Thomas F. Madden is a great historian of Venice and after reading his book, "Venice: A New History," it would seem that yes, at least he believes that the Venetians (and especially their navy) were an integral part in the defense of Europe from ottoman invasion (among other nations, but his focus is clearly on the Venetian contribution).
The Venetians would supply men, money, and crucially, ships for Christian crusades in order to facilitate transport. They also, for quite a while, were the most formidable European navy which had any chance of defeating the Turks until the rise of the colonial powers, and especially after the battle of Lepanto. Until then, Venice was a serious thorn in the side of the superior Ottomans and played a crucial role in stemming the ottoman tide.
I highly suggest picking up Madden's book, if was fascinating to read and gives a wonderful, comprehensive analysis of the Venetian place in the world and its role in world history.
The Venetians were stuck in a perpetual cat-and-mouse game with the Ottoman Empire, not in the least exacerbated by the fact that from the first Ottoman-Venetian war onwards, Venetian power had already begun its slow decline. However, it is incorrect to depict the relationship as entirely adversarial; the vast majority of relations between the Ottomans and Venetians were amicable. The Venetian mercantile economy relied extensively on free passage in Istanbul and other Ottoman cities, while the Ottomans relied on Venetians as their main export market.
Wars between the Venetians and the Ottomans have to be understood within the context which sparked them. often, external events, periods of internal upheaval within the Empire, or events in Europe that diminished the comparative importance of Venetian merchants, were the decisive factors in sparking confrontation rather than some sort longstanding rivalry. Indeed, the First Ottoman-Venetian War falls within the wider Ottoman conquest of the near east, which in addition to removing the Venetians from their overseas possession of Negropont also involved the conquest of Trebizond as well as Byzantine rump-states in Morea and Epirus. The Venetian government had in fact attempted to normalize relations with the Ottomans after the fall of Costantinople, however Morea in particular housed a number of Venetian outposts and concessions; confrontation with the Venetians was an inevitable consequence of Ottoman power consecration in the Aegean (and the Venetians themselves took advantage of instability in the region to seize the island of Cyprus). Similarly, the second Ottoman-Venetian war in 1499 was a principally naval conflict which resulted in the cession of the last Venetian outposts in the Agean, definitively curbing the Republic's ability to project power in the Aegean Sea.
However, twenty years separated the first two wars between the Ottoman Empire and the Republic of Venice. The two polities would not confront each other again for another thirty years, until the Sixth Italian War in the 1530s, when the Venetians sided with Charles the V of Spain and Austria against the French King Francis; Francis convinced the Ottomans to tie up the Venetians for him while he attempted to wrest the Duchy of Milan from Charles' Empire.
By 1570 (forty years after the pervious war) tensions between Venice and the Ottomans would come to a head again when Sultan Selim II had come to power on the tail end of wars started against the Habsburg empire by his father Suleman (the "Magnificent"). Stepping into his father's shoes, Selim sought to prove himself to his father's advisors by seizing Cyprus from the Venetians. However, Cyprus was not the default objective for the new Sultan; initially, he and his advisors had postulated sending an expedition to southern Spain, hoping to prop up a client state there with help from the restless native population. Secret negotiations had even been opened with the Venetians to evaluate the feasibility of a potential collaboration. Logistics would come to be the scape tipping factor in the selection of Cyprus as a target, and in spite of the Venetian diplomatic machine's ability to assemble an improbable Italo-Iberian alliance which defeated the Ottoman fleet at sea, the unwieldy alliance was unable to organize relief for the besieged Island.
The next conflict would take place little over sixty years later (starting in 1645) over the last Venetian island possession: Crete (at the time also called Candia). The war, which was astoundingly long (lasting until 1669), saw the Venetians generally victorious at sea but unable to combat the gradual Ottoman occupation of the Island. Here too, the cause can be generally attributed to the Sultan Murad IV and his need to legitimize himself to his supporters and advisors, having re-taken the throne from his uncle Ahmed. The Thirty Years War had sizably reduced an important market for Venetian merchants, making good relations with Venice less integral to the Empire's well-being, and reduced tax income had made the Republic vulnerable.
The Sixth Ottoman-Venetian War in 1689 was in fact part of a wider conflict whose principal combatants were the Austrian Empire against the Ottomans. The Venetians, who intervened alongside the Austrians, gained the Peloponnesus, at the time called Morea. However, by 1714 the Ottoman Empire had re-gained its confidence; the Sultan Ahmed III had intervened successfully against Russia during the Russo-Swedish War in 1710, while the War of Spanish Succession occupied the attentions of Spain and Austria, leaving Venice isolated against aggression.
If you're interested, I answered a more in-depth question on Venice in the 18th century here, Venice in the 16th century here, as well as this question on the Venetian navy, and this question on the early Venetian army.