyl-labelled fatty acids and proposed β-oxidation, and in the 1950s Lynen and others worked out its enzymatic steps.

脂肪酸在线粒体中经 β-氧化，每轮切下两个碳生成乙酰辅酶 A，进入三羧酸循环，一分子棕榈酸完全氧化约生成 106 个 ATP。人体不能合成亚油酸（ω-6）和 α-亚麻酸（ω-3），必须从食物获得。鱼油中的 EPA 和 DHA 是 ω-3 脂肪酸，与心血管和大脑健康相关。

In mitochondria fatty acids undergo β-oxidation, losing two carbons as acetyl-CoA in each round, which enters the citric acid cycle; complete oxidation of one palmitate yields about 106 ATP. Humans cannot make linoleic acid (ω-6) or α-linolenic acid (ω-3) and must get them from food. EPA and DHA in fish oil are ω-3 fatty acids linked to cardiovascular and brain health.